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Strachan T, Read AP. Human Molecular Genetics. 2nd edition. New York: Wiley-Liss; 1999.

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Human Molecular Genetics. 2nd edition.

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Chapter 2Chromosomes in cells

DNA functions in a context. Human DNA is structured into chromosomes that function within cells. Cells divide, differentiate and drive the development of the whole organism. Cell biology and development are huge subjects that we cannot possibly do justice to here, but we start with very brief overviews of these areas to help orient those students who come to human molecular genetics from a nonbiological direction. The main part of this chapter is an introduction to chromosomes - what they are, what they do, and how they can go wrong.

2.1 Organization and diversity of cells

All organisms more complex than viruses consist of cells, aqueous compartments bounded by membranes, which under restricted conditions are capable of existing independently. All cells are derived by cell division from other cells. Ultimately, there must be an unbroken chain of cells leading back to the first successful primordial cell that lived maybe 3.5 billion years ago. How that cell formed is an interesting question.

2.1.1 Prokaryotes and eukaryotes represent a fundamental division of living cells

All cellular organisms can be subdivided into two major classes, prokaryotes and eukaryotes, on the basis of the architecture of their cells (Figure 2.1).

Figure 2.1. Prokaryotic and eukaryotic cell anatomy.

Figure 2.1

Prokaryotic and eukaryotic cell anatomy. The top part of the figure shows a typical human cell and typical bacterium drawn to scale. The human cell is 10μm in diameter and the bacterium is rod-shaped with dimensions 1 × 2 μm. Examples (more...)

Prokaryotes lack a defined nucleus and have a relatively simple internal organization. Under the electron microscope they appear relatively featureless. They comprise two kingdoms of life: eubacteria which include most of the bacteria; and the archaea, rather poorly understood organisms that superficially resemble bacteria and often grow in unusual environments, such as in acid hot springs, saturated brines, etc. The genome of a prokaryote typically consists of a single small circular chromosome in which the DNA is not packaged in any obviously organized way. Prokaryotes may be simple, but they are not primitive - they have been through far more generations of evolution than we have.

Eukaryotes have a much more complex intracellular organization with internal membranes, membrane-bound organelles including a nucleus, and a well-organized cytoskeleton. The general features are summarized briefly in Box 2.1. Eukaryotic cells have several linear chromosomes in their cell nuclei, in each of which a single very long DNA molecule is elaborately packaged by histone and other proteins. The number and DNA content of the chromosomes vary greatly between species (Table 2.1). In general the genome size tends to parallel the complexity of the organism, but there are many exceptions. Most mammals have much the same size genome. Humans do not have specially large genomes, while the cells of an onion and a lily contain respectively about five and 30 times as much DNA as a typical human cell. Eukaryotes are thought to have first apeared about 1.5 billion years ago.

Box Icon

Box 2.1

Anatomy of animal cells. Eukaryotic cells have complicated internal anatomies, with many internal membranes. Typically there is a nucleus and cytoplasm, the latter comprising various organelles, membranes and an aqueous compartment known as the cytosol. (more...)

Table 2.1. Variation in chromosome number and genome size.

Table 2.1

Variation in chromosome number and genome size.

2.1.2 Cell size and shape can vary enormously, but rates of diffusion fix some upper limits

Bacteria and some other simple organisms, such as the yeast Saccharomyces cerevisiae, consist of a single cell. Such cells are necessarily able to carry out all the functions that are required to sustain the organism. Multicellular organisms begin life as a single cell but then undergo repeated cell division, cell differentiation and cell turnover. They may end up containing huge numbers of cells. Cell differentiation ensures that individual multicellular organisms are composed of a variety of cell types that can vary greatly in size and shape.

Cells depend on diffusion to coordinate their metabolic activities. As they grow larger, the surface-to-volume ratio decreases. It is thought that the simple internal structure of prokaryotic cells limits their maximum size - typically bacterial cells are 1 μm in diameter. The complex internal membranes and compartmentalization of eukaryotic cells may be important in allowing them to grow larger. Nevertheless, metabolically active internal regions are seldom more than 15–25 μm from the cell surface, so that the limit of cell size is typically 30–50 μm. The average diameter of cells in a multicellular organism falls within the range of 10–30 μm. Some individual human nerve cells can be as long as 1 metre, but the long projections are very thin. However, the ostrich egg warns us not to over-generalize.

2.1.3 Genetic content of cells: ploidy, the cell cycle and the life cycle

Most cells of humans are diploid. They contain two copies of the human genome. The DNA content and chromosome number of a genome are designated C and n respectively. For humans C = 3.5 × 10-12 g, approximately, and n = 23. The DNA content of diploid cells is 2C and they have 2n chromosomes. Almost all mammals are diploid, but among other organisms there are many examples of species that are normally haploid (n chromosomes, DNA content C), tetraploid (4n) or polyploid. Triploidy (3n) is less common because triploids have problems with meiosis (see below).

The diploid cells of our body are derived from the original diploid fertilized egg by repeated rounds of mitotic cell division. Each round can be summarized as one turn of the cell cycle (Figure 2.2). This comprises a short stage of cell division, the M phase (mitosis; see Figure 2.10) and a long intervening interphase. Interphase can be divided into S phase (DNA synthesis), G1 phase (gap between M phase and S phase) and G2 phase (gap between S phase and M phase). From anaphase of mitosis right through until DNA duplication in S phase, a chromosome of a diploid cell contains a single DNA double helix and the total DNA content is 2C. G1 is the normal state of a cell, and the long-term end state of nondividing cells. Cells enter S phase only if they are committed to mitosis; nondividing cells remain in a modified G1 stage, sometimes called G0. The cell cycle diagram can give the impression that all the interesting action happens in S and M phases - but this is an illusion. A cell spends most of its life in G0 or G1 phase, and that is where the genome does most of its work.

Figure 2.2. Human chromosomal DNA content during the cell cycle.

Figure 2.2

Human chromosomal DNA content during the cell cycle. Interphase comprises G1 + S + G2. Chromosomes contain one DNA double helix from anaphase of mitosis right through until the DNA has duplicated in S phase. From this stage until the end of metaphase (more...)

Figure 2.10. Cell division by mitosis.

Figure 2.10

Cell division by mitosis.

A subset of the diploid body cells constitute the germ line (see Figure 2.12). These give rise to specialized diploid cells in the ovary and testis that can divide by meiosis to produce haploid gametes (sperm and egg). In humans (n = 23) each gamete contains 22 autosomes (nonsex chromosomes) plus one sex chromosome. In eggs the sex chromosome is always an X; in sperm it may be an X or a Y. After fertilization the zygote is diploid (2n) with the chromosome constitution 46,XX or 46,XY (Figure 2.3). The other cells of the body, apart from the germline, are known as somatic cells. Somatic cells have no input to succeeding generations - unless animal cloners intervene.

Figure 2.12. Development of the germ line.

Figure 2.12

Development of the germ line. The germ line develops by repeated mitotic division of diploid cells, culminating in production of primary oocytes and primary spermatocytes. These diploid cells can undergo meiosis. Meiosis involves two cell divisions but (more...)

Figure 2.3. Human life, from a chromosomal viewpoint.

Figure 2.3

Human life, from a chromosomal viewpoint. The haploid sperm and egg cells originate by meiosis from diploid precursors (see Figure 2.12). In the fertilized egg the sperm and egg chromosomes initially form separate male and female pronuclei. These combine (more...)

