DNA Replication Lecture 8 - Chapter 5 PDF

DNA Replication Section 5.1 Learning Objectives Describe and compare the proteins and mechanisms that carry out DNA replication at replication forks of bacteria and mammalian cells. Contrast the mechanisms of synthesis of the leading and lagging strands of DNA. Compare origins of replic...

DNA Replication Section 5.1 Learning Objectives Describe and compare the proteins and mechanisms that carry out DNA replication at replication forks of bacteria and mammalian cells. Contrast the mechanisms of synthesis of the leading and lagging strands of DNA. Compare origins of replication in bacteria and mammalian cells. Summarize the action of telomerase. Describe the mechanisms that ensure accurate DNA replication. …and predict outcomes when these processes are perturbed. DNA Replication Key Concepts All known DNA polymerases synthesize DNA in the 5’ to 3’ direction. Leading strand synthesis occurs continuously heading into the replication fork Lagging strand synthesis occurs in Okazaki fragments heading away from the replication fork DNA polymerase starts at origins of replication, which contain binding sites for proteins that initiate the process. DNA polymerase and various other proteins act in a coordinated manner to synthesize both leading and lagging strands. Telomeric repeat sequences at the ends of chromosomes are maintained by the action of telomerase (a reverse transcriptase) that carries its own template RNA. The reaction catalyzed by DNA polymerase Recall that: DNA is ______________ stranded Strands are _____________ A-T pairs → ___ hydrogen bonds G-C pairs → ___ hydrogen bonds Replication is ____________________ New strands of DNA synthesized __________ 5’ _____________ group 3’ _____________ group DNA synthesis requires _________ of energy Which of the following is NOT true of DNA polymerase? A) It catalyzes an endergonic reaction B) It can only synthesize DNA in the 5’ to 3’ direction C) It can only add nucleotides to the free 3’ OH group of an existing strand of nucleotides NOTE CHECK D) It can substitute RNA for DNA nucleotides if necessary Join.nearpod.com pin = Replication begins at an ____________________ where two _____________________ form Origin of replication in _____________ prokaryotic chromosomes Replication origins in ___________ eukaryotic chromosomes In yeast, origin recognition complex (ORC) binds to ARS Complete ARS ~100 base pairs Conserved ACS sequence – binding site for origin recognition complex (ORC) ORC recruits additional proteins to the ARS to initiate replication 5’ to 3’ synthesis results in an asymmetrical replication fork Time Point 1 Time Point 2 Synthesis of new DNA must be initiated with RNA ___________ RNA primers must be replaced with DNA and backbone nicks sealed by ___________ enzyme The issue with 5’→3’ synthesis: The End Replication Problem Image modified from Zlir’a, public domain artist The solution: action of ____________________ Make a prediction Which do you think is most likely to be true regarding telomerase activity? a) It is necessary in prokaryotic cells that have circular genomes b) Cancer cells, which divide rapidly and repeatedly, are likely to be missing functional telomerase c) Abnormally low levels of telomerase can lead to premature aging syndromes NOTE CHECK Join.nearpod.com pin = Telomerase is a drug target in both cancer and aging membrane Nuclear What is the mechanism of DNA replication, and what proteins are involved? Roles of DNA polymerases in E. coli and mammalian cells Polymerase accessory proteins RFC PCNA What other jobs need to be done during replication? Double Strands Must Be (Kept) Separated _____________________ denatures the double helix into two single strands _____________________ coat single strands to keep them apart Tension must be relieved ahead of the replication fork _______________________ relieve the tension that builds up in front of a replication fork. Model of the E. coli replication fork Note check In a process called Polymerase Chain Reaction (PCR), DNA replication can be mimicked in a tube to copy a specific region of DNA. The only enzyme included in the tube is DNA polymerase; various tricks or reagents are used to avoid the need for the other proteins. Heat is used to separate the two strands of DNA; which protein’s job is heat doing? a) Ligase b) Helicase c) Primase NOTE CHECK d) Telomerase Join.nearpod.com pin = DNA polymerase is self-correcting due to _______________ ability Mini Case Study: AZT treatment for HIV HIV is an example of a retrovirus which uses reverse transcriptase to make a DNA copy of its RNA genome, which is then inserted into the genome of its host. AZT, a key HIV treatment drug that targets reverse transcriptase, is shown in comparison to the thymidine nucleotide. AZT is what’s known as a “thymidine analog,” or look-a-like. Which is likely true? a) AZT is more likely to be recognized by an RNA polymerase than a DNA polymerase b) If AZT is incorporated into a growing strand of DNA, it will prevent additional bases from being added afterwards c) If AZT is incorporated into a growing strand of DNA, Join.nearpod.com the next base added will automatically be an adenine pin = Mini Case Study: AZT treatment for HIV HIV’s reverse transcriptase has a much higher affinity for AZT than human DNA polymerase. But even when our cellular DNA polymerase does bind to AZT, it is not frequently incorporated into replicated human DNA. Why do you think this might be? a) Human DNA polymerase incorporates uracil, not thymidine b) Human cells rarely need to replicate DNA c) The proofreading ability of DNA polymerase recognizes incorrect base pairing and removes AZT d) There are additional DNA repair mechanisms in human cells that would remove AZT to continue replication Join.nearpod.com pin =