Module 6.4 Flashcards
Where does synthesis in the DNA fork begin?
- at the replication fork
- a specific nucleotides sequence begins replication: called the origin of replication
- both DNA strands are read simultaneously
- synthesis occurs in a replication bubble
What does it mean for replication to be semi discontinuous?
- leading and lagging strands
- leggings strands: discontinuous, moves in opposite direction of the fork (forms Okazaki fragments)
what are the modes of replication and who uses them?
- circular genomes: theta replication (bacteria eg; E.Coli) and rolling circle replication (viruses)
- linear genomes: linear replication (eukaryotes)
Describe theta replication?
- in E.Coli/bacteria
- single replicon = entire chromosome
- bi-directional erpliatoin: has two replication forks which are semi-discontinuous (leading and lagging)
- product : two circular DNA molecules!
What is rolling circle replication?
- no replication bubble
- uncoupling of replication of the two DNA strands
- replication is continuous!
Describe linear DNA replication?
- multiple replicons, replication bubbles, and origins of replication
- replication is semi discontinuous and bi directional (two forks within one bubble)
What are the four stages of replication?
- initiation, unwinding, elongation, termination
Describe initiation in prokaryotes
- initiator proteins bind to the origin of replication
-a short section unwinds and proteins bind to the ssDNA - single stranded binding proteins keep strands separated
- helicase binds to lagging strand: breaks hydrogen bonds down
Describe unwinding in prokaryotes
- helicase binds to the lagging strand and separates the strands by breaking down hydrogen bonds
- DNA gyrase travels ahead of the fork to alleviate super coiling
Where are DNA gyrase and helicase useful?
- in the unwinding section of DNA replication in prokaryotes
- helicase separates the strands by breaking down hydrogen bonds
- gyrase alleviates super coiling (travels ahead of the fork)
Describe chain elongation in prokaryotes?
- a short section of RNA nucleotides is synthesized by primase
- RNA primer provides a free 3’ OH end for the DNA to use
- RNA primer is later removed and replaced with DNA nucleotides
Describe E.Coli DNA polymerase in prokaryotes
- E.Coli has 5 DNA polymerase
- each with 5’ to 3’ polymerase activity
- Pol 1 - Pol V
- polymerase III is the principal replication enzyme
- Pol I removes and replaces RNA primers with DNA
- some polymerase have exonuclease activity to remove nucleotides that don’t match template strand
Describe replacement of RNA primers with DNA?
- DNA pol I uses exonlucease activity to remove RNA primers
- uses polymerase activity to replace DNA nucletoides
- DNA ligase heals the nicks in the sugar phosphate backbone
Describe termination in prokaryotes
- termination occurs when two replication forks meet or when special sequences are met
Describe initiation in eukaryotes
- origins of replication are activated in clusters of 30-80: known as replication units
- must be licensed by licensing factors
- origin is activated and replication can begin
- once replication is initiated, the origin is deactivated
What are the requirements for an origin of replication to turn on in eukaryotes?
- must be licensed by replication factors
- once origin of replication is activated and replication can begin
- one activated it is then deactivated
Describe the rate of DNA synthesis in prokaryotes and eukaryotes. Which is faster?
- genome of E.Coli is >4.6 million with one origin of replication
- in humans, genome >3 billion with >10,000 origins
- S phase in humans would last more than a month with one origin!! multiple origins of replication means faster replication and going slower means it is less prone to make mistakes
- (prokaryotic replication is faster than eukaryotic)
Why must nucleosomes be disassembled and reassembled?
- chromatin packaged \, must be disassembled to product more histones and then reassembled
What are telomeres? what is there problem?
- telomeres are found on the end of linear chromosomes
- short G rich repeat sequence
- stabilizes end of chromosomes
- removal of primes leaves a gap” fixed by telomerase!!
What is telomerase? What problem does it fix?
- specialized reverse transcriptase
- extends end of parental DNA by RNA templated DNA synthesis
= responsible for replication
