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Chromosome

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A chromosome is a structure inside a cell that carries the cell's DNA and the genes it contains.[1] In plants, animals and other eukaryotes, each chromosome in the nucleus contains a single long, linear DNA molecule.[2] Each gene is found at a particular place on a chromosome, called its locus.[1]

Chromosomes in humans

Humans have 46 chromosomes in most of their body cells. These form two sets of 23: one pair of sex chromosomes, plus two sets of 22 other chromosomes, called autosomes.[1][1][3] Females have two X chromosomes (XX) and males have an X and a Y (XY).[1] Having two sets of chromosomes is called being diploid.[1] Sex cells (gametes), such as eggs and sperm, are haploid: they have just one set of 23 chromosomes. When an egg is fertilised, the double set is restored.[1][4] With about 30,000 genes shared among 23 chromosomes, each chromosome carries on average about 1,300 genes.[5]

The number of chromosomes varies greatly between species and has nothing to do with how complex an organism is. Yeast has 16, the Indian muntjac deer has four, and one species of ant, Myrmecia pilosula, has just one.[2] Horses have 64 chromosomes and donkeys 62.[6]

Structure

Chromosomes are made of about half protein and half DNA. This mixture is called chromatin.[4] The term was first used by Walther Flemming in 1879 for the material in the nucleus that stains with dyes.[7] Most of the protein is made up of histones.[8] The DNA is wound around groups of eight histone molecules, forming bead-like units called nucleosomes.[9]

This packaging is needed because chromosomes are much shorter than the DNA they hold. The DNA in one human cell's set of chromosomes is about a metre long in total. Each chromosome is only a few thousandths of a millimetre long at its most compact, yet holds on average more than 4 cm of DNA.[9][10]

Before a cell divides, its DNA is copied. Each chromosome then consists of two identical sister chromatids joined at a point called the centromere.[11][11] The centromere can be in the middle of the chromosome or nearer one end.[11] The ends of a chromosome are called telomeres. They protect the ends of the DNA.[12][12] Scientists can identify each chromosome by its size, the position of its centromere and the pattern of bands it shows when stained. A picture of a full set arranged in order is called a karyogram.[11][13]

Cell division

In mitosis, the chromosomes condense, line up in the middle of the cell, and are pulled apart so that each new cell gets a complete set of chromosomes.[14][15] In meiosis, which makes gametes, there are two divisions without the DNA being copied in between. Matching (homologous) chromosomes pair up and are separated in the first division, and sister chromatids are separated in the second, giving haploid cells.[16][1]

Sometimes chromosomes fail to separate properly during meiosis. This is called nondisjunction, and it gives gametes with too many or too few chromosomes.[17]

History

Gregor Mendel discovered the basic patterns of inheritance in about 1856–63.[5] By 1909, when genes were given their name by Johannsen, scientists knew that genes are carried on chromosomes. This was because the way chromosomes are passed on during cell division and reproduction exactly matches the way genes behave.[18] The chromosome theory was stated most convincingly in 1903 by W. S. Sutton.[18]

Thomas Hunt Morgan and his group at Columbia University studied the fruit fly Drosophila melanogaster. Between 1911 and 1929 they developed methods for mapping the positions of genes on chromosomes.[18][18] Morgan realised that genes on the same chromosome can be separated by crossing over between pairs of chromosomes.[19][5]

Chromosomes and new species

A hybrid between two species is often sterile, because the chromosomes from the two parents cannot pair properly in meiosis.[20][6] In plants, a hybrid can sometimes double its number of chromosomes, which is called polyploidy. This restores regular pairing and can make a fertile new species.[21][20] The fertile plant species Primula kewensis arose when the chromosome number of a sterile hybrid between two other Primula species doubled.[22]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 33. ISBN 1-4051-0345-0. {{cite book}}: |access-date= requires |url= (help)
  2. ↑ 2.0 2.1 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 255. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  3. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 528. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  4. ↑ 4.0 4.1 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 256. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  5. ↑ 5.0 5.1 5.2 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 34. ISBN 1-4051-0345-0. {{cite book}}: |access-date= requires |url= (help)
  6. ↑ 6.0 6.1 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 320–321. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  7. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 256–257. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  8. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 257. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  9. ↑ 9.0 9.1 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 259. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  10. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 259–260. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  11. ↑ 11.0 11.1 11.2 11.3 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 261. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  12. ↑ 12.0 12.1 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 263. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  13. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 536. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  14. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 265–266. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  15. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 266. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  16. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. pp. 267–268. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  17. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 525. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  18. ↑ 18.0 18.1 18.2 18.3 Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 3. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  19. ↑ Brown, Terry A. (2025). Introduction to Genetics: A Molecular Approach (2nd ed.). Boca Raton, FL: CRC Press. p. 329. doi:10.1201/9781003473862. ISBN 978-1-032-74353-0. {{cite book}}: |access-date= requires |url= (help)
  20. ↑ 20.0 20.1 Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 405. ISBN 1-4051-0345-0. {{cite book}}: |access-date= requires |url= (help)
  21. ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. p. 53. ISBN 1-4051-0345-0. {{cite book}}: |access-date= requires |url= (help)
  22. ↑ Ridley, Mark (2004). Evolution (3rd ed.). Malden, MA: Blackwell Publishing. pp. 53–54. ISBN 1-4051-0345-0. {{cite book}}: |access-date= requires |url= (help)
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