Nucleic acids and protein synthesis
DNA structure and replication
A nucleotide = phosphate + pentose sugar (deoxyribose in DNA, ribose in RNA) + nitrogenous base. Purines (A, G) have two rings; pyrimidines (C, T, U) one. Nucleotides join by phosphodiester bonds (condensation) into a sugar-phosphate backbone. DNA is a double helix of two antiparallel strands joined by hydrogen bonds: A–T (2), C–G (3). Semi-conservative replication: helicase breaks hydrogen bonds; free nucleotides pair with each exposed strand; DNA polymerase joins them 5′→3′ (continuous leading strand; fragments on the lagging strand joined by ligase). Each new molecule has one old and one new strand, shown by the Meselson–Stahl ¹⁵N/¹⁴N experiment. RNA is single-stranded with ribose and uracil.
≈ 45 minTranscription and translation
A gene is a sequence of DNA bases coding for a polypeptide. The code is triplet (3 bases = 1 amino acid), degenerate (most amino acids have several codons), non-overlapping and universal. Transcription (nucleus): RNA polymerase reads the template strand, building pre-mRNA; introns are spliced out, leaving exons. Translation (ribosome): mRNA codons pair with anticodons on tRNA carrying specific amino acids; peptide bonds form; the ribosome moves along until a stop codon. Gene mutations: substitution (may be silent, missense or nonsense), insertion or deletion (frameshift, usually serious). Sickle cell: one base substitution changes glutamic acid to valine in the β chain.
≈ 50 min