DNA Replication PDF

DNA Replication The process by which DNA is duplicated (DNA DNA). Three possible mechanisms: (a) Semiconservative mechanism. DNA replication produces DNA molecules with 1 parental strand and 1 newly made strand. (b) Conservative mechanism. DNA...

DNA Replication The process by which DNA is duplicated (DNA DNA). Three possible mechanisms: (a) Semiconservative mechanism. DNA replication produces DNA molecules with 1 parental strand and 1 newly made strand. (b) Conservative mechanism. DNA replication produces 1 double helix with both parental strands, and the other with 2 new daughter strands. (c) Dispersive mechanism. DNA replication produces DNA strands in which segments of new DNA are with the parental DNA.  Meselson and Stahl Experiment DNA replication Start The Meselson and Stahl Experiment showed that DNA replication is semi- conservative. Replication cycle I A semi-conservative process in which a double helix gives rise to two double helices, each with an old strand and a newly synthesized strand. Replication cycle II Original First round Second round double helix of replication of replication Replication cycle III N14 DNA light N14 DNA/N15 DNA half-heavy N15 DNA heavy DNA replication Semi-conservative Replication Molecular processes:  “Untwisting” of the DNA molecule. This occurs by the breaking of H-bonds between N-bases, promoted by certain enzymes.  Each of two strands now severs as a “template” to synthesize a new complementary strand.  A specific enzyme called “DNA polymerase” helps in the linking of free nucleotides to form a new strand.  The nucleotides are always added at the 3’-end (which has a free-OH group) of the new strand.  The template (old) and complementary (new) strands are antiparallel; the 3’-end of one strand is paired with 5’-end of the other. 5’ 3’ 5’ 3’ DNA replication Semi-conservative Replication Molecular processes:  Both parent strands are replicated, and replication in eukaryotic cells proceeds in many locations of each strand forming DNA pieces.  DNA pieces are linked by an enzyme called “DNA ligase”.  The two DNA molecules formed are identical, i.e. in base sequence, to the parent molecule.  Each DNA molecule has ½ of the old and ½ new molecule - semi-conservative. 5’ 3’ 3’ 5’ DNA replication Molecular processes with involvement of major proteins: Replication fork Leading strand template Lagging strand template Leading strand Lagging strand DNA replication Molecular processes with involvement of major proteins: Replication fork Leading strand Lagging strand DNA helicase Topoisomerase Single-stranded binding proteins DNA primase DNA replication Molecular processes with involvement of major proteins: Replication fork Leading strand Lagging strand Sliding clamp DNA helicase Topoisomerase Single-stranded binding proteins DNA primase DNA clamp(sliding clamp) DNA polymerases DNA replication Molecular processes with involvement of major proteins: Replication fork Leading strand Lagging strand Topoisomerase DNA helicase Single-stranded binding proteins DNA primase DNA clamp(sliding clamp) DNA polymerases (III , I) Okazaki fragments DNA replication Molecular processes with involvement of major proteins: Replication fork Leading strand Lagging strand Topoisomerase DNA helicase Single-stranded binding proteins DNA primase DNA clamp(sliding clamp) DNA polymerases Okazaki fragments DNA ligase  DNA replication Molecular processes with involvement of major proteins: TABLE 11.1 DNA replication Replication origin in the chromosomes of different organisms Bacteria have a single origin, whereas eukaryotes require multiple origins Replication advances at a rate of about 500-1000 nucleotides per second in bacteria, and about 50-100 per second in eukaryotes.