Genetics and Molecular Biology
Genetics and Molecular Biology
This domain examines how genetic information is stored, copied, expressed, and inherited. The exam favours questions that combine molecular detail with probability or pedigree reasoning.
Core rules
- DNA is antiparallel and double-stranded; adenine pairs with thymine (two hydrogen bonds) and guanine with cytosine (three).
- Replication is semi-conservative and proceeds 5′→3′ on the new strand. The leading strand is continuous; the lagging strand is made in Okazaki fragments, later joined by DNA ligase.
- Transcription uses RNA polymerase to make mRNA complementary to the template strand; in eukaryotes introns are spliced out before translation.
- Translation reads mRNA codons in the 5′→3′ direction; tRNA anticodons deliver amino acids, and stop codons do not code for an amino acid.
- Mendelian ratios assume independent assortment, random fertilisation, and no linkage or lethality.
Worked example
Cross AaBb × AaBb with unlinked genes. Probability of A_ bb = P(A_) × P(bb) = 3/4 × 1/4 = 3/16. If 320 offspring are expected, 60 are predicted. Reasoning: each locus is analysed independently, then probabilities are multiplied.
Traps
- Linked genes on the same chromosome do not assort independently; recombination frequency reflects distance.
- Incomplete dominance gives intermediate phenotypes, not a 3:1 ratio.
- A pedigree showing affected males much more often than females suggests X-linked recessive inheritance.
- Do not confuse a codon (three mRNA bases) with an anticodon (three tRNA bases).