SEC G3 Combined Science Biology component K327/K328
B10: Molecular Genetics
Connect DNA, genes, chromosomes, base pairing, and the genetic code to polypeptide sequence.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Molecular genetics links chromosome-scale organisation to DNA structure, genes, base pairing, and the information used to build polypeptides.
DNA, genes, and chromosomes
Chromosomes contain DNA. A gene is a particular sequence within a DNA molecule and occupies a position on a chromosome.
These terms describe nested relationships, not three separate substances with unrelated functions.
Nucleotides and complementary pairing
DNA is a double helix of two nucleotide strands. Each nucleotide contains a sugar, phosphate group, and one of four bases.
Complementary base pairing means A pairs with T and C pairs with G. Use the rule to write the complementary sequence in the opposite strand.
Genes and the genetic code
A gene is a nucleotide sequence, a unit of inheritance, and information for one polypeptide. Different base sequences can therefore specify different polypeptides.
DNA carries the genetic code used to make specific polypeptides. Detailed transcription and translation mechanisms are not required in this Combined scope.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: Write the complementary DNA sequence for A C G T T A.
- Pair A with T.
- Pair C with G and G with C.
- Continue base by base in the aligned strand.
Answer: T G C A A T.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain DNA, genes, and chromosomes with the named terms, evidence, and causal links kept distinct.
- Use Nucleotides and complementary pairing to interpret the evidence given and justify each conclusion.
- Apply Genes and the genetic code to a new example, then check the conclusion against the information given.
Official outcome coverage
K327 B10: 5 mapped outcomes, references B10(a), B10(b), B10(c), B10(d), B10(e). Check the official K327 syllabus.
K328 B10: 5 mapped outcomes, references B10(a), B10(b), B10(c), B10(d), B10(e). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use a DNA model or unfamiliar data to identify nucleotides, complementary base pairing, genes and chromosomes. State which relationship is represented and keep transcription and translation mechanism outside the official requirement.
Exam traps and retrieval check
Avoid these traps
- Calling a gene a whole chromosome.
- Pairing A with G.
- Adding transcription and translation detail beyond the required relationship.
Check from memory
What contains genes?
DNA within chromosomes.
What are the three nucleotide parts?
Sugar, phosphate, and a base.
What does one gene code for here?
One polypeptide.
Official Combined Science scope
This shared Combined Biology owner serves both K327 and K328. DNA, genes, chromosomes, nucleotide structure, complementary pairing, polypeptide coding, and the genetic code without transcription or translation detail.