Although most somatic cells are diploid, there are exceptions. Some terminally differentiated cells, red blood cells for example, have no nuclei and others, such as keratinocytes, are devoid of organelles altogether. Such cells are nulliploid. Other cells are polyploid as a result of DNA replication without cell division (endomitosis). Regenerating cells of the liver and other tissues are naturally tetraploid because of endomitosis, while the giant megakaryocytes of the bone marrow usually contain 8C, 16C or 32C, and individually give rise to thousands of nulliploid platelet cells (Figure 2.4A). Other naturally occurring cells have multiple diploid nuclei as a result of cell fusion (Figure 2.4B).

Figure 2.4. Some cells form by fragmentation or fusion of other cells.

Figure 2.4

Some cells form by fragmentation or fusion of other cells. (A) Platelets are formed by budding from a giant megakaryocyte. They have no nucleus. (B) Muscle cells are formed by fusion of large numbers of myoblast cells.

2.2 Development

The development of any animal from a single fertilized egg cell is vastly complicated, but the early stages are common to all animals (Box 2.2), and at the molecular level development is controlled by a limited repertoire of developmental programs. All development depends on the basic processes of cell division, differentiation, morphogenesis and programed cell death (apoptosis). Differentiation is driven by gene switching: the difference between one cell type and another is primarily in the range of genes that are active in each cell. Morphogenesis, too, is ultimately driven by gene switching, as particular cells develop the capacity to respond to signals from neighboring cells by moving, dividing or dying. Apoptosis is an integral part of development: cells do not just happen to die, they have an inbuilt death program that is triggered in response to external or internal signals. All these developmental programs depend on cascades of signals and responses that have been remarkably highly conserved throughout the animal kingdom. Unraveling these programs is a major part of biological research. Probably the best introduction to how this is done, and why the results matter, is the book by Lawrence (Further reading).

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Box 2.2

A brief outline of animal development. This very brief summary concentrates on origins and cell lineages. We see that the earliest stages of human development are largely concerned with forming extraembryonic structures, and that the cells and tissues (more...)

2.2.1 Only a small percentage of the cells in the early embryo give rise to the mature organism

As shown in Box 2.2, the early stages of human development are largely concerned with establishing the future placenta and extraembryonic membranes, and implanting the conceptus in the uterine endometrium. About 2 weeks of development and many rounds of mitosis pass before the group of cells that will give rise to the embryo has any separate identity. Embryonic development proper begins with gastrulation in the third week after fertilization.

2.2.2 Cells differ in their potential to divide and differentiate

Multicellular animals begin life as a single cell following fertilization of an egg cell (oocyte) by a sperm cell. The fertilized egg proceeds to undergo a series of cell divisions. At the early stages of development, individual cells in the embryo are totipotent: each cell retains the capacity to differentiate into all the different types of cell in the body. As development proceeds cells become more restricted in their capacity to generate different types of descendant cells and are said to be pluripotent. Progenitor cells that can only develop into a single cell type are unipotent cells. The processes of cell differentation lead to individual cells acquiring specialized forms and functions (Box 2.3).

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Box 2.3

The diversity of human cells. Over 200 types of cells are described in histology textbooks. Here we illustrate the variety of size and form among common cell types. Ovum - A large cell, 120 μm in diameter, surrounded by the zona pellucida. The (more...)

Histology textbooks recognize about 210 different types of cell in the human body. Some, such as epithelial cells and fibroblasts, perform essentially the same role in a variety of organ systems. Others, such as hepatocytes, may be restricted to an individual organ. Some mature terminally differentiated cells do not undergo cell division. Other cells (often distinguished by the suffix -blast , as in osteoblasts, chondroblasts, myoblasts, etc.) divide actively and act as precursors of terminally differentiated cells. In some cases, the precursor cells are also capable of undergoing self-renewal and are known as stem cells. Stem cells are an important target of gene therapy (Chapter 22). As an example, Figure 2.5 illustrates the successive commitment of cells in the hemopoietic lineage.

Figure 2.5. Commitment and differentiation in a cell lineage.

Figure 2.5

Commitment and differentiation in a cell lineage. Blood cells are formed from pluripotent hematopoietic stem cells in the bone marrow.

2.2.3 X inactivation is a special feature of female development

Chromosomally-based sex-determination systems, like the human XX/XY system, create a problem for development. Having the wrong number of chromosomes almost always makes an organism develop abnormally, and yet the two sexes must develop normally with different chromosome constitutions. In humans and other mammals the solution is different for the X and the Y chromosomes. The Y chromosome contains very few genes, and these are mostly genes governing male sexual function, so that females can get by perfectly well without a Y chromosome. The X chromosome, however, contains many genes that play vital roles in both sexes, and so some method of dosage compensation is required, to ensure that cells function normally with either one or two X chromosomes.

Mammals achieve X-chromosome dosage compensation by the mechanism of X-inactivation, often called lyonization, after Dr Mary Lyon who first suggested this mechanism (Lyon, 1999, Heard et al., 1997; Figure 2.6). Early in embryonic development (at late blastocyst stage in the mouse, and probably also in humans), cells somehow count their X chromosomes and then inactivate all but one of them. 46,XY cells (and also 45,X cells) leave their single X active; 46,XX cells (and also 47,XXY cells) inactivate one X, while a 47,XXX cell would inactivate two Xs. Thus, regardless of the number of X chromosomes in the karyotype, there is only one set of active X-linked genes in a cell. Males are constitutionally hemizygous for X chromosome genes (that is, they have only a single copy of each gene), while females become functionally hemizygous: they have only a single functional copy of each gene.

Figure 2.6. The process of X chromosome inactivation in mammals.

Figure 2.6

The process of X chromosome inactivation in mammals. In the early XX female zygote, both X chromosomes are active but, around the time embryonic development begins, a choice is made randomly in each cell to inactivate either the paternal or the maternal (more...)

When chromosomes of a female cell are observed at metaphase of mitosis, the active and inactive Xs look the same - but this is because all chromosomes at metaphase of mitosis are condensed and largely inactive. After the end of cell division, the inactive X remains condensed while the other chromosomes decondense and resume transcriptional activity. In some cells the inactive X can be seen as a Barr body or sex chromatin body near the membrane of the interphase nucleus. This allows a simple but not very reliable method of sexing interphase cells.

Which X in a 46,XX cell is inactivated is random (with a few exceptions discussed later), so that in a female embryo, some cells will inactivate the paternal X and some the maternal X. Once the choice is made, it is remembered. When the cell divides, the daughter cells inactivate the same X as the mother cell. An adult female is a mosaic of clones derived from different embryonic cells. Within a clone, all the cells inactivate the same X, but between clones the choice is random. If she happens to be a carrier of an X-linked recessive disease, this can have major implications (Section 3.1.2).

X inactivation is a fascinating and imperfectly understood phenomenon. One would like to know how the cell counts its X chromosomes, how the inactivation works, how it is perpetuated, and how it is reversed during gametogenesis (see Figure 2.6). We know that there is not blanket inactivation of all genes on the inactive X; certain regions (see Section 2.4.3), and certain genes in other regions, escape inactivation. The mechanisms are discussed in more detail in Section 8.5.2

2.3 Structure and function of chromosomes

Chromosomes as seen under the microscope and illustrated in textbooks are rather misleading. When we look at chromosomes in a dividing cell we see the genome of the cell largely switched off and packed up into neat bundles ready for cell division. The processes of cell division are fascinating in their own right, and errors in packaging or dividing up the genome have major medical consequences (Section 2.6). However, it is important to remember that the switched on, functioning interphase chromosome that controls cellular activities is a much more extended and diffuse structure than the metaphase chromosomes seen in Figure 2.17. Importantly, it comprises only a single chromatid and one DNA double helix, not the two-chromatid structure of mitotic chromosomes. As functioning organelles, eukaryotic chromosomes seem to require only three classes of DNA sequence element: centromeres, telomeres and origins of replication. This simple requirement has been verified by the successful construction of artificial chromosomes in yeast: large foreign DNA fragments behave as autonomous chromosomes when ligated to short sequences that specify a functional centromere, two telomeres and a replication origin (Figure 4.16). Recently mammalian artificial chromosomes have been constructed on similar principles (Huxley, 1997; Schindelhauer, 1999).

Figure 2.17. G-banded prometaphase karyogram of mitotic chromosomes from lymphocytes of a normal female.

Figure 2.17

G-banded prometaphase karyogram of mitotic chromosomes from lymphocytes of a normal female. Compare with the idealized ideograms in Figure 2.18. Overall lengths of metaphase chromosomes range between 2 and 10 μm; the DNA of the cell, if stretched (more...)

2.3.1 Packaging of DNA into chromosomes requires multiple hierarchies of DNA folding

In the cell the structure of each chromosome is highly ordered (Manuelidis, 1990). Even in the interphase nucleus the 2 nm DNA double helix is subject to at least two levels of coiling (Figure 2.7).

Figure 2.7. From DNA duplex to metaphase chromosome.

Figure 2.7

From DNA duplex to metaphase chromosome. The figure shows human chromosome 17, as seen in a G-banded, 400 band preparation. The estimated packaging ratios (the degree of compaction of the linear DNA duplex) for human chromosomes are 1:6 for nucleosomes, (more...)

  • The most fundamental unit of packaging is the nucleosome. This consists of a central core of eight histone proteins, small highly conserved basic proteins of 102–135 amino acids. Each core comprises two molecules each of histones H2A, H2B, H3 and H4, around which a stretch of 146 bp of double-stranded DNA is coiled in 1.75 turns. Adjacent nucleosomes are connected by a short length of spacer DNA. Electron micrographs of suitable preparations show a ‘string of beads’ appearance.
  • The string of beads, approximately 10 nm in diameter, is in turn coiled into a chromatin fiber of 30 nm diameter. The interphase chromosome seems to consist of these chromatin fibers, probably organized into long loops as described below.

During cell division, the chromosomes become ever more highly condensed. The DNA in a metaphase chromosome is compacted to about 1/10 000 of its stretched-out length. Loops of the 30 nm chromatin fiber, containing 20–100 kb of DNA per loop, are attached to a central scaffold . This consists of nonhistone acidic proteins, notably topoisomerase II, an enzyme which has the interesting ability to pass one DNA double helix through another by cutting a gap and repairing it. Topoisomerase II and some other chromatin proteins are known to bind to AT-rich sequences, and the chromatin loops may be attached by stretches of several hundred base pairs of highly AT-rich (>65%) DNA (scaffold attachment regions). In the chromatids of a metaphase chromosome the loop-scaffold complex is compacted yet further by coiling (see Figure 2.7).

2.3.2 Chromosomes as functioning organelles: the centromere

Normal chromosomes have a single centromere that is seen under the microscope as the primary constriction, the region at which sister chromatids are joined. The centromere is essential for segregation during cell division. Chromosome fragments that lack a centromere (acentric fragments) do not become attached to the spindle, and so fail to be included in the nuclei of either of the daughter cells.

During late prophase of mitosis, a pair of kinetochores forms at each centromere, one attached to each sister chromatid. Multiple microtubules attach to each kinetochore, linking the centromere of a chromosome and the two spindle poles (see Figure 2.11). At anaphase, the kinetochore microtubules pull the two sister chromatids toward opposite poles of the spindle. Kinetochores play a central role in this process, by controlling assembly and disassembly of the attached microtubules and, through the presence of motor molecules, by ultimately driving chromosome movement.

Figure 2.11. Mitosis: homologous chromosomes align independently on the metaphase plate and spindle fibers then pull the separated sister chromatids to opposite poles.

Figure 2.11

Mitosis: homologous chromosomes align independently on the metaphase plate and spindle fibers then pull the separated sister chromatids to opposite poles. (A) At metaphase, paternal (black) and maternal (blue) homologs of each chromosome pair are independently (more...)

Specific DNA sequences presumably specify the structure and function of centromeres. In simple eukaryotes, the sequences that specify centromere function are very short. For example, in the yeast Saccharomyces cerevisiae the centromere element (CEN) is about 110 bp long, comprising two highly conserved flanking elements of 9 bp and 11 bp and a central AT-rich segment of about 80–90 bp (Figure 2.8). The centromeres of such cells are interchangeable - a CEN fragment derived from one yeast chromosome can replace the centromere of another with no apparent consequence. In mammals, centromeres comprise hundreds of kilobases of repetitive DNA, some nonspecific and some chromosome-specific (Section 7.4.1).

Figure 2.8. The functional elements of a yeast chromosome.

Figure 2.8

The functional elements of a yeast chromosome.

2.3.3 Chromosomes as functioning organelles: origins of replication

The DNA in most diploid cells normally replicates only once per cell cycle. The initiation of replication is controlled by cis-acting sequences that lie close to the points at which DNA synthesis is initiated. Probably these are sites at which trans-acting proteins bind. Eukaryotic origins of replication have been most comprehensively studied in yeast, where the presence of a putative replication origin can be tested by a genetic assay. To test the ability of a random fragment of yeast DNA to promote autonomous replication, it is incorporated into a bacterial plasmid together with a yeast gene that is essential for growth of yeast cells. This construct is used to transform a mutant yeast that lacks the essential gene. The transformed cells can only form colonies if the plasmid can replicate in yeast cells. However, the bacterial replication origin in the plasmid does not function in yeast, therefore the few plasmids that transform at high efficiency must possess a sequence within the inserted yeast fragment that confers the ability to replicate extrachromosomally at high efficiency - that is an autonomously replicating sequence (ARS) element.

ARS elements are thought to derive from authentic origins of replication and, in some cases, this has been confirmed by mapping a specific ARS element to a specific chromosomal location and demonstrating that DNA replication is indeed initiated at this location. ARS elements extend for only about 50 bp and consist of an AT-rich region which contains a conserved core consensus and some imperfect copies of this sequence (Figure 2.8). In addition, the ARS elements contain a binding site for a transcription factor and a multiprotein complex is known to bind to the origin.

Mammalian replication origins have been much less well defined because of the absence of a genetic assay. Some initiation sites have been studied, but such studies have not been able to identify a unique origin of replication. This has led to speculation that replication can be initiated at multiple sites over regions tens of kilobases long. Mammalian artificial chromosomes seem to work without specific ARS sequences being provided. Computer analysis of regions encompassing several eukaryotic origins of replication, including some human and other mammalian examples, identified a consensus DNA sequence WAWTTDDWWWDHWGWHMAWTT where W = A or T; D = A or G or T; H = A or C or T; and M = A or C (Dobbs et al., 1994).

2.3.4 Chromosomes as functioning organelles: the telomeres

Telomeres are specialized structures, comprising DNA and protein, which cap the ends of eukaryotic chromosomes. They have several likely functions:

  • Maintaining the structural integrity of a chromosome. If a telomere is lost, the resulting chromosome end is unstable. It has a tendency either to fuse with the ends of other broken chromosomes, to be involved in recombination events or to be degraded. The loop structure of human telomeres (see below) means that natural chromosomes have no free DNA end.
  • Ensuring complete replication of the extreme ends of chromosomes. During DNA replication, synthesis of the lagging strand is discontinuous and requires the presence of some DNA ahead of the sequence which is to be copied to serve as the template for an RNA primer (see Figure 1.9). However, at the extreme end of a linear molecule, there can never be such a template, and a different mechanism is required to solve the problem of replicating the ends of a linear DNA molecule (see below).
  • Helping establish the three-dimensional architecture of the nucleus and/or chromosome pairing. Chromosome ends appear to be tethered to the nuclear membrane, suggesting that telomeres help position chromosomes.

Eukaryotic telomeres consist of a long array of tandem repeats. One DNA strand contains TG-rich sequences and terminates in the 3′ end; the complementary strand is CA-rich. Unlike centromeres, the sequence of telomeres has been highly conserved in evolution - there is considerable similarity in the simple sequence repeat, for example TTGGGG (Paramecium), TAGGG (Trypanosoma) TTTAGGG (Arabidopsis) and TTAGGG (Homo sapiens) (see also Figure 2.8).

The problem of replicating the ends of a chromosome has been solved by extending the synthesis of the leading strand using a specialized enzyme, telomerase. This RNA-protein complex carries within its RNA component a short sequence which will act as a template to prime extended DNA synthesis of telomeric DNA sequences on the leading strand. Further extension of the leading strand provides the necessary template for DNA polymerase α to complete synthesis of the lagging strand (Figure 2.9). This mechanism leaves the telomere itself with a protruding 3′ end. In mammalian chromosomes, the single-stranded end is believed to loop round and invade the double helix several kilobases proximally, producing a triple-stranded structure resembling the mitochondrial D-loop (Figure 7.2), which is stabilized by binding telomere-specific proteins (Greider, 1999). However, the actual nature of the telomere sequence may not be important. The telomere length is known to be highly variable and is subject to genetic control.

Figure 2.9. Telomerase extends the TG-rich strand of telomeres by DNA synthesis using an internal RNA template.

Figure 2.9

Telomerase extends the TG-rich strand of telomeres by DNA synthesis using an internal RNA template.

Just internal to the essential telomeric repeats, eukaryotic chromosomes also have a more complex set of repeats called subtelomeric or telomere-associated repeats. Their sequences are not conserved in eukaryotes and their function is unknown.

2.3.5 Heterochromatin and euchromatin

In the interphase nucleus most of the chromatin (euchromatin) exists in an extended state, dispersed through the nucleus and staining diffusely. However, some chromatin remains highly condensed throughout the cell cycle and forms dark-staining regions (heterochromatin). Genes located in euchromatin may or may not be expressed, depending on the cell type and its metabolic requirements, but genes that are located within heterochromatin, either naturally or as the result of a chromosomal rearrangement, are very unlikely to be expressed. There are two classes of heterochromatin:

Figure 2.18. Banding pattern of human chromosomes.

Figure 2.18

Banding pattern of human chromosomes. This is a compilation of the best banding patterns that might be seen on each chromosome, and not a picture of how chromosomes appear in any one cell under the microscope. Chromosomes are numbered in order of size, (more...)

In euchromatin, the G bands (Section 2.5.2) partake of some of the properties of heterochromatin, but to a lesser degree. G band chromatin in metaphase chromosomes is more condensed than R band chromatin, and data on CpG island distribution (Section 7.1.2; Figure 7.4) show that G bands are relatively poor in genes. The subset of R bands that are revealed by T-banding have a particularly high density of genes. Section 1.3.5 discusses the different structures of chromatin in transcriptionally active and inactive chromosomal regions.

2.4 Mitosis and meiosis are the two types of cell division

2.4.1 Mitosis is the normal form of cell division

As a person develops from an embryo, through fetus and infant to an adult, cell divisions are needed to generate the large numbers of cells required. Additionally, many cells have a limited lifespan, so there is a continuous requirement to generate new cells in the adult. All these cell divisions occur by mitosis. Mitosis is the normal process of cell division, from cleavage of the zygote to death of the person. In the lifetime of a human there may be something like 1017 mitotic divisions (Section 9.2.1).

The M phase of the cell cycle (Figure 2.2) consists of the various stages of nuclear division (prophase, prometaphase, metaphase, anaphase and telophase of mitosis), and cell division (cytokinesis), which overlaps the final stages of mitosis (Figure 2.10). In preparation for cell division, the previously highly extended chromosomes contract and condense so that, by metaphase of mitosis, they are readily visible under the microscope. Even though the DNA was replicated some time previously, it is only at prometaphase that individual chromosomes can be seen to comprise two sister chromatids, attached at the centromere.

The mitotic spindle (Figure 2.11) is formed from tubulin-based microtubules and microtubule-associated proteins. Polar fibers, which extend from the two poles of the spindle towards the equator, develop at prophase while the nuclear membrane is still intact. Kinetochore fibers do not develop until prometaphase. These fibers attach to the kinetochore, a large multiprotein structure attached to the centromere of each chromatid, and extend in the direction of the spindle poles. The interaction between the different spindle fibers pulls the chromosomes towards the center, and by metaphase each chromosome is independently aligned on the equatorial plane (metaphase plate). Paternal and maternal homologs do not associate at all during mitosis. Following centromere division at anaphase, the spindle fibers pull the separated sister chromatids of each chromosome to opposite poles (Figure 2.11). The DNA of the two sister chromatids is identical, barring any errors in DNA replication. Thus the effect of mitosis is to generate daughter cells that contain precisely the same DNA sequences.

2.4.2 Meiosis is a specialized form of cell division giving rise to sperm and egg cells

Primordial germ cells migrate into the embryonic gonad and engage in repeated rounds of mitosis (many more in males than in females, which may be a significant factor in explaining sex differences in mutation rates - see Figure 9.4) to form oogonia in females and spermatogonia in males. Further growth and differentiation produces primary oocytes in the ovary and primary spermatocytes in the testis. These specialized diploid cells can undergo meiosis (Figure 2.12). Meiosis involves two successive cell divisions but only one round of DNA replication, so the products are haploid. In males, the product is four spermatozoa; in females, however, the cytoplasm divides unequally at each stage: the products of meiosis I (the first meiotic division) are a large secondary oocyte and a small cell (polar body). The secondary oocyte then gives rise to the large mature egg cell and a second polar body.

There are two crucial differences between mitosis and meiosis (Table 2.2).

Table 2.2. Mitosis and meiosis compared.

Table 2.2

Mitosis and meiosis compared.

  • The products of mitosis are diploid; the products of meiosis are haploid.
  • The products of mitosis are genetically identical; the products of meiosis are genetically different.

Mitosis involves a single turn of the cell cycle (Figure 2.2). The DNA is replicated in S phase and the two copies are divided exactly equally between the daughter cells in M phase. Meiosis is also preceded by one round of DNA synthesis, but then there are two cell divisions without intervening DNA synthesis, so that the products end up haploid. The second division of meiosis is identical to mitosis, but the first division has important differences whose purpose is to generate genetic diversity between the daughter cells. This is done by two mechanisms, independent assortment of paternal and maternal homologs (Figure 2.13), and recombination.

