Molecular Biology and Genetics: Replication, Expression, and Inheritance
Core concepts
Build a single coherent picture from DNA to phenotype. Know semiconservative replication: helicase unwinds, primase lays RNA primers, DNA polymerase III extends 5' to 3' on the leading strand continuously and the lagging strand in Okazaki fragments, ligase seals nicks. Transcription uses the template strand read 3' to 5', producing RNA 5' to 3'; RNA polymerase needs no primer. Eukaryotic processing adds a 5' cap and poly(A) tail and splices introns via the spliceosome. Translation: initiation at AUG, tRNA anticodon pairing, elongation at the A, P, and E sites, termination at stop codons. Know the central dogma exceptions (reverse transcription, RNA replication) and mutation classes: missense, nonsense, frameshift, silent. Mendelian genetics, linkage, and Hardy–Weinberg (p² + 2pq + q² = 1) round out the domain, along with regulation: operons in prokaryotes, enhancers and chromatin in eukaryotes.
Traps
Directionality is the classic failure point: polymerases synthesize 5' to 3' and read the template 3' to 5'. A frameshift changes everything downstream; a nonsense mutation truncates. Do not confuse the coding (sense) strand with the template strand. In Hardy–Weinberg, allele frequencies apply to populations, not families. Remember that a silent mutation can still alter splicing or codon usage.
Worked example
A recessive allele has q = 0.2, so p = 0.8. Carrier frequency is 2pq = 2 × 0.8 × 0.2 = 0.32, and affected frequency is q² = 0.04. If the population is not in equilibrium, these predictions fail - check assumptions of random mating, no selection, and large size first.