Ch 6 covers the molecular basis of inheritance — DNA structure, replication, transcription, genetic code, translation (protein synthesis), gene regulation (lac operon), and the Human Genome Project.
DNA: double helix, antiparallel strands, complementary base pairing (A=T, G≡C). Replication: semi-conservative (Meselson-Stahl experiment, N¹⁵/N¹⁴). Origin of replication, replication fork. Leading strand: continuous (5'→3'). Lagging strand: Okazaki fragments joined by ligase. Enzymes: helicase, primase, DNA polymerase III, ligase.
Central dogma: DNA → RNA (transcription) → Protein (translation). Transcription: RNA polymerase reads template 3'→5', synthesises mRNA 5'→3'. Eukaryotes: post-transcriptional processing (capping, tailing, splicing). Genetic code: triplet, degenerate, universal, non-overlapping. 61 sense codons + 3 stop (UAA, UAG, UGA). AUG = start (Met). Translation: initiation → elongation → termination at ribosomes. tRNA: anticodon + amino acid.
Lac operon (E. coli): structural genes (lacZ, lacY, lacA) + promoter + operator + regulator. Without lactose: repressor binds operator → no transcription. With lactose: allolactose binds repressor → operator free → transcription. Human Genome Project: 3.2 billion bp, ~20,000-25,000 genes, <2% codes for proteins.
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The central dogma states: DNA → RNA → Protein. DNA is transcribed into mRNA (transcription), which is translated into protein at ribosomes (translation). DNA can also be replicated. Exceptions exist: reverse transcription (RNA → DNA, by reverse transcriptase in retroviruses) and RNA replication (in some RNA viruses). Proposed by Francis Crick in 1958.
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