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.