A chromosome is large unit of packaged DNA.
The term chromosome can be confusing because it means different things at different stages of a cell's life cycle:
- Before replication: each chromosome is made up of a single DNA strand.
- During replication: DNA is stretched out to allow space for the enzymes to build new strands.
- After replication: each chromosome is made up of two identical DNA strands, called sister chromatids.
This version forms the classic 'X-shaped' chromosome. However, chromosomes only exist in this state for the short period of time between DNA replication and cell division!
Eukaryotic DNA is associated with proteins called histones that help to condense chromosomes. Prokaryotic chromosomes are circular and have a much looser structure.
In diploid cells, chromosomes come in pairs. These pairs of chromosomes are called homologous chromosomes.
Homologous chromosomes are not identical. They carry different alleles (versions) of the same genes.
An individual inherits one chromosome in each homologous pair from either parent.
In a pair of homologous chromosomes, one chromosome may carry the allele for brown eyes while the other carries the allele for blue eyes.
Genes on a pair of homologous chromosomes lie on the same locus (plural loci). This means that the two genes occupy the same position on both chromosomes.
Important! The sex chromosomes X and Y are not homologous. They do not contain the same genes.
Ploidy refers to the number of sets of chromosomes in a cell.
Haploid cells have one set of chromosomes. Gametes (sex cells) in sexually reproducing organisms are haploid.
Diploid cells have two sets of chromosomes (called homologous chromosomes). One set of chromosomes comes from each parents. In most animals, all somatic cells (body cells) are diploid.
Human sperm cells are haploid. A sperm cell contains a single copy of each chromosome (23 total). A human skin cell is diploid. It contains two copies (a pair) of each chromosome (46 total).
Some cells are polyploid and have more than two sets of chromosomes. Plants often have polyploid tissue. Polyploidy occurs if cells fail to divide properly during meiosis or mitosis.
Plants exhibit high tolerance for polyploidy. This means that polyploidy offspring can survive and reproduce, explaining the high incidence of polyploidy in these populations.