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GCSE Biology 07 — Inheritance, Variation and Evolution

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Topics include DNA Structure and the Genome, Protein Synthesis, Genetic Mutations, Cell Division: Mitosis and Meiosis, Inheritance and Monohybrid Crosses, Sex Determination, Pedigrees, and Codominance, Variation, and Evolution by Natural Selection.

Biology EN
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DNA Structure and the Genome

The genome is the entire genetic material of an organism, composed of DNA arranged in a double helix structure of repeating nucleotides.

Key points

  • DNA is a polymer made of two strands coiled into a double helix.
  • The strands are linked by complementary base pairs: Adenine (A) with Thymine (T), and Cytosine (C) with Guanine (G).
  • A nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base.
  • The genome is the entire DNA of an organism; a gene is a small section of DNA that codes for a protein.
  • DNA strands run in opposite directions (antiparallel).

Worked example

Question

A sample of DNA contains 20% Thymine. Calculate the percentage of Guanine in the sample.

Solution

1. In DNA, A pairs with T, so A = T = 20%.
2. A + T = 40%.
3. Remaining bases (C + G) = 100% - 40% = 60%.
4. Since C pairs with G, they are equal. G = 60% / 2 = 30%.

Common pitfalls

  • Confusing the genome (all DNA) with a gene (section of DNA).
  • Thinking RNA contains Thymine (it contains Uracil).
  • Forgetting that the sugar-phosphate backbone is on the outside, with bases pairing in the middle.

Prerequisites

  • Basic cell structure (nucleus)
  • Polymers and monomers
Further resources