A level Biology B · Edexcel 9BI0 · Topic 1E
Nucleic acids and DNA replication
Key points
- A nucleotide contains a pentose sugar, a phosphate group and a nitrogenous base. DNA uses deoxyribose and bases A, T, C, G; RNA uses ribose and U instead of T.
- Nucleotides join by phosphodiester bonds to form a sugar–phosphate backbone.
- DNA is a double helix of two antiparallel strands held by hydrogen bonds between complementary bases: A–T (two bonds) and C–G (three bonds).
- Semi-conservative replication: DNA helicase unzips the strands; free nucleotides pair with each exposed template; DNA polymerase joins them; each new molecule has one old and one new strand.
- Meselson and Stahl showed this using heavy (N) and light (N) nitrogen: after one generation all DNA was intermediate density.
Topic notesPremium
DNA holds the instructions for building every protein in an organism, and it must be copied accurately each time a cell divides. Its double-helix structure explains both its stability and how it is replicated.
Unlock the full notes with Premium
Premium includes full topic notes, the paper generator, self-quizzes and progress tracking.
Practise this topicPremium
Premium members can generate an exam-style paper with a mark scheme, or take a quick quiz, on Nucleic acids and DNA replication.