Edexcel IGCSE Biology

Revision Notes

3.2.1 DNA & the Genome

The Genome

  • The entire set of the genetic material of an organism is known as its genome
  • In 2003, scientists completed a 13-year project in which they sequenced the genes that make up the whole human genome
    • This project was named the human genome project
    • It helped to develop medical understanding and treatment of genetic diseases as well as facilitating the study of human migration patterns
  • Each gene within the genome is made up of a small section of DNA that codes for a particular sequence of amino acids
  • These sequences of amino acids form different types of proteins
  • Alleles are different versions of a gene
    • e.g. there are different alleles that code for brown or blue eye colour
    • Having 2 alleles of the same type is called homozygous
    • Having 2 alleles that are different to one another is called heterozygous
  • There are many different types of proteins but some examples of these could be:
    • Structural proteins such as collagen found in skin cells
    • Enzymes
    • Hormones
  • Genes control our characteristics as they code for proteins that play important roles in what our cells do

The Structure of DNA

  • DNA, or deoxyribonucleic acid, is the genetic material found in the nucleus of a cell
  • DNA, is a polymer made up of two strands coiled around to make a double helix
  • The strands are formed from a sugar phosphate backbone with bases joined together by complementary base pairing
    • Adenine pairs with thymine
    • Guanine pairs with cytosine
  •  Cell division and protein synthesis both rely on these base-pairing rules because each half of the DNA double helix acts as a template to be copied in order to create a complete new double helix

DNA double helix formation, downloadable AS & A Level Biology revision notes

The structure of DNA and complementary base pairs


  • In the nucleus of a cell, the DNA double helix supercoils to form structures called chromosomes
  • They are only visible during cell division
  • Ordinary human body cells contain 23 pairs of chromosomes
    • This is a diploid number (often shown as 2n)
    • One chromosome from a pair is inherited from each parent
    • Each chromosome pair is called a homologous pair
  • The 23rd pair of chromosomes are the sex chromosomes
    • In females, the sex chromosomes are the same, a homozygous genotype (XX)
    • In males, the sex chromosomes are different, a heterozygous genotype (XY)
  • Gametes contain just 23 individual chromosomes
    • This is half the full number of a body cell and is called the haploid number (or n)
  • Genes are found in specific locations on the chromosomes, these locations were identified in the human genome project

DNA, genes and chromosomes, IGCSE & GCSE Biology revision notes

Genes are short lengths of DNA that code for a protein. They are found on chromosomes.

Exam Tip

Remember that the number of chromosomes found in each species differs, for example, humans have 23 pairs whereas dogs have 39 pairs and rice has 12 pairs.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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