Figure 2.13. Meiosis: independent assortment of maternal and paternal homologs at meiosis I produces the first level of genetic diversity.

Figure 2.13

Meiosis: independent assortment of maternal and paternal homologs at meiosis I produces the first level of genetic diversity. There are 223 or 8.4 million different ways of picking one chromosome from each of the 23 pairs in a diploid cell. Gametes A-E show (more...)

Independent assortment of paternal and maternal homologs

During meiosis I the maternal and paternal homologs of each chromosome pair form a bivalent by pairing together (synapsis) (Figure 2.14). Each chromosome consists of two sister chromatids following DNA replication, so that the bivalent is a four-stranded structure at the metaphase plate. Spindle fibers then pull one complete chromosome (two chromatids) to either pole. However, for each of the 23 homologous pairs, the choice of which homolog enters which daughter cell is independent. This allows 223 or about 8.4 × 106 possible combinations of parental chromosomes to be produced by one person.

Figure 2.14. Meiosis: the five stages of prophase in meiosis I.

Figure 2.14

Meiosis: the five stages of prophase in meiosis I. Two representative pairs of homologs are shown. There are two crossovers in the chromosome 1 bivalent and one in the chromosome 17 bivalent. For the sake of clarity, the two crossovers on chromosome 1 (more...)

Recombination

During prophase of meiosis I the synapsed homologs within each bivalent exchange segments in a random way. At the zygotene stage, each pair of homologs begins to form a synaptonemal complex consisting of the two chromosomes in close apposition, separated by a long linear protein core. Completion of this complex marks the start of the pachytene stage, which is when recombination (or crossover) occurs. Crossing-over involves physical breakage of the double helix in one paternal and one maternal chromatid, and joining of maternal and paternal ends. Overall, the combination of recombination between homologs in prophase I plus independent assortment of homologs at anaphase I ensures that a single individual can produce an almost unlimited number of genetically different gametes.

The mechanism allowing alignment of the homologs is not understood. However, it is thought that such close apposition is required for recombination. Recombination nodules, very large multiprotein assemblies located at intervals on the synaptonemal complex, are thought to mediate the recombination events. The two homologs can be seen to be physically connected at specific points. Each such connection is described as a chiasma (plural chiasmata) and marks a crossover point. There are an average of 55 chiasmata in a male meiotic cell, and maybe 50% more in female meiosis. The genetic consequences of crossing over are considered in Chapter 11.

In addition to their role in recombination, chiasmata are thought to be essential for correct chromosome segregation at meiosis I. By holding the maternal and paternal homologs of each chromosome pair together on the spindle until anaphase I (Figures 2.14 and 2.15), they have a role that is analogous to that of the centromeres in mitosis and meiosis II. There is genetic evidence that children with wrong numbers of chromosomes are often the product of gametes where a bivalent lacked crossovers. Meiosis II appears identical to mitosis, except that there are only 23 chromosomes instead of 46. Each chromosome consists of two chromatids, and these are separated in anaphase II. However, there is one difference. The sister chromatids of a mitotic chromosome are identical, being copies of each other. The two chromatids of a chromosome in meiosis II may be genetically different as a result of crossovers in meiosis I (Figure 2.15).

Figure 2.15. Meiosis: from metaphase I to the gametes.

Figure 2.15

Meiosis: from metaphase I to the gametes. The figure follows on from Figure 2.14. (A) From metaphase to cell division in meiosis I. The figure shows one possible segregation pattern of the two bivalents. (B) Meiosis II. Although the two sister chromatids (more...)

2.4.3 X-Y pairing and the pseudoautosomal regions

In female meiosis, each chromosome has a fully homologous partner, and the two Xs synapse and cross over just like any other pair of homologs. In male meiosis there is a problem. The human X and Y sex chromosomes are very different from one another. Nevertheless, they do pair in prophase I in males, thus ensuring that at anaphase I each daughter cell receives one sex chromosome, either the X or the Y. X-Y pairing is end-to-end rather than along the whole length, and it is made possible by a 2.6 Mb region of homology between the X and Y chromosomes at the tips of their short arms. Pairing is sustained by an obligatory crossover in this region. Genes in the pairing segment have some interesting properties:

  • they are present as homologous copies on the X and Y chromosomes;
  • they are not subject to X-inactivation (as expected since each sex has two copies);
  • because of the crossing over, alleles at these loci do not show the normal X-linked or Y-linked patterns of inheritance, but segregate like autosomal alleles.

Because of this behavior, this region is known as the major pseudoautosomal region. A second smaller pseudoautosomal region of 320 kb is located at the tips of the long arms of both chromosomes, but pairing and crossing-over in this minor pseudoautosomal region is not an obligatory feature of male meiosis.

2.5 Studying human chromosomes

2.5.1 Mitotic chromosomes can be seen in any dividing cell, but most conveniently in lymphocytes; meiotic chromosomes are hard to study in humans

Chromosomes can only be seen in dividing cells, and obtaining dividing cells directly from the human body is difficult. Bone marrow is a possible source, but it is much easier all round to take an accessible source of nondividing cells and culture them in the laboratory. Blood is the material of choice - most people don't mind giving a few millilitres, and the T lymphocytes in blood can be easily induced to divide by treatment with lectins such as phytohemagglutinin. Other common sources include fibroblasts grown from skin biopsies, and (for prenatal diagnosis) chorionic villi or fetal cells shed into the amniotic fluid.

Although chromosomes were described accurately in some organisms as early as the 1880s, for many decades all attempts to prepare spreads of human chromosomes produced a tangle that defied analysis. The key to getting analyzable spreads was a new technique, growing cells in liquid suspension and treating them with hypotonic saline to make them swell. This allowed the first good quality preparations to be made in 1956. White cells from blood are put into a rich culture medium laced with phytohemagglutinin and allowed to grow for 48–72 hours, by which time they should be dividing freely. Nevertheless, because M phase occupies only a small part of the cell cycle, few cells will be actually dividing at any one time. The mitotic index (proportion of cells in mitosis) is increased by treating the culture with a spindle disrupting agent such as colcemid. Cells reach M phase of the cycle, but are unable to leave it, and so cells accumulate in metaphase of mitosis. Often it is preferable to study prometaphase chromosomes, which are less contracted and so show more detail. Cell cultures can be prevented from cycling by thymidine starvation; when the block is released the cells progress through the cycle synchronously. By trial and error, the time after release can be determined when a good proportion of cells are in the desired prometaphase stage.

Meiosis can only be studied in testicular or ovarian samples. Female meiosis is especially difficult, as it is active only in fetal ovaries, whereas male meiosis can be studied in a testicular biopsy from any post-pubertal male who is willing to give one. The results of meiosis can be studied by analyzing chromosomes from sperm, although the methodology for this is cumbersome. Meiotic analysis is used for some investigations of male infertility.

