chromosome

The 23 pairs of chromosomes of a normal human male.

Human chromosomes, hugely magnified. A chromosome is a threadlike structure of nucleic acids and protein, carrying a set of linked genes, and (in the case of humans and most other higher organisms) paired with an identical copy.

The structure of a chromosome. The chromosome is made up of two identical chromatids joined by a centromere. Each chromatid is made up of coiled DNA.

Structures in a cell nucleus that carry the many thousands of genes, in sequence, that determine the characteristics of an organism. There are 46 chromosomes in a normal human cell. Each chromosome normally consists of one very long strand (or molecule) of DNA, coiled and folded to produce a compact structure. The exception is just before cell division when each chromosome contains two strands of DNA, a result of the copying of each molecule of DNA. The point on a chromosome where a particular gene occurs is known as its locus. Most higher organisms have two copies of each chromosome, together known as a homologous pair (they are diploid) but some have only one (they are haploid). See also mitosis and meiosis.

In a working cell chromosomes exist in a less dense form called chromatin and cannot be seen using a laboratory light microscope. However, during cell division they get shorter and fatter and so become visible. When pictures are seen of chromosomes, they have usually been taken in this state during mitosis. In such pictures of human chromosomes it can be seen that most chromosomes in a cell have a matching chromosome of exactly the same size. These are known as homologous pairs. However, one pair is not matched. These are the sex chromosomes - a short, male determining one called the Y chromosome and the X chromosome. Males have an X and a Y chromosome and females have two Xs.

In a sex cell (gamete) that has been produced by meiosis, the number of chromosomes is halved. Only one chromosome from each pair of homologous chromosomes is found in a gamete.

Artificial chromosomes The first artificial human chromosome was built by US geneticists in 1997. They constructed telomeres, centromeres, and DNA containing genetic information, which they removed from white blood cells, and inserted into human cancer cells. The cells assembled the material into chromosomes. The artificial chromosome was successfully passed onto all daughter cells.

DNA sequencing The first human gene to have its DNA sequenced was chromosome 22. Its 545 genes and 33.5 million bases were sequenced by UK researchers by the end of 1999, leaving only a few gaps. It is one of the smallest human chromosomes, and has been linked to a number of disorders, including schizophrenia and trisomy 22 (a common cause of miscarriage).