Microbiology 251: Chapter 5, Lectures 1 and 2 Notes
Microbiology 251: Chapter 5, Lectures 1 and 2 Notes BMB 251
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This 3 page Class Notes was uploaded by Julianna Sickafus on Sunday September 18, 2016. The Class Notes belongs to BMB 251 at Pennsylvania State University taught by SCOTT LINDNER in Fall 2016. Since its upload, it has received 52 views. For similar materials see Molecular and Cell Biology I in Microbiology at Pennsylvania State University.
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Date Created: 09/18/16
Microbiology 251 Chapter 5: DNA Replication, Repair, and Recombination Lectures 1,2 9/21/16-9/25/16 Lecture 1: DNA Synthesis and Machinery -riboses are attached by a phosphate that bridges 3’ and 5’ carbons -nucleotides are always added from 5’ to 3’ -can only be added to the 3’ end -DNA polymerase builds in a 5’ to 3’ direction -requires primer to begin -occasionally adds wrong nucleotide -1 mistake per 100,000 nucleotides -doesn’t correct incorrect nucleotide itself -marks incorrect base -another enzyme is responsible for the correction -DNA primase -makes short RNA primers that provide the 3’ OH for DNA polymerase to start synthesis -Okazaki fragment: RNA primer + nucleotides -connecting fragments involves degrading RNA primer and ligating the nick -RNase H degrades RNA in the heteroduplexed region -DNA polymerase fills gap (from degraded primers) -DNA ligase ligates the nick -ATP used -AMP released -DNA helicase -can mechanically separate 2 DNA strands -uses ATP -Single Stranded DNA Binding Protein (SSB) -prevents hairpins from forming in single stranded DNA -even though bases are on same strand, they can still bind with the correct opposite base if not prohibited from doing so -sliding clamp protein -regulates association of DNA polymerases with template -placed on DNA by clamp loader -ATP binding opens sliding clamp so DNA can engage -ADP + P released -when DNA polymerase unwinds DNA at the fork, the DNA ahead of the fork overwinds -produces stress in DNA -topoisomerases (topo’s) -untangles DNA by introducing transient breaks in DNA -topo 1 -1 strand -cuts 1 spot -forms covalent bond with DNA backbone -no ATP needed -topo 2 -2 strand -needs ATP -makes reversible covalent attachment to opposite DNA strands -both topo’s hold onto DNA so it doesn’t float away and is lost -strand directed mismatch repair: -MutS - finds mismatches -MutL -detects nick in new DNA strand -able to tell which strand is newly synthesized -can accurately remove and replace Lecture 2: DNA Replication in Cells -in E. coli: -replication origin is AT rich and associates with initiator proteins -recruit DNA helicase -AT forms 2 H bonds -easier to pull apart than 3 between C and G -eukaryotic chromosomes have more than 1 origin of replication -if eukaryotes had 1, chromosome replication would take a month -some origins are more frequently used than others -some origins are weaker and are used as back ups -each origin of replication has 2 forks with each having a leading and lagging strand -forks merge to form whole chromosome -firing of replication origins regulated by cell cycle -origin recognition complex (ORC) serves as initiator proteins in eukaryotes -bonded to DNA at all times -only activated by kinase to transition into S-phase -recruits a DNA helicase -also activated by kinase -once origin has fired, ORC remains inactive until cell divides -cell doubles amount of histones produced -produced before and during S-phase -chromatin forms right after passage of DNA polymerase to make a fully formed chromosome -replication of chromosomes involves: -dissociating histones ahead of fork -distributing those histone dimers behind fork -telomerase is an RNA template-directed DNA polymerase -reverse transcription -RNA -> DNA -used so lagging strand can be completed
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