2.5.2 Chromosomes are identified by their size, centromere position and banding pattern

Until the 1970s chromosomes were identified on the basis of their size and the position of the centromeres. This allowed chromosomes to be classified into groups (Table 2.3) but not unambiguously identified. The introduction of banding techniques (Box 2.4) finally allowed each chromosome to be identified, as well as permitting more accurate definition of translocation breakpoints, subchromosomal deletions, etc. Banding resolution can be increased by using more elongated chromosomes, for example chromosomes from prometaphase or earlier, rather than metaphase. Typical high-resolution banding procedures for human chromosomes can resolve a total of 400, 550 or 850 bands (Figures 2.16, 2.18). The chromosome constitution is described by a karyotype that states the total number of chromosomes and the sex chromosome constitution. Human females and males are 46,XX and 46,XY respectively. When there is a chromosomal abnormality the karyotype also describes the type of abnormality and the chromosome bands or sub-bands affected. See Box 2.5 for details of chromosome nomenclature. Chromosomes are displayed as a karyogram (often loosely described as a karyotype). Karyograms such as Figure 2.17 are prepared by cutting up a photograph of the spread, matching up homologous chromosomes and sticking them back down on a card - or nowadays more often by getting an image analysis computer to do the job.

Table 2.3. Human chromosome groups.

Table 2.3

Human chromosome groups.

Box Icon

Box 2.4

Chromosome banding. G-banding - the chromosomes are subjected to controlled digestion with trypsin before staining with Giemsa, a DNA-binding chemical dye. Dark bands are known as G bands. Pale bands are G negative (Figures 2.17, 2.18). Q-banding - the (more...)

Figure 2.16. Different chromosome banding resolutions can resolve bands, sub-bands and sub-sub-bands.

Figure 2.16

Different chromosome banding resolutions can resolve bands, sub-bands and sub-sub-bands. (A) G-banded chromosome 1 at different banding resolutions. (B) Numbering of bands, sub-bands, and sub-sub-bands. Reproduced from Wolstenholme (1992) in Human Cytogenetics: (more...)

Box Icon

Box 2.5

Chromosome nomenclature. The International System for Human Cytogenetic Nomenclature (ISCN) is fixed by the Standing Committee on Human Cytogenetic Nomenclature (see Mitelman, 1995). The basic terminology for banded chromosomes was decided at a meeting (more...)

Chromosome banding picks out structural organization on a 1–10 Mb scale

Various treatments involving denaturation and/or enzymatic digestion, followed by incorporation of a DNA-specific dye, can cause human and other mitotic chromosomes to stain as a series of light and dark bands (Box 2.4, Figure 2.18; see Craig and Bickmore, 1993). Banding patterns are interesting (as well as being useful to cytogeneticists) because they provide evidence of some sort of structure over 1–10 Mb regions. The banding patterns correlate with other properties. Regions that stain as dark G bands replicate late in S phase of the cell cycle and contain more condensed chromatin, while R bands (light G bands) generally replicate early in S phase, and have less condensed chromatin. Genes are mostly concentrated in the R bands, while the later replicating, more condensed G-band DNA is less active transcriptionally. There are also differences in the types of dispersed repeat elements found in G and R bands (Sections 7.4.5 and 7.4.6).

Bands similar to G bands can be produced by staining with quinacrine, which preferentially binds to AT-rich DNA, while the R-banding pattern can be elicited using chromomycin, which preferentially binds GC-rich DNA. However, the AT content of human G band DNA is only a few per cent higher than R band DNA. Saitoh and Laemmli (1994) suggested the difference depends on differences in the scaffold-loop structure (see Figure 2.7). Chromatin loops are thought to attach to the chromosome scaffold at special scaffold attachment regions (SARs). According to the Saitoh and Laemmli model, there are more SARs per unit length of DNA in G bands than in R bands. G bands have smaller loops and a tighter ‘queue’ of SARs along the scaffold, so that there are more SARs per unit length of chromosome, leading to stronger staining with AT-selective stains like Giemsa.

2.6 Chromosome abnormalities

Chromosome abnormalities might be defined as changes resulting in a visible alteration of the chromosomes. How much can be seen depends on the technique used. The smallest loss or gain of material visible by traditional methods on standard cytogenetic preparations is about 4 megabases of DNA. However, fluorescence in situ hybridization (FISH, Section 10.1.4) allows much smaller changes to be seen; the development of molecular cytogenetics has removed any clear dividing line between changes described as chromosomal abnormalities and changes thought of as molecular or DNA defects. An alternative definition of a chromosomal abnormality is an abnormality produced by specifically chromosomal mechanisms. Most chromosomal aberrations are produced by misrepair of broken chromosomes, by improper recombination or by malsegregation of chromosomes during mitosis or meiosis.

2.6.1 Types of chromosomal abnormality

A chromosomal abnormality may be present in all cells of the body (constitutional abnormality), or may be present in only certain cells or tissues (somatic or acquired abnormality). Constitutional abnormalities must have been present very early in development, most likely the result of an abnormal sperm or egg, or maybe abnormal fertilization or an abnormal event in the early embryo. By contrast, an individual with a somatic abnormality is a mosaic (see Figure 3.9), containing cells with two different chromosome constitutions, with both cell types deriving from the same zygote. Chromosomal abnormalities, whether constitutional or somatic, mostly fall into two categories: numerical and structural abnormalities (Box 2.6). Occasionally, abnormalities have been identified in which chromosomes have the correct number and structure, but represent unequal contributions from the two parents (Section 2.6.4). Perhaps unexpectedly, correct parental origin matters.

Box Icon

Box 2.6

Nomenclature of chromosome abnormalities. This is a short nomenclature; a more complicated nomenclature is defined by the ISCN that allows complete description of any chromosome abnormality - see Further reading.

2.6.2 Numerical chromosomal abnormalities involve gain or loss of complete chromosomes

Three classes of numerical chromosomal abnormalities can be distinguished: polyploidy, aneuploidy and mixoploidy.

Polyploidy

Between 1 and 3% of recognized human pregnancies are triploid. The most usual cause is two sperm fertilizing a single egg (dispermy); sometimes the cause is a diploid gamete (Figure 2.19). Triploids very seldom survive to term, and the condition is not compatible with life. Tetraploidy is much rarer and always lethal. It is usually due to failure to complete the first zygotic division: the DNA has replicated to give a content of 4C, but cell division has not then taken place as normal. Although constitutional polyploidy is rare and lethal, all normal people have some polyploid cells (Section 2.1.3).

Figure 2.19. Origins of triploidy and tetraploidy.

Figure 2.19

Origins of triploidy and tetraploidy. About two-thirds of human triploids arise by fertilization of a single egg by two sperm (A). Other causes are a diploid egg (B) or sperm (C). Most human triploids abort spontaneously; very rarely they survive to term, (more...)

Aneuploidy

Euploidy means having complete chromosome sets (n, 2n, 3n, etc.). Aneuploidy is the opposite, that is, one or more individual chromosomes extra or missing from a euploid set. Trisomy means having three copies of a particular chromosome in an otherwise diploid cell, for example trisomy 21 (47,XX or XY, +21) in Down syndrome. Monosomy is the corresponding lack of a chromosome, for example monosomy X (45,X) in Turner syndrome. Cancer cells often show extreme aneuploidy, with multiple chromosomal abnormalities (Figure 18.6). Aneuploid cells arise through two main mechanisms:

  • Nondisjunction: failure of paired chromosomes to separate (disjoin) in anaphase of meiosis I, or failure of sister chromatids to disjoin at either meiosis II or at mitosis. Nondisjunction in meiosis produces gametes with 22 or 24 chromosomes, which after fertilization by a normal gamete make a trisomic or monosomic zygote. Nondisjunction in mitosis produces a mosaic.
  • Anaphase lag: failure of a chromosome or chromatid to be incorporated into one of the daughter nuclei following cell division, as a result of delayed movement (lagging) during anaphase. Chromosomes that do not enter a daughter cell nucleus are lost.

