DNA Replication

Complementary Strands DNA arranged in double helix (Rosalind Franklin’s work) Antiparallel – run 5’  3’ on 1 strand and 3’  5’ on other 2 strands are complementary; ie 3’—AGTAC—5’ 3’—AGTAC—5’ 5’—TCATG—3’ 5’—TCATG—3’

DNA Replication is semiconservative Confirmed by Messelson-Stahl: w- hill.com/sites/ /student_view0/c hapter14/animations. html# w- hill.com/sites/ /student_view0/c hapter14/animations. html#

Steps in DNA Replication Begins at specific sites called origins of replication DNA helicase unwinds double helix by breaking H-bonds – forms replication forks Single-stranded binding proteins hold strands open

DNA polymerase adds nucleotides to 3’- end of growing DNA strand Synthesis is always in 5’  3’ direction Synthesis is always in 5’  3’ direction Requires a RNA primer to build off of Requires a RNA primer to build off of DNA primase synthesizes a short complementary RNA strand for DNA polymerase to build on to DNA primase synthesizes a short complementary RNA strand for DNA polymerase to build on to

DNA Replication occurs on both strands at the same time Strands run in opposite directions DNA only replicates in 5’  3’ direction Therefore, only 1 strand can replicate toward the replication fork: leading strand Strand replicating away from the fork is called the lagging strand Can only synthesize short pieces at a time Can only synthesize short pieces at a time Okazaki fragments Okazaki fragments

Synthesis on leading strand is continuous Synthesis on lagging strand requires multiple primers When RNA primer of previous Okazaki fragment is reached, DNA polymerase breaks it down DNA ligase seals Okazaki fragments together hill.com/sites/ /student_view0/chapte r14/animations.html# hill.com/sites/ /student_view0/chapte r14/animations.html#

Proofreading DNA polymerase proofreads new nucleotides against template If mistake made, DNA polymerase repairs mistake If mistake not corrected, mutation has occurred

Pro vs. Eukaryotes Prokaryotes have 1 origin of replication DNA synthesis proceeds in both directions around the circular chromosome until 2 replication forks meet DNA synthesis proceeds in both directions around the circular chromosome until 2 replication forks meet Eukaryotic chromosomes have multiple origins of replication Replication bubbles eventually meet and merge Replication bubbles eventually meet and merge Speeds up process Speeds up process