Eukaryotic DNA (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

Chromosome structure

  • In the nucleus of eukaryotic cells, DNA molecules are very long, linear

  • Together, a DNA molecule and its associated proteins form a chromosome

    • The main proteins present are the large, positively charged globular proteins called histones. Their role is to organise and condense 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 chromatids, and therefore chromosomes, are made of

  • Chromatids are organised into sections, which is a series of genes

  • The ends of the chromatids in chromosomes are ‘sealed’ with protective structures called telomeres

Diagram depicting chromosome structure, showing chromatids, chromatin, and DNA wrapped around histone proteins, emphasising genetic organisation.
DNA is coiled around histone proteins to make chromatin
Diagram of a chromosome showing chromatids, centromere, and telomeres, linked to DNA structure with highlighted genes 1 and 2.
Simplified diagram of the structure of a chromosome

Examiner Tips and Tricks

It is important to distinguish when the terms chromatin, chromatids and chromosomes are used.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding