Genetics Review
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Course Description



Genetics Review

Types of Databases

Format of Sequence Record



3-D Structures

Genomes and Maps

Librarian Roles

WWW Sites

Glossaries and Dictionaries


Chromosome back to top

Source:  image from the National Human Genome Research Institute (NHGRI) Genetic Illustrations.

Definition back to top

A chromosome is one of the threadlike "packages" of genes and other DNA in the nucleus of a cell.

Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and two sex chromosomes. Each parent contributes one chromosome to each pair, so children get half of their chromosomes from their mothers and half from their fathers.

Source:  definition from the National Human Genome Research Institute (NHGRI) Glossary of Genetic Terms.

More... back to top

Haploid vs. Diploid

A cell in which the chromosomes are present as single chromosomes is called haploid.

A cell in which the chromosomes are paired is called diploid.

Number of chromosomes

Some organisms contain a single chromosome, and others contain multiple chromosomes.  For example:

  • the bacterium E. coli contains a single, circular chromosome
  • a yeast (S. cerevisiae) cell contains 16 chromosomes
  • a mouse cell contains 20 pairs of chromosomes (19 autosomes plus the X/Y sex chromosome)
  • as noted above, a human cell contains 23 pairs of chromosomes (22 autosomes plus the X/Y sex chromosome).
Humans do not necessarily have the largest number of chromosomes, however. The other great apes, for example, have 48 chromosomes, and some plants have hundreds of chromosomes. Members of the genus Ophioglossum (adder's tongue fern, an apparently rather ancient lineage) have 100's of chromosomes. Ophioglossum reticulatum, for example, is estimated to have from 240 to 510 chromosomes in the haploid state, or up to 1020 chromosomes in the diploid state!

If you are interested in the genome sizes (in base pairs) of various organisms, one helpful resource is DOGS: Database of Genome Sizes.

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  Revised September 28, 2000
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