Glossary

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Chromosome

Carrier of genetic information in the cell nucleus

Chromosomes are long, thread-like structures inside the cell nucleus, consisting of DNA (the genetic information) and proteins. Before cell division, when the chromosomes are allotted to the daughter cells, the chromosomes are condensed into tightly packed units. Chromosomes can easily be stained with dye during the cell division stages, making them visible under a light microscope. This is the origin of their name, derived from the Greek words “chroma” (colour) and “soma” (body).

Every animal and plant species has a characteristic number of chromosomes. In the body cells of mammals, chromosomes always occur in pairs (diploid), with the exception of the x/y sex chromosomes in male mammals. One chromosome originates from the father, the other from the mother. They share the same form, structure and sequence of gene loci, hence the term homologous (similar) chromosomes.

However, they are not identical, since genes at the corresponding loci may exist in different forms (alleles). Organisms with more chromosomes are not necessarily more complex. Humans, for example, have 46 chromosomes, wheat 42, whilst carp have 104. A round worm gets by with only two chromosomes and the fruit fly Drosophila melanogaster has eight; on the other hand, Euglena, a tiny alga, has 200 chromosomes.