A chromosome is a threadlike structure of genes in a linear organisation in eukaryotes, or a circular one in prokaryotes, containing either just DNA or a complex of DNA, DNA-bound proteins, such as histones, and various other proteins and RNAs. Chromosomes carry DNA sequences between 1,000 and 1,000,000,000 nucleotides in length. Various genes as well as non-coding 'junk' DNA are found at different loci (fixed points) along the chromosome.

Chromosomes are found in all living cells and must be faithfully replicated and transmitted between cells in order to maintain genetic continuity between generations. In eukaryotes, chromosomes are found as linear structures in the nucleus and in prokaryotes they are usually, though not always, found as smaller circular structures. Eukaryotic organelles, such as mitochondria and chloroplasts, also contain their own DNA held in chromosomes.

Chromosomes, when replicated and ready for division, contain two identical sister chromatids, joined by a region of repetitive DNA called a centromere. The centromere is important in attaching to the microtubules of the spindle during mitosis or meiosis (via kinetochore proteins), such that the two sister chromatids can be separated from one another in the formation of daughter cells. Most chromosomes have a 'short arm' (denoted p) and a 'long arm' (denoted q), drawn by convention with the short arm at the top and the long arm at the bottom. Sister chromatids will have one of these each. The position of the centromere is one way of classifying different types of chromosome:

  • Metacentric chromosomes have the centromere roughly in the middle, such that the two arms of the chromosome are equal in length
  • Submetacentric chromosomes have the centromere not in the middle; arm lengths are unequal
  • Acrocentric chromosomes have the centromere near to one end, such the short arm is so short as to almost be unobservable
  • Telocentric chromosomes have the centromere located right at the terminus of the chromosome, perhaps with a telomere after it
  • Holocentric chromosomes have the whole chromosome acting as a centromere

At either end of (mainly eukaryotic) chromosomes are structures called telomeres. Telomeres are also regions of repetitive DNA. They serve to protect the chromosome from losing genetic information after successive rounds of replication.

The level of packaging in chromosomes - that is, the degree of association between the DNA and DNA-binding proteins - varies at different stages in the cell cycle to make certain genes more or less accessible to replication enzymes and transcription factors, depending on whether the cell needs certain genes to be either replicated or transcribed.

The number of full sets of chromosomes in a cell is called the ploidy of that cell: human somatic cells are diploid, carrying two full sets of chromosomes, one set from the maternal line and one from the paternal line. Chromosomes are often arranged into pairs called homologous chromosomes, where the genes on each chromosome in the pair are identical, although the alleles of those genes may differ.