Transcription proceeds through initiation, elongation, and termination. Promoters contain conserved sequences: in bacteria, the -10 (Pribnow) box and -35 region; in eukaryotes, the TATA box (bound by TBP), CAAT box, and GC box. Enhancers and silencers, distant regulatory elements, modulate transcription through DNA looping and mediator complexes.
The replication machinery is a multi-protein complex. Helicase unwinds the DNA ahead of the fork, while single-strand binding proteins (SSBs) prevent reannealing. Topoisomerases (e.g., gyrase) relieve supercoiling stress. DNA polymerase I removes RNA primers and fills gaps, and DNA ligase seals nicks. Eukaryotic replication is more complex due to linear chromosomes and multiple origins; telomerase solves the end-replication problem by extending telomeres using an internal RNA template. Francis Crick’s central dogma states that genetic information flows from DNA → RNA → protein. Transcription is the first step: RNA polymerase synthesizes an RNA strand complementary to a DNA template. In bacteria, a single RNA polymerase (with sigma factor for promoter recognition) produces all RNAs. In eukaryotes, three distinct RNA polymerases exist: Pol I (most rRNA), Pol II (mRNA and some snRNAs), and Pol III (tRNA, 5S rRNA). biologia molecolare del gene zanichelli pdf
Protein folding, post-translational modifications (phosphorylation, glycosylation, ubiquitination), and targeting (signal sequences for the ER) complete the journey from gene to functional molecule. Not all genes are expressed at all times. Regulation occurs at multiple levels. The replication machinery is a multi-protein complex
The double helix is antiparallel: the two strands run in opposite directions (5’→3’ and 3’→5’), a feature essential for the action of polymerases. The major and minor grooves created by the helix provide binding sites for regulatory proteins, allowing sequence-specific recognition without strand separation. DNA replication must be extraordinarily accurate (error rate ~1 in 10⁹ nucleotides) and rapid. In E. coli , replication begins at a single origin ( oriC ) and proceeds bidirectionally. The key enzyme, DNA polymerase III, synthesizes new strands only in the 5’→3’ direction. This creates a fundamental problem: the two template strands are antiparallel. The leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized discontinuously as Okazaki fragments, each requiring a new RNA primer. DNA polymerase I removes RNA primers and fills
is exemplified by the lac operon. In the absence of lactose, the Lac repressor binds the operator, blocking transcription. Allolactose (an inducer) binds repressor, causing a conformational change that releases DNA. Additionally, when glucose is low, cAMP accumulates and binds CAP (catabolite activator protein); the cAMP-CAP complex binds the CAP site near the promoter, enhancing RNA polymerase binding. This dual control ensures efficient lactose metabolism only when necessary.