Flashcards in DNA Replication, Repair And Recombination 1 Deck (38):
What can result from high mutation rates in somatic cells?
What is the reaction that DNA polymerase catalyzes?
DNA(n) + dNTP -> DNA(n +1) + P2O7
In order for DNA polymerase to begin replication, what does it require?
A free 3' -OH
What is the only direction that DNA polymerase can syntesize DNA in?
5' to 3' direction
What is the difference between the leading strand and lagging strand?
The leading strand is synthesized continuously, whereas the lagging strand is synthesized in segments.
What takes place immediately after incorrect bases are added?
A 3' to 5' exonuclease clips off unpaired residues at 3' primer terminus.
In order to synthesize the lagging strand, what is initially needed?
An RNA primer. This allows DNA polymerase to make an Okazaki fragment.
Once an Okazaki fragment is made, how is the RNA primer removed?
By RNAseH. It replaces the RNA primer with DNA.
What enzyme unwinds DNA at the replication fork?
What is the function of single-stranded DNA binding proteins?
They bind tightly and cooperatively to exposed SS DNA.
They help stabilize unwound DNA and prevent the formation of hairpins. The DNA bases also remain exposed.
What is the function of a sliding clamp?
It keeps DNA polymerase on DNA when moving; it releases when double stranded DNA is encountered.
What does the assembly of a sliding clamp require?
A clamp loader. It hydrolyzes ATP as it loads the clap onto a primer-template junction.
As DNA polymerase moves along the leading strand, what other structure remains with the DNA polymerase?
Why is a clamp loader needed at the lagging strand?
So it can assemble a new clamp at the start of each Okazaki fragment.
What is mismatch repair?
Repair that removes almost all errors missed by proofreading by detecting distortion caused by mispairing.
In mismatch repair, does MutS or MutL bind to the mismatch?
MutL scans for the nick and triggers degradation of nicked strand.
When occurs when there is a mutation in the mismatch repair gene?
Cells accumulate mutations at a high rate.
What enzyme breaks a phosphodiester bond to change superhelicity and thereby relieving supercoiling?
What is the mechanism of type I topoisomerases?
They create a single strand break in DNA.
This allows the DNA on either side of the break to rotate freely relative to each other.
Resealing then rapidly occurs.
What is the mechanism of type II topoisomerase?
They make a double stranded break in the DNA.
A second strand passes through.
The break is resealed and dissociates.
What is the replication origin?
A-T rich regions where sequence attacts initiator proteins to pry open DNA.
What is involved in the initiation of DNA replication in bacteria?
Initiation proteins bind to specific sites in ORI and form a complex.
DNA helicase goes to the complex and binds to SS DNA.
Helicase unwinds DNA. A primarse makes RNA primer on the leading strand and the other proteins create 2 replication forks.
When does eukaryotic DNA replication occur?
During DNA synthesis phase (S) which lasts eight hours for mammalian cells.
Regions of gemone with ___ condensed chromatin replicate first.
What are the minimum requirements for sequence to be ORI?
Must have a binding site for ORC (origin recognition complex)
Must have an A-T rich stretch for easy unwinding
Must have binding site for proteins that help attact ORC
In the S phase, activated Cdks lead to
Dissociation of helicase loading proteins
Activation of helicase
Unwinding of DNA
Loading of DNA polymerase
When are histone proteins synthesized?
Mainly in S phase
For efficient replication, what type of proteins are needed to destabilize the DNA-histone interface?
As the replicationf ork passes through chromatin, histone octamer breaks into:
An H3 - H4 tetramer, distributed randomly to daughter duplexes.
2 H2A - H2B dimers which are released from the DNA.
Reassembly of DNA requires what type of chaperones?
Histone chaperones. THey are directed to DNA with sliding clamp called PCNA.
Daughter nucleosomes generally contain what type of histones?
Old and new histones.
What is a problem for replication on the lagging strand?
There is no place for an RNA primer.
What special sequence is repeated thousands of times at the end of each chromosomes?
An enzyme called telomerase replenishes these sequences by elongating parental strand in 5' to 3' direction using an RNA template on the enzyme.
What is the function of telomerase?
It extends the parental strand, which allow completion of replication of the lagging strand by DNA polymerase. It uses the extension as a template.
It ensures that the 3' end is longer, leaving a protruding SS end that loops back and tucks into the repeat.
What are T loops?
Structures that protect ends and distinguishes them from broken ones that need to be repaired.
True or false: each chromosome end in a given cell contains variable numbers of telomere repeats depending on age.
Why is it risky for an organism to control cell proliferation via telomeres?
Not all cells will stop dividing, and some cells may give rise to variant cells that lead to cancer.