Quest_ReplicationConverted - T extbook Many proteins work together in DNA Replication and Repair The Basic Principle Base Pairing to a Template Strand

Textbook – Many proteins work together in DNA Replication and Repair The Basic Principle: Base Pairing to a Template Strand: • Complementary strands, antiparallel to each other • Each strand serves as a template for ordering nucleotides into another complementary strand • Semiconservative Model – When a double helix replicates, each of the 2 daughter molecules will have one old strand (from parent) and one new strand • Conservative Model – 2 parent strands come back together and daughter molecules form new helix • Dispersive Model – All 4 strands of DNA after replication have a mixture of new & old DNA • Matthew Meselson & Franklin Stahl (1958) – proved that semiconservative model is correct *Grew E. coli in a medium containing labeled 15N (heavy nitrogen) for several generations, then transferred to medium containing 14N (light nitrogen) *They took 2 samples from the culture; 1 after 1 st replication, another after 2 nd *Centrifuged samples - 1 st sample: got a hybrid band; told them it wasn’t conservative model - 2 nd sample: both light DNA & hybrid bands; told them it wasn’t dispersive “Getting Started”: • Replication of DNA begins at sites called origins of replication (short stretches of DNA having a specific sequence of nucleotides) • The E. coli chromosome is circular and contains 1 origin • Proteins that initiate replication recognize the sequence and attach to the DNA, separating the 2 strands and opening a replication “bubble” • Replication proceeds in both directions until the entire DNA is copied (in E. coli) • Eukaryotic chromosomes have 100s-1000s of origins and replication also proceeds in both directions from the origin • At the end of the bubble is a replication fork (Y-shaped region where parental strands are unwound) • Helicases – enzymes that untwist the double helix at the replication forks (separates