Chromosomes

Definition

Thread-like structures of nucleic acids and protein found in the nucleus of eukaryotic cells, carrying genetic information in the form of genes. In insects and arachnids, chromosomes determine sex through various systems (XX-XO, XX-XY, or haplodiploidy), influence development, and vary in number and morphology across taxa—ranging from a few large chromosomes in some Diptera to numerous small chromosomes in certain Lepidoptera.

Etymology

From Greek chroma (color) + soma (body), referring to their staining properties with basic dyes.

Example

In Drosophila melanogaster, the four pairs of chromosomes (three autosomes plus sex chromosomes XX or XY) have been foundational to genetics research, with polytene chromosomes in larval salivary glands providing visible banding patterns used to map gene locations.

Related Terms

• genome
• Karyotype
• haplodiploidy
• polytene chromosomes
• Sex determination
• mitosis
• meiosis
• Gene
• nucleus
• DNA

Usage Notes

The number and morphology of chromosomes (karyotype) vary widely among arthropod groups and are used in taxonomic and phylogenetic studies. Specialists distinguish somatic chromosomes in body cells from germ-line chromosomes. In some contexts, 'chromosome' refers to the condensed, visible state during cell division versus the extended chromatin of interphase. Bacterial and archaeal genetic material is not organized into true chromosomes by the eukaryotic definition, though the term is sometimes applied loosely to their nucleoid structures or plasmids.