Biology unit guide
Continuity and change
DNA structure and replication, protein synthesis, mutation, inheritance, evolution and gene technology.
Unit 4 is the most examined unit in biology and the one where precise language matters most. Transcription and translation are asked about almost every session, and the marks go to answers that name the enzyme, the template and the product rather than describing the process loosely. The genetics half is procedural: Punnett squares are marks you either collect reliably or lose to presentation, and the fix is a fixed layout you always use.
Topics in this unit
What gets examined
- The structure of DNA, complementary base pairing, and why the strands are antiparallel.
- Semi-conservative replication, and the evidence that established it.
- Transcription and translation, including the role of RNA polymerase, tRNA and the ribosome.
- Using the genetic code to read a codon table, and the consequences of different mutation types.
- Monohybrid and dihybrid crosses, sex linkage, and codominance, set out in a Punnett square.
- Natural selection as an explanation for a described change in a population.
How to revise it
Use one Punnett square layout every time
Parental genotypes, gametes, grid, then genotype and phenotype ratios written out. Marks are awarded along that structure, so a consistent layout collects them even when the reasoning wobbles.
Name the molecules in transcription and translation
RNA polymerase, template strand, mRNA, tRNA, anticodon, ribosome. Describing the process without naming its parts is the routine way to score half.
Write natural selection as a sequence
Variation exists, selection pressure acts, advantageous alleles survive and reproduce, allele frequency changes over generations. Four steps, and mark schemes want all four.
Say what a mutation does to the protein
Not just "a base changes" - say whether the amino acid changes, and whether that changes the protein's shape and function.
Where marks get lost
- Saying organisms adapt during their lifetime rather than populations evolving across generations.
- Confusing codon with anticodon, or mRNA with tRNA.
- Forgetting that a substitution can be silent because the code is degenerate.
- Giving a phenotype ratio where the genotype ratio was asked for.
- Describing DNA replication as conservative rather than semi-conservative.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.