Genetics and Molecular Biology
Genetics and Molecular Biology examines the molecular basis of inheritance and gene expression: DNA structure and semi-conservative replication, transcription and translation, mutation, Mendelian ratios, linkage, and the chi-square test. STPM questions often require you to predict ratios and explain deviations from them.
Core rules
- DNA strands are antiparallel; adenine pairs with thymine by two hydrogen bonds, guanine with cytosine by three.
- Replication is semi-conservative: each daughter molecule has one parental and one new strand.
- Transcription uses the template strand to make mRNA; translation reads codons 5′ to 3′ on mRNA, with tRNA anticodons pairing antiparallel.
- A gene is a DNA sequence coding for a polypeptide or functional RNA; alleles are alternative versions at the same locus.
- Independent assortment applies to genes on different chromosomes; linked genes tend to be inherited together, and crossing over produces recombinants.
Worked example
A dihybrid cross between heterozygotes (AaBb × AaBb) with independent assortment gives a 9:3:3:1 phenotypic ratio. Reasoning: each parent produces four gamete types in equal frequency (AB, Ab, aB, ab); random fertilisation gives 16 equally likely combinations; the 9 class is A_B_, while the 3:3:1 classes are A_bb, aaB_ and aabb. A test cross with aabb is better for revealing the genotype of an unknown dominant phenotype because it gives 1:1:1:1 when the unknown is AaBb.
Traps
- Do not confuse the template strand with the coding strand; mRNA matches the coding strand except U replaces T.
- “Missense” changes one amino acid; “nonsense” creates a stop codon; “silent” does not change the amino acid.
- Linked genes do not give 9:3:3:1; recombination frequency below 50% indicates linkage.