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Chromosome

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!

Each strand is joined by a centromere.
Each strand is joined by a centromere.

Eukaryotic DNA is associated with proteins called histones that help to condense chromosomes. Prokaryotic chromosomes are circular and have a much looser structure.