Eukaryotic Chromosomes
- Chromosomes in eukaryotic cells are one, very long DNA molecule associated with proteins
- The main proteins present are the large, positively charged globular proteins called histones, their role is to organise and wrap the DNA tightly so that it fits into the nucleus
- The other proteins are enzymes used in copying and repairing the DNA
- The tightly coiled combination of DNA and proteins is called chromatin – this is what chromosomes are made of
DNA is coiled around histone proteins to form chromatin
The replication of chromosomes
- During interphase (the period before mitosis) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere
- The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
- It is important that the sister chromatids are identical in order to produce genetically identical daughter cells via mitosis
- During anaphase of mitosis one chromatid ends up in one daughter cell while the other chromatid ends up in the other daughter cell
- After the centromere is split apart at the start of anaphase the chromatids are referred to as individual chromosomes again
Diagram illustrating the structure of a chromosome at different stages of mitosis
Examiner Tip
It is important to distinguish between the terms chromatid, sister chromatids and chromosomes.