Molecular Biology and Gene Expression
Molecular Biology and Gene Expression
This domain tests DNA structure, semi-conservative replication, transcription, translation and mutation. Expect questions on base pairing, the central dogma, and the effect of a changed base on a polypeptide.
Core rules
- DNA is a double helix of antiparallel strands. Adenine pairs with thymine by two hydrogen bonds; guanine pairs with cytosine by three. Replication is semi-conservative: each new molecule keeps one parental strand.
- Transcription uses RNA polymerase and one DNA template strand to make mRNA. Translation occurs at ribosomes; tRNA anticodons pair with mRNA codons, and each codon specifies one amino acid.
- The genetic code is triplet, degenerate and non-overlapping. A substitution may be silent, missense or nonsense. Insertions or deletions of bases not in multiples of three cause frameshift mutations.
Worked example
A template strand reads 3′-TAC-5′. The mRNA is complementary and antiparallel: 5′-AUG-3′. AUG is the start codon and codes for methionine. Reasoning: RNA bases pair A with U and C with G, and the new strand runs opposite in direction to the template.
Traps
The coding strand has the same sequence as mRNA except T is replaced by U; it is not the template. mRNA codons are read 5′ to 3′. Replication copies the whole genome, whereas transcription copies a gene. Do not write thymine in mRNA.