O-Level and SEC G3 Biology K325
B12: Molecular Genetics
Connect DNA, genes, chromosomes, the polypeptide code, transgenic insulin production, and ethical evaluation.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Molecular Genetics connects DNA structure, genes, chromosomes, the genetic code, and genetic engineering. K325 requires the relationship from a gene to a specific polypeptide but explicitly does not require transcription and translation detail, so this chapter keeps that boundary visible.
DNA, genes and chromosomes
DNA consists of two nucleotide strands in a double helix. Each nucleotide contains a sugar, a phosphate group, and one of four bases. Complementary base pairing joins adenine with thymine and cytosine with guanine.
A gene is a nucleotide sequence within a DNA molecule, codes for one polypeptide, and acts as a unit of inheritance. A chromosome is a long DNA molecule containing many genes. Keep DNA, gene, and chromosome at their correct scales.
Genetic code and transgenic organisms
DNA carries a genetic code used to synthesise specific polypeptides. K325 requires this information relationship but not the detailed stages of transcription and translation. Do not add RNA processing, codon tables, or ribosome mechanisms unless a question supplies them as data.
A gene can be transferred from cells of one organism to cells of another to form a transgenic organism. The transferred gene can direct production of a desired polypeptide when it functions in the host cell.
Insulin production and ethical evaluation
To produce human insulin, the human insulin gene is isolated and inserted into bacterial DNA, commonly a plasmid. The recombinant DNA is transferred into bacteria, successful cells are cultured, and the insulin they produce is harvested and purified. Explain each tool through its purpose rather than listing names alone.
Genetic engineering can support medicine and economically important crops or animals, but an evaluation must consider safety, environmental effects, access, welfare, ownership, and ethical concerns in the stated context. A benefit does not automatically remove a risk, and a theoretical risk is not evidence that harm has occurred.
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: One DNA strand has the sequence . Write the complementary DNA sequence and state the pairing rule used.
- Match adenine with thymine and cytosine with guanine.
- Read each base in the supplied order and write its complement.
- Keep this as DNA, so use thymine rather than uracil.
Answer: The complementary DNA sequence is .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Connect DNA, genes, and chromosomes and apply complementary base pairing.
- State that a gene codes for one polypeptide without adding excluded transcription and translation detail.
- Outline insulin production in transgenic bacteria and evaluate benefits and ethical considerations.
Official outcome coverage
K325 B12: 8 mapped outcomes, references B12(a), B12(b), B12(c), B12(d), B12(e), B12(f), B12(g), B12(h). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret models, sequence data, or phenotype evidence without claiming that a correlation proves one gene fully determines a trait.
Exam traps and retrieval check
Avoid these traps
- Calling a gene an entire chromosome.
- Adding detailed transcription and translation mechanisms that K325 explicitly does not require.
- Listing genetic-engineering tools without stating what each one does in the transfer process.
Check from memory
Which DNA base pairs with cytosine?
Guanine.
What does one gene code for at this level?
One polypeptide.
What enzyme joins DNA fragments?
DNA ligase.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.