Units 5-6 cover heredity and gene expression — from meiosis and inheritance patterns to the molecular mechanisms of DNA replication, transcription, translation, and gene regulation.
Meiosis: two divisions producing 4 haploid cells. Genetic variation from: crossing over (prophase I), independent assortment, random fertilisation. Mendel\'s laws: segregation (alleles separate in meiosis), independent assortment (genes on different chromosomes). Dominance, codominance, incomplete dominance, multiple alleles, polygenic inheritance, epistasis. Sex-linked traits: X-linked (more common in males). Chi-square test: compare observed to expected ratios.
DNA replication: semiconservative, helicase (unwinds), primase (RNA primer), DNA polymerase III (5\'→3\' synthesis), leading strand (continuous), lagging strand (Okazaki fragments), ligase (joins). Transcription: RNA polymerase, promoter, template strand → mRNA. RNA processing (eukaryotes): 5\' cap, 3\' poly-A tail, splicing (remove introns, keep exons). Translation: ribosome reads mRNA codons, tRNA brings amino acids, start codon AUG (Met), stop codons (UAA, UAG, UGA).
Prokaryotic: operon model (lac operon — inducible, trp operon — repressible). Eukaryotic regulation at multiple levels: chromatin remodelling (epigenetics, methylation, acetylation), transcription factors, RNA processing (alternative splicing), mRNA degradation, translational regulation, post-translational modification. Mutations: point (substitution — silent, missense, nonsense), frameshift (insertion/deletion). Biotech: PCR, gel electrophoresis, restriction enzymes, plasmids, CRISPR gene editing.
The lac operon in E. coli controls genes for lactose metabolism. Without lactose: the repressor protein (produced by lacI gene) binds to the operator, blocking RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA). With lactose present: allolactose (a lactose isomer) binds to the repressor, changing its shape so it cannot bind the operator → RNA polymerase transcribes the genes → enzymes produced to digest lactose. Additionally, when glucose is low, cAMP levels rise → cAMP binds CAP protein → CAP-cAMP complex binds promoter → enhances transcription. So the operon is maximally active when lactose is present AND glucose is absent.
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