In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

Chromosome Composition:

In eukaryotes, chromosomes consist of a single molecule of DNA associated with:

  • many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA.
  • a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.

Chromosome Structure:

  • For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.
  • Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).
  • As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.
  • These duplicated chromosomes are called dyads. image
  • When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains 1–10 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.
  • While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.
  • The kinetochore is a complex of >80 different proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.
  • The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.
  • Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands.
  • G bands are numbered and provide "addresses" for the assignment of gene loci.

Chromosome Numbers

  • All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number.
  • These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.
  • The gametes contain the haploid number (n) of chromosomes.

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

Diploid numbers of some commonly studied organisms
(as well as a few extreme examples)

Homo sapiens (human)


Mus musculus (house mouse)


Drosophila melanogaster (fruit fly)


Caenorhabditis elegans (microscopic roundworm)


Saccharomyces cerevisiae (budding yeast)


Arabidopsis thaliana (plant in the mustard family)


Xenopus laevis (South African clawed frog)


Canis familiaris (domestic dog)


Gallus gallus (chicken)


Zea mays (corn or maize)


Muntiacus reevesi (the Chinese muntjac, a deer)


Muntiacus muntjac (its Indian cousin)


Myrmecia pilosula (an ant)


Parascaris equorum var. univalens (parasitic roundworm)


Cambarus clarkii (a crayfish)


Equisetum arvense (field horsetail, a plant)



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:

  • 22 pairs of autosomes
  • 1 pair of X chromosomes

The karyotype of the human male contains:

  • the same 22 pairs of autosomes
  • one X chromosome
  • one Y chromosome

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

The X and Y chromosomes are called the 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

  • the Philadelphia chromosome (Ph1) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML)
  • a translocation between chromosomes 8 and 14 that causes Burkitt’s lymphoma
  • a translocation between chromosomes 18 and 14 that causes B-cell leukemia


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.

Additional Resourses (Free download study materials)

1. Chromosome structure and Cell division stages Study materials

2. Chromosome Structure


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