DNA and protein synthesis
DNA and protein synthesis tests DNA structure, replication, transcription, translation, the genetic code and mutation. You need to apply complementary base pairing and explain how a gene becomes a polypeptide.
DNA and replication
DNA is a double helix of antiparallel polynucleotide strands. Adenine pairs with thymine (two hydrogen bonds); guanine pairs with cytosine (three). Replication is semi-conservative: helicase unwinds, each strand acts as a template, and DNA polymerase joins free nucleotides.
Transcription and translation
Transcription makes mRNA from the template strand using RNA polymerase; uracil replaces thymine. In eukaryotes, mRNA is spliced before leaving the nucleus. Translation occurs at ribosomes: tRNA anticodons pair with mRNA codons, and peptide bonds form between amino acids. The genetic code is degenerate, non-overlapping and read in triplets.
Worked example
A DNA template strand reads 3'-TAC-5'. The complementary mRNA codon is 5'-AUG-3', because A pairs with T, U pairs with A, and G pairs with C. AUG is a start codon and codes for methionine. The coding strand would read 5'-ATG-3'.
Traps
Do not write thymine in mRNA. The template strand is read 3' to 5'. Codons are on mRNA; anticodons are on tRNA. A mutation may be silent because the genetic code is degenerate.