A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences.
Chromosomes can be divided into two types—autosomes, and sex chromosomes. Human cells have 23 pairs of large linear nuclear chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 chromosomes.
The specific location of a gene or DNA sequence on a chromosome is called a locus (plural : loci). A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map. Gene mapping is the procession of determining the locus for a particular biological trait.
Diploid and polyploid cells whose chromosomes have the same allele of a given gene at some locus are called homozygous with respect to that gene, while those that have different alleles of a given gene at a locus, are called heterozygous with respect to that gene.
The number of genes and base pairs per chromosome varies among the different sources available on the net. The following list shows statistics from the Major Assembly GRCh37, patch 7, released by the Genome Reference Consortium on February 11, 2012.
| No | Sequenced | # Genes | % DNA | # base pairs (millions) |
| 1 | May 2006 | 3.511 | 8 | 250 |
| 2 | April 2005 | 2.368 | 8 | 243 |
| 3 | April 2006 | 1.926 | 6.5 | 198 |
| 4 | April 2005 | 1.444 | 6 | 191 |
| 5 | September 2004 | 1.633 | 6 | 181 |
| 6 | October 2003 | 2.057 | 5.5 | 171 |
| 7 | July 2003 | 1.882 | 5 | 159 |
| 8 | January 2006 | 1.315 | 4.5 | 146 |
| 9 | May 2004 | 1.534 | 4.5 | 141 |
| 10 | May 2004 | 1.391 | 4.5 | 136 |
| 11 | March 2006 | 2.168 | 4.5 | 135 |
| 12 | March 2006 | 1.714 | 4.5 | 134 |
| 13 | March 2004 | 720 | 3.5 | 115 |
| 14 | January 2003 | 1.532 | 3.5 | 107 |
| 15 | March 2006 | 1.249 | 3.5 | 103 |
| 16 | December 2004 | 1.326 | 3 | 90 |
| 17 | April 2006 | 1.773 | 2.5 | 81 |
| 18 | March 2004 | 557 | 2.5 | 78 |
| 19 | March 2004 | 2.066 | 2 | 59 |
| 20 | December 2001 | 891 | 2 | 63 |
| 21 | May 2000 | 450 | 1.5 | 48 |
| 22 | December 1999 | 855 | 1.5 | 51 |
| X | March 2005 | 1.672 | 5 | 155 |
| Y | June 2003 | 429 | 2 | 59 |
| Total : | 36.463 | 100 | 3.094 |
The following list gives links to different chromosome repositories :