Glossary
chromosome
A chromosome is one long DNA molecule wound around histone proteins, and a human body cell carries 46 of them, in 23 pairs.
A chromosome is one long molecule of DNA wound around proteins called histones, which pack it small enough to fit inside a nucleus. A human body cell carries 46 chromosomes, arranged as 23 pairs. One of each pair came from each parent, and the two members of a pair are the homologous pair: they carry the same genes in the same order, though not necessarily the same alleles. The 23rd pair is the sex chromosomes — XX in a female, XY in a male — and the rest are autosomes. Before a cell divides it copies its DNA, so each chromosome arrives at division as two identical sister chromatids joined at a centromere; mitosis then separates the sisters, so each daughter cell keeps the full diploid number. Meiosis separates the homologous pairs instead, so each gamete ends with half the number, 23. A karyotype is a picture of that set, arranged in pairs, and it is how a missing or extra chromosome is seen.
Taught in
Related terms
- gene — A gene is a section of DNA that carries the instructions for a trait or a protein, and the alternative versions of a gene are its alleles.
- mitosis — Mitosis is the division that grows and repairs a body: one cell becomes two, and each daughter cell carries the same set of chromosomes as the parent.
- meiosis — Meiosis is the division that makes gametes: two divisions after one DNA replication turn one diploid cell into four haploid cells.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)