Molecular Biology and Gene Expression
Core concepts
Know the structure of DNA (double helix, antiparallel strands, complementary base pairing A–T, G–C, hydrogen bonds) and RNA (single-stranded, uracil replaces thymine). Master the central dogma: replication, transcription, translation. Know semiconservative replication and the roles of helicase, polymerase, and ligase at a conceptual level. Transcription makes mRNA from a DNA template; translation at the ribosome uses codons and tRNA anticodons to assemble amino acids. Know the genetic code features: triplet, degenerate, non-overlapping, universal. Understand gene regulation (operon concept, promoter and operator) and mutation effects on protein structure.
Traps
DNA polymerase adds nucleotides to the 3' end; strands are antiparallel. Transcription uses RNA polymerase and produces RNA, not DNA. Codons are on mRNA; anticodons are on tRNA. The genetic code is degenerate (several codons per amino acid) but not ambiguous (one codon, one amino acid). A frameshift is more damaging than a substitution. Distinguish the template strand from the coding strand. Do not say translation occurs in the nucleus.
Worked example / rule of thumb
Rule of thumb: "DNA to RNA to protein; count triplets." Worked example: a template DNA triplet 3'-TAC-5' transcribes to mRNA 5'-AUG-3', which codes for methionine (start codon). A sequence of 30 nucleotides in mRNA corresponds to 10 codons. If a substitution changes one codon but the amino acid stays the same, it is a silent mutation.