CC VI MOLECULAR BIOLOGY DNA REPLICATION Ms A

CC - VI – MOLECULAR BIOLOGY – DNA REPLICATION Ms. A. M. THAFSHILA AAFRIN ASSISTANT PROFESSOR PG & RESEARCH DEPARTMENT OF BIOCHEMISTRY

DNA is anti-parallel • Two strands run parallel to each other but with opposite alignments (directions) • Mc. Graw-Hill DNA • Why is being antiparallel an advantage to the DNA molecule?

DNA REPLICATION Semi-conservative = each one of the parent DNA strands is passed to the daugher DNA + one new strand for each Semi-conservative DNA (30 secs) SNEAK PREVIEW: DNA REPLICATION PLAYERS (enzyme review)

Fill out DNA Replication Enzymes CHART • Lots of enzymes are needed to start each step

Topoisomerase

Enzyme Helicase: separates (breaking HHelicase bonds) double helix at the replication fork

DNA Helicase • The enzyme is unwinding the chain and breaking the H-bonds between the complementary base pairs (A-T, G-C).

RNA Primer • Nucleotides for the starting point for DNA replication • Short strands of RNA

DNA replication is initiated at many points in eukaryotic chromosomes. They will eventually all meet to form whole replicated strand

DNA Replication Bubble: DNA duplicates in both directions

SSB’s single strand binding proteins • Stabilize the DNA strands as they are being replicated • Prevents rejoining of DNA strands

DNA Polymerases • DNA Polymerase I • Cuts off RNA primers and fills in with DNA (between Okazaki fragments) –lagging strand • Can proofread • DNA Polymerase III • Elongates the strand by adding DNA nucleotides on leading strand • Also proofreads and corrects the DNA strand

Anti-parallel strand builds in the opposite direction (but always in 5’ to 3’ direction)

Leading Strand Lagging • Strand Template strand • Other DNA strand of DNA • Continuous addition of nitrogenous bases • in 5’ to 3’ direction • Mc. Graw-Hill Replication Fork • Forms short strands of Okazaki fragments (that will be joined later) • in the 5’ to 3’ direction • DNA Replication You Tube (1: 35)

OKAZAKI FRAGMENTS The short strands of newly made DNA fragments on the lagging strand are called Okazaki fragments after the Japanese Biochemist Reiji Okazaki.

DNA LIGASE is the enzyme that joins the Okazaki fragments (sugar -phosphate backbone) with covalent bonds DNA REPLICATION (look for ligase) 2: 00