IP Biology Upper Sec 12: Molecular Genetics

These notes align with SEAB GCE O-Level Biology (6093) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 — scope unchanged; remains the reference for this note.

What you must know

• DNA is a double helix of two antiparallel strands; nucleotide = deoxyribose sugar + phosphate + base (A, T, C, G); complementary pairing A–T, C–G via hydrogen bonds.
• Genes are sections of DNA on chromosomes; each gene codes for a polypeptide; base sequence determines amino acid sequence.
• Basic flow: DNA sequence → mRNA copy (concept), mRNA read to assemble amino acids into polypeptide (no detailed steps needed at this level).
• Genetic engineering: isolate target gene (e.g., human insulin), insert into vector (plasmid) using restriction enzymes and ligase, transfer to host (bacteria) to express protein. Consider benefits (medicine, crops) vs risks/ethics (allergies, escape, equity).

Detailed notes

• DNA structure: double helix, antiparallel strands, sugar-phosphate backbone outside, bases inside paired A–T, C–G via hydrogen bonds; sequence stores information.
• Gene–protein link: base triplet codes for one amino acid; order of bases → order of amino acids → protein shape/function. Mutation (base change) can alter protein (sometimes silent).
• Replication (concept): strands separate; complementary nucleotides pair; ensures identical copies before cell division.
• Genetic engineering steps (IP level): identify gene → cut with restriction enzyme → splice into plasmid vector with ligase → insert into host (e.g., bacteria) → express protein → harvest. Advantages (insulin, vaccines, resistant crops); risks (allergies, gene escape, biodiversity, access).

Worked walkthroughs

• Explain how a base substitution might change an amino acid and affect protein function (or be silent).
• Outline insulin GM production at high level: human insulin gene into plasmid → bacteria produce insulin → purified for patients.
• Describe one benefit and one risk of GM pest-resistant crops (higher yield vs possible non-target effects or resistance development).

Pitfalls and fixes

• Saying DNA bases pair randomly—must be complementary (A–T, C–G).
• Ignoring that sequence order matters; not just base types.
• Describing GM without mentioning vector/host change.
• Overstating certainty of risks/benefits—keep balanced.

Practice drills

• Given a short DNA sequence, write its complementary strand.