Molecular Biology: DNA, RNA, Replication, Transcription and Translation
This section covers DNA structure and the central dogma. The exam tests base pairing, semiconservative replication, transcription, the genetic code, and translation.
Core rules
DNA is an antiparallel double helix. Adenine pairs with thymine by two hydrogen bonds; guanine pairs with cytosine by three. Replication is semiconservative: each daughter duplex keeps one parental strand. DNA polymerase adds nucleotides to a 3-prime end and needs a primer. The leading strand is synthesized continuously; the lagging strand forms Okazaki fragments. Transcription uses the template strand to make RNA in the 5-prime to 3-prime direction; RNA polymerase does not need a primer. The genetic code is triplet, degenerate, non-overlapping, and nearly universal. AUG is the start codon and also codes methionine; UAA, UAG, and UGA are stop codons. Translation occurs on ribosomes, with tRNA anticodons pairing with mRNA codons.
Worked example
A DNA template reads 3-prime TAC 5-prime. RNA polymerase makes the complementary mRNA 5-prime AUG 3-prime. AUG is the start codon and codes methionine. If a student mistakenly uses the coding strand, it reads 5-prime ATG 3-prime, which corresponds to mRNA AUG only after replacing T with U, but the direct template rule is safer: template T pairs with A, A with U, and C with G.
Traps that cost marks
Do not confuse the template strand with the coding strand. Do not say RNA polymerase needs a primer. Do not write thymine in RNA. Do not count a stop codon as an amino acid. Remember that the code is degenerate but not ambiguous: one codon specifies one amino acid, while one amino acid may have several codons.