DNA Replication 2007-2008 http://highered.mheducation.com/sites/0072943696/student_view0/chapter3/animation__dna_replication__quiz_1_.html

Copying DNA Why replicate or copy DNA? Make an exact copy of the genetic material for cell division Where does replication occur? Nucleus of eukaryotic cells cytoplasm of prokaryotic cells When does replication occur? Before cell division: interphase (s phase) of mitosis/meiosis Replication of DNA base pairing allows each strand to serve as a template for a new strand new strand is 1/2 parent template & 1/2 new DNA

How Replication Occurs: 1st step Unwind DNA DNA helicase enzyme Breaks hydrogen bonds unwinds part of DNA helix Forms a replication fork helicase replication fork

Replication: 2nd step Build daughter DNA strand using DNA template 5’ Replication: 2nd step Build daughter DNA strand using DNA template add new complementary nucleotides Enzyme DNA polymerase Builds new DNA in 5’ to 3’ direction 3’ DNA Polymerase http://video.search.yahoo.com/yhs/search;_ylt=a0levvftbqtaowsayp4pxqt.?p=dna+replication+animation&fr=sfp&fr2=piv-web&hspart=iry&hsimp=yhs-fullyhosted_003&type=wbf_mdaffmarmarie_17_36&vm=r#id=19&vid=c311b184edb1a39a9d02ea903fe1b679&action=view 5’ 3’

Leading & Lagging strands Okazaki Leading & Lagging strands Limits of DNA polymerase can only build onto 3 end of an existing DNA strand  5 Okazaki fragments 5 5 3 5 3 5 3 ligase Lagging strand 3 growing replication fork 3 5 Leading strand  3 5 Lagging strand Non-continuous Okazaki fragments joined by ligase “spot welder” enzyme 3 DNA polymerase Leading strand continuous synthesis

Editing & proofreading DNA 1000 bases/second = lots of typos! DNA polymerase proofreads & corrects typos repairs mismatched bases removes abnormal or mutated repairs damage throughout life reduces error rate from 1 in 10,000 to 1 in 100 million bases

Fast & accurate! It takes E. coli bacteria<1 hour to copy 5 million base pairs in its single chromosome Forms 2 replication forks Human cells copy 3.2 billion bases & divide into daughter cells in only 8 hours Forms multiple replication forks remarkably accurate ~1 error per 100 million bases ~30 errors per cell cycle