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The development and function of an organism is in large part controlled by genes. Mutations can lead to changes in the structure of an encoded protein or to a decrease or complete loss in its expression. Because a change in the DNA sequence affects all copies of the encoded protein, mutations can be particularly damaging to a cell or organism. In contrast, any alterations in the sequences of RNA or protein molecules that occur during their synthesis are less serious because many copies of each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
A fundamental genetic difference between organisms is whether their cells carry a single set of chromosomes or two copies of each chromosome. The former are referred to as haploid; the latter, as diploid. Many simple unicellular organisms are haploid, whereas complex multicellular organisms (e.g., fruit flies, mice, humans) are diploid.
).
Recessive mutations inactivate the affected gene and lead to a loss of function. For instance, recessive mutations may remove part of or all the gene from the chromosome, disrupt expression of the gene, or alter the structure of the encoded protein, thereby altering its function. Conversely, dominant mutations often lead to a gain of function. For example, dominant mutations may increase the activity of a given gene product, confer a new activity on the gene product, or lead to its inappropriate spatial and temporal expression. Dominant mutations, however, may be associated with a loss of function. In some cases, two copies of a gene are required for normal function, so that removing a single copy leads to mutant phenotype. Such genes are referred to as haplo-insufficient. In other cases, mutations in one allele may lead to a structural change in the protein that interferes with the function of the wild-type protein encoded by the other allele. These are referred to as dominant negative mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Recessive and dominant mutations can be distinguished because they exhibit different patterns of inheritance. To understand why, we need to review the type of cell division that gives rise to gametes (sperm and egg cells in higher plants and animals). The body (somatic) cells of most multicellular organisms divide by mitosis (see Figure 1-10), whereas the germ cells that give rise to gametes undergo meiosis. Like body cells, premeiotic germ cells are diploid, containing two of each morphologic type of chromosome. Because the two members of each such pair of homologous chromosomes are descended from different parents, their genes are similar but not usually identical. Single-celled organisms (e.g., the yeast S. cerevisiae) that are diploid at some phase of their life cycle also undergo meiosis (see Figure 10-54).
A premeiotic germ cell has two copies of each chromosome (2n), one maternal and one paternal. Chromosomes are replicated during the S phase, giving a 4n chromosomal complement. During the first meiotic division, each replicated chromosome (actually two sister chromatids) aligns at the cell equator, paired with its homologous partner; this pairing off, referred to as synapsis, permits genetic recombination (discussed later). One homolog (both sister chromatids) of each morphologic type goes into one daughter cell, and the other homolog goes into the other cell. The resulting 2n cells undergo a second division without intervening DNA replication. During this second meiotic division, the sister chromatids of each morphologic type separate and these now independent chromosomes are randomly apportioned to the daughter cells. Thus, each diploid cell that undergoes meiosis produces four haploid cells, whereas each diploid cell that undergoes mitosis produces two diploid cells (see Figure 1-10).
depicts the major events in
meiosis. One round of DNA replication, which makes the cell
4n, is followed by two separate cell
divisions, yielding four haploid (1n) cells that contain only
one chromosome of each homologous pair. The apportionment, or segregation, of homologous
chromosomes to daughter cells during the first meiotic division is random; that
is, the maternally and paternally derived members of each pair, called homologs,
segregate independently, yielding germ cells with different mixes of paternal
and maternal chromosomes. Thus parental characteristics are reassorted randomly
into each new germ cell during meiosis. The number of possible varieties of
meiotic segregants is 2n, where n is the haploid
number of chromosomes. In the case of a single chromosome, as illustrated in
Figure 8-2, meiosis gives rise to
two types of gametes; one type carries the maternal homolog and the other
carries the paternal homolog.
Crosses between genotypically normal individuals (blue) and mutants (yellow) that are heterozygous for a dominant mutation (a) or homozygous for a recessive mutation (b) produce different ratios of normal and mutant phenotypes in the F1 generation. Although all the F1 progeny from a cross between a normal individual and an individual homozygous for a recessive mutation will have a normal phenotype, one-quarter of the progeny from the intercross between F1 progeny will have a mutant phenotype. Observation of segregation patterns like these led Gregor Mendel (1822 – 1884) to conclude that each gamete receives only one of the two parental alleles, a conclusion known as Mendel’s first law.
, half the gametes
from an individual heterozygous for a dominant mutation in a particular gene
will have the wild-type allele, and half will have the mutant allele. Since
fertilization of female gametes by male gametes occurs randomly, half the first
filial (F1) progeny resulting from the cross between a normal
wild-type individual and a mutant individual carrying a single dominant allele
will exhibit the mu-tant phenotype. In contrast, all the gametes produced by a
mutant homozygous for a recessive mutation will carry the mutant allele. Thus,
in a cross between a normal individual and one who is homozygous for a recessive
mutation, none of the F1 progeny will exhibit the mutant phenotype
(Figure 8-3b). However, one-fourth
of the progeny from parents both heterozygous for a recessive mutation will show
the mutant phenotype.
(a) Point mutations, which involve alteration in a single base pair, and small deletions generally directly affect the function of only one gene. A wild-type peptide sequence and the mRNA and DNA encoding it are shown at the top. Altered nucleotides and amino acid residues are highlighted in green. Missense mutations lead to a change in a single amino acid in the encoded protein. In a nonsense mutation, a nucleotide base change leads to the formation of a stop codon (purple). This results in premature termination of translation, thereby generating a truncated protein. Frameshift mutations involve the addition or deletion of any number of nucleotides that is not a multiple of three, causing a change in the reading frame. Consequently, completely unrelated amino acid residues are incorporated into the protein prior to encountering a stop codon. (b) Chromosomal abnormalities involve alterations in large segments of DNA. Presumably these abnormalities arise owing to errors in the mechanisms for repairing double-strand breaks in DNA. Chromosomes (I or II) are shown as single thick lines with the regions involved in a particular abnormality highlighted in green or purple. Inversions occur when a break is rejoined to the correct chromosome but in an incorrect orientation; deletions, when a segment of DNA is lost; translocations, when breaks are rejoined to the wrong chromosomes; and insertions, when a segment from one chromosome is inserted into another chromosome.
