Q.What is DNA repair?

DNA replication is semiconservative: the two parental strands of the double helix separate, and each serves as a template for a new complementary strand, so every daughter duplex retains exactly one original parental strand. Taylor, Woods and Hughes proved this experimentally in Vicia faba root cells using 3H-thymidine labelling and autoradiography: after one replication round both sister chromatids of every chromosome were labelled, but after a second round only one of the two sister chromatids per chromosome remained labelled, exactly matching the semiconservative prediction. In eukaryotes, replication begins simultaneously at many chromosomal origins of replication, each activated by a prereplication complex that includes the origin recognition complex (ORC); the site of active unwinding and synthesis is the replication fork, where helicase unwinds the duplex, replication protein A (RPA) stabilises the separated single strands, and topoisomerase relieves supercoiling ahead of the fork. DNA polymerase alpha/primase lays down short RNA primers (needed because DNA polymerase requires a free 3' OH to start); synthesis then always runs 5' to 3', so the leading strand is made continuously (same direction as fork movement) while the lagging strand is made discontinuously as Okazaki fragments (opposite direction to fork movement), later joined by DNA ligase. Three polymerases handle nuclear replication: DNA Pol alpha (primers), DNA Pol delta (the main replicating enzyme), and DNA Pol epsilon (strand extension); DNA Pol beta plays no role in norma …