Chapter 11: Meiosis & Sexual Reproduction

The central dogma of molecular biology, describing the flow of genetic information from DNA to RNA to protein, begins with the foundational concept of the gene, which was redefined from being a determinant of traits by Mendel to a sequence of DNA by Watson and Crick, with key insights into its function provided by Garrod's "inborn errors of metabolism" and Beadle and Tatum's "one gene–one polypeptide" hypothesis derived from experiments on Neurospora. Gene expression relies on transcription, catalyzed by DNA-dependent RNA polymerases, which synthesizes various RNAs: messenger RNA (mRNA) carries the code, ribosomal RNA (rRNA) provides structure and catalysis, and transfer RNA (tRNA) acts as an adaptor. Prokaryotes employ a single RNA polymerase that uses a sigma factor (σ) to recognize promoter elements like the Pribnow box, facilitating transcription that is tightly coupled with translation. Eukaryotes feature three distinct RNA polymerases, with Polymerase II generating heterogeneous nuclear RNA (hnRNA), the precursor to mRNA, relying on general transcription factors to form a preinitiation complex at sites like the TATA box. Eukaryotic mRNA undergoes complex processing, including the addition of a 5' methylguanosine cap and a 3' poly(A) tail. A critical step is RNA splicing, which excises non-coding intervening sequences, or introns (often thousands of bases long), from the coding exons, a split-gene structure discovered through R-loop experiments on viral and globin gene transcripts. Splicing requires the spliceosome, a large dynamic machine containing small nuclear ribonucleoproteins (snRNPs) and their associated snRNAs, operating via a lariat intermediate similar to self-splicing group II introns. The catalytic capability of RNA molecules (ribozymes), evidenced by rRNA processing and the peptidyl transferase center of the ribosome, strongly supports the theory of an ancient "RNA world." Gene regulation also involves small noncoding RNAs: double-stranded RNA is cleaved by Dicer into small interfering RNAs (siRNAs) that guide the Argonaute-containing RISC complex to destroy complementary target RNAs, a defense mechanism called RNA interference (RNAi); microRNAs (miRNAs) modulate translation by binding partially complementary regions of target mRNAs; and piwi-interacting RNAs (piRNAs) specifically suppress transposable element movement in germ cells. Translation decodes the genetic information, which is nonoverlapping, triplet-based (codons), and degenerate. Charged tRNAs, covalently linked to their amino acids by aminoacyl-tRNA synthetases, recognize codons via their anticodon loop, following the flexible base-pairing rules described by the wobble hypothesis. Protein synthesis initiates at an AUG codon (in bacteria guided by the Shine–Dalgarno sequence, in eukaryotes by scanning from the 5' cap) and proceeds through elongation, where GTP-hydrolyzing factors drive tRNA cycling through the A, P, and E sites of the ribosome, synthesizing peptide bonds via the ribosomal ribozyme. Termination occurs when stop codons (UAA, UAG, UGA) are recognized by release factors, and cellular quality control employs nonsense-mediated decay (NMD) to eliminate messages containing premature termination codons. Finally, the chapter examines DNA origami, a nanotechnology that leverages the structural rigidity of DNA to engineer complex, defined nanostructures using a single-stranded scaffold and numerous staple strands.