Biologia Molecolare Del Gene Zanichelli Pdf Site

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.

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. biologia molecolare del gene zanichelli pdf

Ribosomes have three sites: A (aminoacyl-tRNA binding), P (peptidyl-tRNA binding), and E (exit). Initiation involves the small subunit binding the Shine-Dalgarno sequence in bacteria (or scanning from the 5’ cap in eukaryotes). Elongation cycles: a new tRNA enters the A site; peptidyl transferase (an RNA enzyme in the large subunit) forms a peptide bond; the ribosome translocates three nucleotides. Termination occurs when a stop codon (UAA, UAG, UGA) is recognized by release factors, releasing the polypeptide. The double helix is antiparallel: the two strands