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In eukaryotes, chromosomes consist of a single molecule of DNA [Link to visual proof] associated with:


Chromosome Numbers

(In plants, the haploid stage takes up a larger part of its life cycle - Link)

Diploid numbers of some commonly studied organisms
(as well as a few extreme examples)
Homo sapiens (human)46
Mus musculus (house mouse)40
Drosophila melanogaster (fruit fly)8
Caenorhabditis elegans (microscopic roundworm)12
Saccharomyces cerevisiae (budding yeast)32
Arabidopsis thaliana (plant in the mustard family)10
Xenopus laevis (South African clawed frog)36
Canis familiaris (domestic dog)78
Gallus gallus (chicken)78
Zea mays (corn or maize)20
Muntiacus reevesi (the Chinese muntjac, a deer)23
Muntiacus muntjac (its Indian cousin)6
Myrmecia pilosula (an ant)2
Parascaris equorum var. univalens (parasitic roundworm)2
Cambarus clarkii (a crayfish)200
Equisetum arvense (field horsetail, a plant)216


The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis and all the chromosomes are present as dyads.

The karyotype of the human female contains 23 pairs of homologous chromosomes: The karyotype of the human male contains:

(A gene on the Y chromosome designated SRY is the master switch for making a male.)

Link to a karyotype of a normal human male stained by the trypsin-giemsa method (the image is 84K)

The X and Y chromosomes are called the sex chromosomes.)

Discussion of sex chromosomes

Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy 21. It is an example of aneuploidy


Karyotype analysis can also reveal translocations between chromosomes. A number of these are associated with cancers, for example


Location of the gene for muscle glycogen phosphorylase on human chromosome 11

This image (courtesy of David C. Ward) provides dramatic evidence of the truth of the story of chromosomes. A piece of single-stranded DNA was prepared that was complementary to the DNA of the human gene encoding the enzyme muscle glycogen phosphorylase. A fluorescent molecule was attached to this DNA. The dyads in a human cell were treated to denature their DNA; that is, to make the DNA single-stranded. When this preparation was treated with the fluorescent DNA, the complementary sequences found and bound each other. This produced a fluorescent spot close to the centromere of each sister chromatid of two homologous dyads (of chromosome 11, upper right). This analytical procedure, which here revealed the gene locus for the muscle glycogen phosphorylase gene, is called fluorescence in situ hybridization or FISH.

DNA Content

The molecule of DNA in a single human chromosome ranges in size from 50 x 106 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 106nucleotide pairs in the largest (which would extend 8.5 cm).

Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters.

See some of the DNA molecule released from a single human chromosome.

In the intact chromosome, however, this molecule is packed into a much more compact structure. [Link]. The packing reaches its extreme during mitosis when a typical chromosome is condensed into a structure about 5 µm long (a 10,000-fold reduction in length).

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17 April 2014