chromosome
Meaning of chromosome: Packaging is the point, since the DNA in one cell would stretch about two meters and chromosomes fold it to fit inside a nucleus.
Definition of chromosome
A chromosome is a single long molecule of DNA wound around packaging proteins, carrying many genes in a fixed order along its length. Human body cells normally hold 46, arranged as 23 pairs, with one member of each pair from each parent. Twenty-two pairs are the same in both sexes, and the twenty-third pair determines sex.
How is a chromosome built?
DNA is wrapped around spool-shaped proteins called histones, and the resulting beaded string coils on itself repeatedly. That material, chromatin, stays loosely spread through the nucleus most of the time and condenses into the familiar X-shaped structures only when a cell is about to divide, which is when they can be seen and counted under a microscope. Each has a pinched waist, the centromere, where the two copies stay joined until they are pulled apart, and protective caps at both ends called telomeres that shorten a little with each division.
What can go wrong?
| Change | What it means |
|---|---|
| Trisomy | Three copies of one chromosome instead of two, as in Down syndrome |
| Monosomy | Only one copy, as in Turner syndrome affecting the X chromosome |
| Translocation | A piece breaks off and joins a different chromosome |
| Deletion or duplication | A segment is missing or present twice over |
| Mosaicism | Some cells carry the change and others do not |
How are chromosomes examined?
A karyotype arranges the condensed chromosomes by size and banding pattern so that number and gross structure can be checked at a glance. Finer methods use fluorescent probes or sequencing to detect segments too small to see. Prenatal testing and the assessment of a child with developmental differences are the classic settings, since these changes are usually inherited in only a minority of cases and often arise fresh during the formation of egg or sperm. Cancer supplies the other main use: a swapped segment between two chromosomes creates the Philadelphia chromosome in chronic myeloid leukemia, producing a fusion gene that both defines the disease and gives treatment a direct target.
Used in a sentence
Chromosome analysis of the sample identified an extra copy of one chromosome.
Good to know
An extra or missing copy of a single chromosome, rather than a change affecting an entire set, is the basis of several well known chromosomal conditions, which is why chromosome count and structure are both examined during genetic testing.
Word origin
Where “chromosome” comes from
Formed from Greek chroma, meaning color, and soma, meaning body, since chromosomes were first identified by their strong uptake of colored laboratory stains.