Mixoploidy

Mixoploidy includes mosaicism (an individual possesses two or more genetically different cell lines all derived from a single zygote) and chimerism (an individual has two or more genetically different cell lines originating from different zygotes - see Figure 3.9). Abnormalities that would be lethal in constitutional form may be compatible with life in mosaics.

Aneuploidy mosaics are common. For example, mosaicism resulting in a proportion of normal cells and a proportion of aneuploid (e.g. trisomic) cells can be ascribed to nondisjunction or chromosome lag occurring in one of the mitotic divisions of the early embryo (any monosomic cells that are formed usually die out). Polyploidy mosaics (e.g. human diploid/triploid mosaics) are occasionally found. As gain or loss of a haploid set of chromosomes by mitotic nondisjunction is most unlikely, human diploid/triploid mosaics most probably arise by fusion of the second polar body with one of the cleavage nuclei of a normal diploid zygote.

Clinical consequences of numerical abnormalities

Having the wrong number of chromosomes has serious, usually lethal, consequences (Table 2.4). Even though the extra chromosome 21 in a man with Down syndrome is a perfectly normal chromosome, inherited from a normal parent, its presence causes multiple congenital abnormalities. Autosomal monosomies have even more catastrophic consequences than trisomies. These abnormalities must be the consequence of an imbalance in the levels of gene products encoded on different chromosomes. Normal development and function depend on innumerable interactions between gene products, including many that are encoded on different chromosomes. Altering the relative numbers of chromosomes will affect these interactions.

Table 2.4. Consequences of numerical chromosomal abnormalities.

Table 2.4

Consequences of numerical chromosomal abnormalities.

Having the wrong number of sex chromosomes has far fewer ill effects than having the wrong number of any autosome. 47,XXX and 47,XYY people often function within the normal range; 47,XXY men have relatively minor problems compared to people with any autosomal trisomy, and even monosomy, in 45,X women, has remarkably few major consequences. In fact, since normal people can have either one or two X chromosomes, and either no or one Y, there must be special mechanisms that allow normal function with variable numbers of sex chromosomes. In the case of the Y chromosome, this is because it carries very few genes, whose only important function is to determine male sex. For the X chromosome, the special mechanism of lyonization (Section 2.2.3) controls the level of X-encoded gene products independently of the number of X chromosomes present in the cell.

Autosomal monosomy is invariably lethal at the earliest stage of embryonic life. On every chromosome there are probably a few genes where a halving of the level of the gene product is incompatible with development. Also, while such a halving is not obviously pathogenic for most genes (Section 16.4.3), it may have minor effects, and the combination of hundreds or thousands of these minor effects could be enough to disrupt normal development of the embryo. Trisomies make a smaller change than monosomies in relative levels of gene products, and their effects are somewhat less. Trisomic embryos survive longer than monosomic ones, and trisomies 13, 18 and 21 are compatible with survival until birth. Interestingly, these three chromosomes seem to be relatively poor in genes (Section 7.1.2). It is not so obvious why triploidy is lethal in humans and other animals. With three copies of every autosome, the dosage of autosomal genes is balanced and should not cause problems. Triploids are always sterile because triplets of chromosomes cannot pair and segregate correctly in meiosis, but many triploid plants are in all other respects healthy and vigorous. The lethality in animals is probably explained by imbalance between products encoded on the X chromosome and autosomes, which lyonization is unable to compensate.

2.6.3 Structural chromosomal abnormalities result from misrepair of chromosome breaks or from malfunction of the recombination system

Chromosome breaks occur either as a result of damage to DNA (by radiation or chemicals, for example) or as part of the mechanism of recombination. In G2 phase of the cell cycle (Figure 2.2) chromosomes consist of two chromatids. Breaks occurring at this stage are manifest as chromatid breaks, affecting only one of the two sister chromatids. Breaks occurring in G1 phase, if not repaired before S phase, appear later as chromosome breaks, affecting both chromatids. Cells have enzyme systems that recognize and if possible repair broken chromosome ends. Repair can be either by joining two broken ends together, or by capping a broken end with a telomere. Cell cycle checkpoint mechanisms (Section 18.7.3) normally prevent cells with unrepaired chromosome breaks from entering mitosis; if the damage cannot be repaired, the cell commits suicide (apoptosis).

Structural abnormalities arise when breaks are repaired incorrectly. Provided there are no free broken ends, the cell cycle checkpoints can be negotiated satisfactorily, but in fact the wrong broken ends may have been joined together. Any resulting chromosome that has no centromere (acentric) or two centromeres (dicentric) will not segregate stably in mitosis, and will eventually be lost. Chromosomes with a single centromere can be stably propagated through successive rounds of mitosis, even if they are structurally abnormal. Meiotic recombination between mispaired chromosomes is a common cause of translocations, especially in spermatogenesis. Table 2.5 summarizes the main stable structural abnormalities and Figures 2.20 and 2.21 illustrate how they arise.

Table 2.5. Structural abnormalities resulting from misrepair of chromosome breaks or recombination between nonhomologous chromosomes.

Table 2.5

Structural abnormalities resulting from misrepair of chromosome breaks or recombination between nonhomologous chromosomes.

Figure 2.20. Possible stable results of two breaks on a single chromosome.

Figure 2.20

Possible stable results of two breaks on a single chromosome.

Figure 2.21. Origins of translocations.

Figure 2.21

Origins of translocations. Dicentric and acentric chromosomes are not stable through mitosis. Robertsonian translocations are produced by exchanges between the proximal short arms of the acrocentric chromosomes 13, 14, 15, 21 and 22. Both centromeres (more...)

An additional rare class of structural abnormality not shown in Table 2.5 are isochromosomes. These are symmetrical chromosomes consisting of either two long arms or two short arms of a particular chromosome. They are believed to arise from an abnormal U-type exchange between sister chromatids just next to the centromere of a chromosome. Isochromosomes are rare except for i(Xq); i(21q) are an occasional cause of Down syndrome.

Structural chromosomal abnormalities are balanced if there is no net gain or loss of chromosomal material, and unbalanced if there is net gain or loss. In general, balanced abnormalities (inversions, balanced translocations) have no effect on the phenotype, although there are important exceptions to this:

  • a chromosome break may disrupt an important gene;
  • the break may affect expression of a gene even though it does not disrupt the coding sequence. It may separate a gene from a control element, or it may put the gene in an inappropriate chromatin environment, for example translocating a normally active gene into heterochromatin;
  • balanced X-autosome translocations cause problems with X-inactivation (see Figure 15.9).

Robertsonian translocations are sometimes called centric fusions, but this is misleading because in fact the breaks are in the proximal short arms. The translocation chromosome is really dicentric, but because the two centromeres are very close together they function as one, and the chromosome segregates regularly. The distal parts of the two short arms are lost as an acentric fragment. Short arms of acrocentric chromosomes contain only arrays of repeated ribosomal RNA genes, and the loss of two short arms has no phenotypic effect. Because there is no phenotypic effect, Robertsonian translocations are regarded as balanced, even though in fact some material has been lost.

