DNA replication and Repair 1C part 2 Flashcards
DNA replication in bacterial chromosomes (3)
initiation: unwinding and separation of the two template DNA at the organ of replication (forms a replication bubble)
elongation: simultaneous synthesis of the two new DNA strands from the template strands by DNA polymerase
Termination: DNA replication stops when reaches a termination site or at the end of the chromosome (linear)
is there a rate difference in leading and lagging?
yes lagging strand is slower and this is a problem as it has a prolonged time for DNA strand to be exposed. This is fixed by having dna loop to couple the leading and lagging strands
number of DNA replication site of eukaryotic chromosomes
Have to deal with much larger genomes and linear chromosomes so there are multiple organs so replication can be completed on time.
PCR steps
- denature at 95 degree to break H bond and separate two strands
- Drop temp to 50-65 degree to allow DNA primer to bind, usually this is a RNA.
- raise temp to around 72 degree so that Taq polymerase (taken from cells that are in hot springs) to extend the primer and make the new daughter strand. The optimal temperature for DNA polymerase to act. DNA polymerase extends the primers, adding nucleotides onto the primer in a sequential manner, using the target DNA as a template.
what is PCR used to do
amplify dna segments
End replication problem (2)
Occur only in Eukaryotes.
The chromosomes of eukaryotes are linear (rod-shaped), meaning that they have ends. These ends pose a problem for DNA replication. The DNA at the very end of the chromosome cannot be fully copied in each round of replication, resulting in a slow, gradual shortening of the chromosome.
n most cases, the primers of the Okazaki fragments can be easily replaced with DNA and the fragments connected to form an unbroken strand. When the replication fork reaches the end of the chromosome, however, there is (in many species, including humans) a short stretch of DNA that does not get covered by an Okazaki fragment—essentially, there’s no way to get the fragment started because the primer would fall beyond the chromosome end
telomere (4)
in eukaryotes only
non coding single stranded DNA added to the 3’ end of the chromosome by telemerase.
usually repeats of 5-8 G and T
Human have 10000 base pair long
when telemerase region is gone
cell stops dividing
telomerase (2)
an enzyme that restores shortened telomeres
not usually active in most somatic cells (cells of the body), but it’s active in germ cells (the cells that make sperm and eggs) and some adult stem cells. it is usually shut off early after giving length to new cells
cancer and telomerase
many cancers acquire mutations that activates the telomerase gene to negate the limitations of rapid cell division
defective genes
failure to maintain high fidelity of DNA replication
DNA pol 3 proofreading (2)
DNA pol 3 detects mistake and uses 3’-5’ exonuclease activity to remove the most recent mist ached nucleotides
DNA pol 3 replaces and correct nucleotide and resumes synthesis of the new DNA strand
DNA mismatch repair MMR (7)
covers for reposition errors not corrected by proofreading
Recognition of mismatch damage by DNA binding MutS and Mut L
(Mut L recognizes the methylated strand and Mut S first identifies the mismatch
MutH endonuclease daughter strand several nucleotides away from mismatch
Mut H needs to bind with Mut L for it to be activated. This causes the DNA to bend. It causes an incision.
Exo 1 5’-3’ exonuclease excises region of daughter strand surrounding the mismatch (cleaves all terminal phosphodiester bond) by the incision
DNA pol 3 fills the gap and repairs the mismatch (5’-3’ daughter)
The nick left after gap is sealed by DNA ligase
