02/04/2025 DNA Replication Start Flashcards
what is the conservative model
both parental strands stay together after DNA replication
what is the semi conservative model
the double-stranded DNA contains one parent + one daughter
what is the dispersive model
the parent and daughter are interspersed between the strands
for each generation of replication in the dispersive model, by how much does the parent DNA (heavy) decrease by
it decreases by (1/2)^n
Will the dispersive model ever show 0 for the heavy DNA
no, there will always be some remnants
In the Meselson and Stahl Experiment, why did they grow the E.coli in the presence of N15
to make the DNA heavy
does the fact that Meselson and Stahl used an N15 isotope matter in terms of radioactivity
No, we only interested in the weight differences between the N15 and N14
Why did we add the N15 labeled E coli cells to the N14 culture medium
every newly synthesized DNA strand would have to incorporate the new N14 as there is no more N15 to use except for in the parent DNA
What was the purpose of the CsCl gradient in the Melelson and Stahl experiment
the CsCl creates a gradient with the Cl on the top and Cs on the bottom, this allowed the DNA strands to be separated by weight as the lighter DNA strands could not sink to the bottom
what was the purpose of UV light in the Meselson and Stahl Experiment
the DNA molecules would absorb the UV light and form a dark band, indicating their presence at a particular region in the CsCl gradient
for the semi conservative model of dna replication, what did the first generation show? what did the second show? third?
the first generation showed one band (100% density) of the half-heavy DNA
the second generation showed two bands, with 50% being the half heavy and the other 50% being light
the third generation showed the same two bands as the second generation, but the light had an increased concentration of 75% to 3/4
for each generation of the semi conservative model, by how much did the concentration of the half heavy band decrease
it decreased by 1/2 every time
for the semi conservative model, would we ever see a 0% concentration for the heavy strand?
no, there would always be some reminants
for the conservative model, what would the first generation show? what about the second?
the first generation would show two bands, one would be 50% concentration in heavy and the other would be 50% light
the second generation would show two bands with the light having a higher concentration than the heavy
for the dispersive model, what does the first generation show? what about the second?
the first generation would show one band at half heavy
the second generation would show one band at 1/4 heavy as the total heaviness decreases by 1/2 each time
how much does the heaviness decrease by in the dispersive model? how many bands will there be
there will always be one band and the heaviness will decrease by (1/2)^2 each time
after two generations, what two types of DNA are seen in the semi-conservative model
light and half heavy
what models would show 100% half heavy after one round of replication
semi conservative and dispersive
what models would show 50% heavy and 50% light?
conservative
what models would show a band for light and a band for 1/2 heavy after 2 rounds of replication
semi conservative
what models would show two bands, one with higher concentrations of light and a lower concentration of heavy at the second generation
the conservative
what models would show 1/4 heavy at the second generation
the dispersive
where does DNA synthesis begin
the origin of replication
how many origins of replication do bacterial chromosomes have
1
how does synthesis of DNA proceed in terms of direction
bidirectional
what ends synthesis of DNA in a bacterial chromosome
the replication forks will meet at the opposite side of the bacterial chromosome and it ends replication
what is the oriC in E.coli
the oriC is the origin of chromosomal replication
what are the three sequences of DNA in the oriC of E coli that are functionally significant?
AT Rich region
DnaA boxes
GATC methylation sites
why are the AT rich regions significant in bacterial replication?
A-T rich regions have 2 H-bonds, this makes them easier to cleave
why are the DnaA boxes significant
the DnaA sequences serve as recognition sequences for the protein DnaA. there are multiple of these proteins that will bind simultaneously
what are the requirements to open the origin of replication
AT rich region
the GATC regions need to be fully methylated by methylase and the DnaA boxes must ALL be occupied
under what conditions can the DnaA boxes be occupied
when the GATC are fully methylated
what is the first step of initiation in bacterial replication
the DnaA proteins find to the boxes with the help of other proteins to buckle the strands open to make the replication fork
what is the role of helicase
it disrupts the helical recgiosn of the dna and breaks the H-bonds
what happens ahead of the fork and why is it a problem
posiitve supercoils are formed and can cause breakage if not relieved
what relieves the positive supercoils ahead of the replication fork in bacterial replicatio
gyrase/ topoisomerase II
what are single stranded binding proteins
they are proteins that keep the DNA from collapsing on itself
what is the purpose of DNA primase
it synthesizes an RNA primer and supplies a 3’ OH group for the DNA polymerase to start synthesizing on to