Unbalanced abnormalities can arise directly, through deletion or, rarely, duplication, or indirectly by malsegregation of chromosomes during meiosis in a carrier of a balanced abnormality. Carriers of balanced structural abnormalities can run into trouble during meiosis, if the structures of homologous pairs of chromosomes do not correspond:

  • A carrier of a balanced reciprocal translocation can produce gametes that after fertilization give rise to an entirely normal child, a phenotypically normal balanced carrier, or various unbalanced karyotypes that always combine monosomy for part of one of the chromosomes with trisomy for part of the other (Figure 2.22). It is not possible to make general statements about the relative frequencies of these outcomes. The size of any unbalanced segments depends on the position of the breakpoints. If the unbalanced segments are large, the fetus will probably abort spontaneously; unbalance for smaller segments may result in liveborn abnormal babies.
  • A carrier of a balanced Robertsonian translocation can produce gametes that after fertilization give rise to an entirely normal child, a phenotypically normal balanced carrier, or a conceptus with full trisomy or full monosomy for one of the chromosomes involved (Figure 2.23).
  • A carrier of a pericentric inversion may produce unbalanced offspring because when the inverted and noninverted homologs pair they form a loop so that matching segments pair along the whole length of the chromosomes. If a crossover occurs within the loop, the result is a chromosome carying an unbalanced deletion and duplication. Paracentric chromosome inversions form similar loops, but any crossover within the loop generates an acentric or dicentric chromosome, which is unlikely to survive. For details of meiosis in carriers of inversions, see the book by Gardner and Sutherland (Further reading) or any other cytogenetics text.

Figure 2.22. Results of meiosis in a carrier of a balanced reciprocal translocation.

Figure 2.22

Results of meiosis in a carrier of a balanced reciprocal translocation. Other modes of segregation are also possible, for example 3:1 segregation. The relative frequency of each possible gamete is not readily predicted. The risk of a carrier having a (more...)

Figure 2.23. Results of meiosis in a carrier of a Robertsonian translocation.

Figure 2.23

Results of meiosis in a carrier of a Robertsonian translocation. Carriers are asymptomatic but often produce unbalanced gametes which can result in a monosomic or trisomic zygote. The bracketed monosomic and trisomic zygotes in this example would not (more...)

2.6.4 Apparently normal chromosomal complements may be pathogenic if they have the wrong parental origin

The rare abnormalities described below demonstrate that it is not enough to have the correct number and structure of chromosomes; they must also have the correct parental origin. 46,XX conceptuses in which both genomes originate from the same parent (uniparental diploidy) never develop correctly. For some individual chromosomes, having both homologs derived from the same parent (uniparental disomy) also causes abnormality. A small number of genes are imprinted with their parental origin (Section 8.5) and are expressed differently according to the origin. It is assumed that the abnormalities of uniparental disomy and uniparental diploidy are caused by abnormal expression of such imprinted genes.

Uniparental diploidy is seen in hydatidiform moles, abnormal conceptuses with a 46,XX karyotype of exclusively paternal origin. Molar pregnancies show widespread hyperplasia of the trophoblast but no fetal parts, and have a significant risk of transformation into choriocarcinoma. Genetic marker studies show that most moles are homozygous at all loci, indicating that they arose by chromosome doubling from a single sperm. Ovarian teratomas are the result of maternal uniparental diploidy. These rare benign tumors of the ovary consist of disorganized embryonic tissues, without extraembryonic membranes. They arise by activation of an unovulated oocyte.

Uniparental disomy (UPD), affecting a single pair of homologs, goes undiagnosed if the result is not abnormal, but is detected for chromosomes for which it produces characteristic syndromes (see Box 16.6). UPD can be isodisomy, where both homologs are identical, or heterodisomy, where they are derived from both homologs in one parent. The usual cause is thought to be trisomy rescue: a conceptus that is trisomic and would otherwise die, occasionally loses one chromosome by mitotic nondisjunction or anaphase lag from a totipotent cell. The euploid progeny of this cell form the embryo, while all the aneuploid cells die. If each of the three copies has an equal chance of being lost, there will be a two in three chance of a single chromosome loss leading to the normal chromosome constitution and a one in three chance of uniparental disomy (either paternal or maternal). Uniparental isodisomy may possibly arise by selection pressure on a monosomic embryo to achieve euploidy by selective duplication of the monosomic chromosome.

Further reading

  1. Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD (1994) Molecular Biology of the Cell, 3rd edn. Garland Publishing, New York. A comprehensive text on cell biology.
  2. Gardner RJM, Sutherland GR (1996) Chromosome Abnormalities and Genetic Counseling, 2nd edn. OUP, Oxford. A thorough introduction to the nature, origin and consequences of human chromosomal abnormalities.
  3. Lawrence PA (1992) The Making of a Fly. Blackwell, Oxford. Some of the detail is out of date, but the overall picture is superbly presented.
  4. Moore KL, Persaud TVN (1998) The Developing Human: Clinically Oriented Embryology, 6th edn. WB Saunders, Philadelphia. Excellently illustrated description of development, week by week.
  5. Rooney DE, Czepulkowski BH (1992) (eds) Human Cytogenetics: a Practical Approach, Vols 1 and 2. IRL Press, Oxford. Detailed laboratory protocols.
  6. Therman E, Susman M (1992) Human Chromosomes: Structure, Behavior and Effects, 3rd edn. Springer, New York. An excellent compact introduction; emphasis on scientific bases rather than clinical implications.
  7. Tyler-Smith C, Willard H F. Mammalian chromosome structure. Curr. Opin. Genet. Dev. (1993);3:390–397. [PubMed: 8353411]

References

  1. Craig J M, Bickmore W A. Chromosome bands - flavours to savour. Bioessays. (1993);15:349–354. [PubMed: 8343145]
  2. Dobbs D L, Shaiu W -L, Benbow R M. Modular sequence elements associated with origin regions in eukaryotic chromosomal DNA. Nucleic Acids Res. (1994);22:2479–2489. [PMC free article: PMC308199] [PubMed: 8041609]
  3. Greider C W. Telomeres do D-loop-T-loop. Cell. (1999);97:419–422. [PubMed: 10338204]
  4. Heard E, Clerc P, Avner P. X-chromosome inactivation in mammals. Annu. Rev. Genet. (1997);31:571–610. [PubMed: 9442908]
  5. Huxley C. Mammalian artificial chromosomes and chromosome transgenics. Trends Genet. (1997);13:345–347. [PubMed: 9287486]
  6. Lyon M F. X-chromosome inactivation. Curr. Biol. (1999);9:R235–R237. [PubMed: 10209128]
  7. Manuelidis L. A view of interphase chromosomes. Science. (1990);250:1533–1540. [PubMed: 2274784]
  8. Migeon B R. X-chromosome inactivation: molecular mechanisms and genetic consequences. Trends Genet. (1994);10:230–235. [PubMed: 8091502]
  9. Mitelman F (ed.) (1995) An International System for Human Cytogenetic Nomenclature (ISCN). Karger, Basel.
  10. Saitoh Y, Laemmli U K. Metaphase chromosome structure: bands arise from a differential folding path of the highly AT-rich scaffold. Cell. (1994);76:609–622. [PubMed: 7510215]
  11. Schindelhauer D. Construction of mammalian artificial chromosomes: prospects for defining an optimal centromere. BioEssays. (1999);21:76–83. [PubMed: 10070257]
  12. Wolstenholme J (1992) In: Human Cytogenetics: a Practical Approach (DE Rooney, BH Czepulkowski eds), Vol. 1, 2nd edn, pp. 1–30. IRL Press, Oxford.
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