). Changes in a single base
pair may produce one of three types of mutation:
Missense mutation, which results in a protein in which one amino acid is substituted for another
Nonsense mutation, in which a stop codon replaces an amino acid codon, leading to premature termination of translation
Frameshift mutation, which causes a change in the reading frame, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon
Small deletions have effects similar to those of frameshift mutations, although one third of these will be in-frame and result in removal of a small number of contiguous amino acids.
).Mutations arise spontaneously at low frequency owing to the chemical instability of purine and pyrimidine bases and to errors during DNA replication. Natural exposure of an organism to certain environmental factors, such as ultraviolet light and chemical carcinogens (e.g., aflatoxin B1), also can cause mutations.
The replication of only one strand is shown; the other strand is replicated normally, as shown at the top. A replication error may arise in regions of DNA containing tandemly repeated sequences (in this case, GTC) when a portion of the newly synthesized strand (light blue) loops out into a single-stranded form. This slippage displaces the newly synthesized strand back along the template strand (dark blue), with its 3′ end still paired with the template. As a result, the DNA-synthesizing enzymes copy a region of the template strand a second time, leading to an increase in length of nine nucleotides (yellow) in this example. A subsequent round of DNA replication results in the production of one normal duplex DNA molecule and one mutant duplex containing the additional nucleotides.
illustrates how one type of copying error can
produce a mutation. In the example shown, the mutant DNA contains nine
additional base pairs.
In order to increase the frequency of mutation in experimental organisms, researchers often treat them with high doses of chemical mutagens or expose them to ionizing radiation. Mutations arising in response to such treatments are referred to as induced mutations. Generally, chemical mutagens induce point mutations, whereas ionizing radiation gives rise to large chromosomal abnormalities.
(a) EMS alkylates guanine at the oxygen on position 6 of the purine ring, forming O6-ethylguanine (Et-G), which base-pairs with thymine. (b) Two rounds of DNA replication of a strand containing Et-G yields a mutant DNA in which a G·C base pair is replaced with an A·T pair. Cells also have repair enzymes that can remove the ethyl group from Et-G (Chapter 12).
). During subsequent DNA replication,
O6-ethylguanine directs incorporation of
deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in
which a G·C base pair is replaced with an A·T base pair
(Figure 8-6b). The causes of
mutations and the mechanisms cells have for repairing alterations in DNA are
discussed further in Chapter
12.
Many common human diseases, often
devastating in their effects, are due to mutations in single genes. Genetic
diseases arise by spontaneous mutations in germ cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell
anemia, which affects 1 in 500 individuals of African descent, is
caused by a single missense mutation at codon 6 of the β-globin gene;
as a result of this mutation, the glutamic acid at position 6 in the normal
protein is changed to a valine in the mutant protein. This alteration has a
profound effect on hemoglobin, the oxygen-carrier protein of erythrocytes, which
consists of two α-globin and two β-globin subunits (see Figure 3-11). The deoxygenated form of the
mutant protein is insoluble in erythrocytes and forms crystalline arrays. The
erythrocytes of affected individuals become rigid and their transit through
capillaries is blocked, causing severe pain and tissue damage. Because the
erythrocytes of heterozygous individuals are resistant to the parasite causing
malaria, which is endemic in Africa, the mutant allele has been maintained. It
is not that individuals of African descent are more likely than others to
acquire a mutation causing the sickle-cell defect, but rather the mutation has
been maintained in this population by interbreeding.
Tumors arise from retinal cells that carry two mutant Rb− alleles. (a) In hereditary retinoblastoma, a child receives a normal Rb+ allele from one parent and a mutant Rb− allele from the other parent. A single mutagenic event in a heterozygous somatic retinal cell that inactivates the normal allele will result in a cell homozygous for two mutant Rb− alleles. (b) In sporadic retinoblastoma, a child receives two normal Rb+ alleles. Two separate somatic mutations, inactivating both alleles in a particular cell, are required to produce a homozygous Rb−/Rb− retinal cell.
). When an Rb heterozygous
retinal cell undergoes somatic mutation, it is left with no normal allele; as a
result, the cell proliferates in an uncontrolled manner, giving rise to a
retinal tumor. A second form of this disease, called sporadic
retinoblastoma, results from two independent mutations disrupting
both Rb alleles (Figure
8-7b). Since only one somatic mutation is required for tumor
development in children with hereditary retinoblastoma, it occurs at a much
higher frequency than the sporadic form, which requires acquisition of two
independently occurring somatic mutations. The Rb protein has been shown to play
a critical role in controlling cell division (Chapter 13).
In a later section, we will see how normal copies of disease-related genes can be isolated and cloned.
Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry only one copy.
Mutations are alterations in DNA sequences that result in changes in the structure of a gene. Both small and large DNA alterations can occur spontaneously. Treatment with ionizing radiation or various chemical agents increases the frequency of mutations.
Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.
). The members of
each pair of homologous chromosomes segregate independently during
meiosis, leading to the random reassortment of maternal and paternal
alleles in the gametes.
).
