2200 set 3 Flashcards
how can we ensure that the leading and lagging strands are getting made at the same ratee
each replisome complex carries out replication of the leading and lagging strand simultaneously. the two dna pol III core enzymes are each tethered to a larger complex by the tau protien
what are the three subunits of pol III complex
a (dnaE gene) has the polymerase activity
E (dnaQ) has 3’-5’ exonucease activity
theta (holE) stimulates the E subunits proofreading
how many protein subunits does the pol III holoenzyme contain
11 including the two pol III core polymerases
what are the tau proteins joined to
a protein complex called the clamp loader
how many protein subunits form the sliding clamp
two or three
what keeps pol III core attatched to the dna strands, preventing dissociation and enhancing the rate of productive synthesis
the sliding clamp
what ensures greater processivity of pol III
the sliding clamp
how many subunits form the sliding clamp in bacteria
two identical subunits (ring has a top half and a bottom half)
how wide is the central hole in the sliding clamp
35 A
how many subunits form the sliding clamp in eukaryotes called PCNA
Three identical subunits
what does PCNA stand for
proliferating cell nucelar antigen
what helps the sliding clamp get onto the dna
the clamp loader complex does this by attaching to the sliding clamp and temporarily opening the subunits so that dna can be put in the subunit
what is the clamp loader complex similar to
dna c
what model represents dna replication
the trombone model of dna replication
what is the problem of chromatin
in eukaryotic cells, the presence of nucleosomes presents a barrier to replisomes and dna replication and you would also need to double the amount of nucelosomes and need to make new histones
how is the problem of chromatin solved
as the replication fork passes, old nucleosomes dissasmble into H2A-H2B dimers and H3-H4tetramers and then the old and new tetramers associate with the dimers that may be a mix of old and new
how many real errors are present in replication
one in every billion nuceloties in e coli
how many mistakes does dna polymerases make
one per every hundred thousand nucleotides
what is the proofreading repair ability of dna polymerase enzymes due to
the 3’-5’ exonuclease activity
what has the 3’-5’ exonuclease activity
pol I (the third out of three activities) pol III (E subunit)
what was the original idea that caused errors in dna
because a base transiently adopted it tauomer conformation and this led to pairing with the “wrong” partner base
what actually causes erros in dna replication
- mispairing between different (non tautomeric) chemical forms of bases (with extra proton, A with G)1
- normal bases that bond inappropriately because of a slight shift in position of the nucleotide in space
what are mispairings called
wobble
why does wobble occur
because dna dobule helix is flexible and able to accomidate slightly misshaped pairings
how many rounds of synthesis does it take ot generate a heritable mutation due to wobble
it takes a second round of synthesis
where do the mi-shaped/ wobble pairs arise on which strand
could occur on either the parental or offspring strand
transition mutationos
keep chemical nature the same (AT-GC)
transversion mutations
change the chemical nature of the baes (AT-CG)
what are the steps to proofreading in dna polymerase
- newly incorporated mismatched nucelotide destabilizes the base pair at the growing end of the strand
- the mimatched end of the just synthesized dna strand is displaced into the proofreading site of the enzyme(3’-5’ exonuclease)
- several nucleotides att eh growing end are removed by the exonuclease
- the 3’ end returns to the polymerae active site and incorporation of new nucleotides resumes
what untangles and seperates the two daughter dna strands from each other
special topoisomerases
the end problem
the challenges that linear/human chromosomes present when they are being copied because without a solution, the chromosomes will continue to get shorter
what are the necesarry steps to solve the end problem in dna
use permuted or concatenated end sequences or temporary circulazation like found in some bacteriophages.
use special end sequences and a special enzyme (tomerase)
what are telomeres
repetitive sequences at the end of chromosomes that consist of hundreds of 6bp repeats, 2 to 20 kb in length overall
what is the telomere repeat in humans
TTAGGG
what is the telomere repeat in tetrahymena
TTGGGG
what synthesizes telomeres
telomerase
what is telomerase made of
a protein part and a RNA part
what does the rna part of telomerase do
acts as a template for copying to synthesize the DNA telomere sequences
what is an example of reverse transcriptase
telomerase- RNA directed DNA polymerase
who discoverdd that chromosomes are protected by telomeres and the enzyme telomerase
elizabeth blackburn
carol greider
jack szotak
what happens after the dna strand is sufficiently elongated by telomerase
dna polymerase can complete the lagging strand against the dna made by telomerase
what does TERT stand for
telomerase reverse transcriptase
what cuases mice to have developmental defects
mice that are homozygous for loss of function mutations of the TERT gene have developmental defects
when are the TERT defects observed in mice
fourth and fifth generations due to loss of telomere length with each generation. by the 4th or 5th generations, shortening is critial and apoptosis is induced
what is apoptosis
programmed cell death
why is telomere length important
its important for chromosome stability, cell longevity and reproductive sucess