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Lecture 6

Lecture 6

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Jane Mitchell

Lecture 6 DNA Replication Part 21Issues in replication2DNA repairA quick key word reviewoLeading strand is synthesized continuously from single RNA primeroLagging strand is synthesized discontinuously from multiple primersoOkazaki fragments consist of the RNA primer and the DNA strandoDNA synthesis proceeds in 5 to 3 directionoPrimosome consists of primase and helicaseoThe predominant helicase is on lagging strandIssues in DNA replicationoWhat happens at the ends of eukaryotic linear chromosomes during replicationoHow is DNA unwoundoHow are mistakes found and correctedWhat happens at the ends of chromosomesoAs you migrate through the double stranded DNA particularly in lagging strand which is initiated by primers the primers run out of templateoThe leading strand rolls along happilyThe problem at ends of chromosomesoThe primer space is programmed by primosome to space them in specific distancesoOver successive rounds of replication a gradual shortening of 5 ends of the newly synthesized strands particularly in lagging strandsyou lose sequence informationWhat happens at the endsoAdds repeated sequences in the 3 ends of strands which allows the replication machinery to faithfully reproduceoThe ends are called telomeres and they provide access points for primers to be initiatedoThe tloops are usually tucked back into the chromosome The reason for looplike structure is that cells dont like to see chromosomes with single stranded DNA strung out at the end because theyll think its a broken chromosome and theyll do their best to eliminate themtelomerase to the rescueoThe enzymes which adds these 3 extensions is called telomerase and its able to add nucleotides to the 3 ends of the DNA in the absence of a primer because it has its own builtin primer and it uses the RNA molecule template for the addition of nucleotides to the 3 endoBecause its not a long stretch of RNA you get a small stretch of RNA as a template that is repeated a lot of times So you get a Grich strings being added to the 3 end which is using multiple copies using this templateTelomere replicationoWith RNA template you end up with complimentary copies added to the chromosomesoTelomerase binds and recognized the 3 end with complimentary base pairing Now it starts to add the replicates of this particular complimentary sequence to the RNA templateoDepending on the cell type a small rounds or very many rounds of additionsoThe typical DNA polymerase can then establish a primer in a particular location and continue using DNA polymerase and complete the sequenceoThese type of enzymes essentially resemble a reverse transcriptase a type of polymerase which is able to add nucleotidesoThis telomerase generates Grich ends and adds nucleotides to 3 ends to parental strands template
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