Continuity and Change
This section covers the IB Biology SL domain Continuity and Change. The exam tests how genetic information is copied, expressed, inherited and changed, and how cell division, homeostasis, natural selection and climate change produce continuity or change.
Core rules
- DNA replication is semi-conservative: helicase unwinds the helix, and DNA polymerase adds complementary nucleotides in the 5' to 3' direction.
- Protein synthesis involves transcription in the nucleus and translation at the ribosome; codons on mRNA pair with anticodons on tRNA.
- Mutations include substitutions and insertions or deletions; effects range from silent to harmful or beneficial, and gene editing can alter sequences.
- Mitosis produces genetically identical diploid cells for growth and repair; meiosis produces haploid gametes with crossing over and independent assortment.
- Monohybrid inheritance can be predicted with Punnett squares; alleles may be dominant, recessive or codominant.
- Homeostasis uses negative feedback to maintain a set point, such as body temperature or blood glucose; natural selection changes allele frequencies in populations over time.
Worked example
Two heterozygous parents, Bb x Bb, each produce B and b gametes. A Punnett square gives BB, Bb, Bb and bb. The genotype ratio is 1 BB : 2 Bb : 1 bb, and the phenotype ratio is 3 dominant : 1 recessive. If a recessive phenotype appears among the offspring of two dominant parents, both parents must carry the recessive allele. This is why a test cross with a homozygous recessive individual can reveal an unknown dominant genotype.
Traps
- Mitosis maintains chromosome number; meiosis halves it and generates variation.
- Dominant alleles are not necessarily more common in a population.
- Mutations are not always harmful; some are neutral or beneficial.
- Semi-conservative replication means each new DNA molecule keeps one original strand, not that both original strands remain together.
- Homeostasis maintains a dynamic set point, not a perfectly constant internal environment.