Nucleic Acid Metabolism and Molecular Biology: Replication, Transcription, Translation and Gene Expression
What this domain tests
Molecular biology questions test DNA replication, transcription, translation, and gene expression, especially directionality, enzymes, and the genetic code.
Core rules
- Replication is semiconservative and bidirectional. DNA polymerase adds nucleotides to a 3'-OH end, so synthesis is 5' to 3'. The leading strand is continuous; the lagging strand is made as Okazaki fragments. Helicase unwinds, topoisomerase relieves supercoiling, primase makes RNA primers, ligase seals nicks.
- Transcription uses RNA polymerase and a DNA template read 3' to 5'; RNA is made 5' to 3'. Promoters position the polymerase. In eukaryotes, mRNA is capped, polyadenylated, and spliced.
- The genetic code is triplet, degenerate, and nearly universal. AUG is start; UAA, UAG, and UGA are stops.
- Translation occurs on ribosomes. tRNA anticodons pair with mRNA codons; initiation, elongation, and termination require factors and energy. The central dogma is DNA to RNA to protein.
Worked example
Template DNA: 3'-TAC GGA TTC-5'. Complementary mRNA is 5'-AUG CCU AAG-3'. Reading codons: AUG = Met, CCU = Pro, AAG = Lys. The peptide is Met-Pro-Lys. If the template were the coding strand instead, the mRNA would be 5'-UAC GGA UUC-3', giving Tyr-Gly-Phe; this is why template and coding strands must not be confused.
Traps
The coding strand has the same sequence as mRNA except T is replaced by U. RNA polymerase does not need a primer, but DNA polymerase does. Replication is semiconservative, not conservative. A point mutation may be silent because the code is degenerate, so it does not always change protein sequence.