Chapter 14- Part 2 DNA Replication Models Flashcards
What is the Watson-Crick model of DNA replication?
Semi-conservative model of DNA replication.
H Bonds break between nitrogenous bases to allow strand separation.
Each DNA strand is a template for the synthesis of a new strand.
Template (parental strand) determines the sequence of bases in the new strand (daughter) = complementary base pairing rules.
Briefly describe the semi-conservative model of replication.
H Bonds break between nitrogenous bases to allow strand separation.
Each DNA strand is a template for the synthesis of a new strand.
Template (parental strand) determines the sequence of bases in the new strand (daughter) = complementary base pairing rules.
What is the replication fork?
Like a zipper it breaks the H-bonds between nitrogenous bases to create a template.
What are the other two models of replication, in addition to the semi-conservative model?
COnservative replication model- The two strands of the original molecule serve as templates for the two strands of a
new DNA molecule; then, they rewind into an all “old” molecule.
Dispersive Replication model- Neither parental strand is conserved, and both chains of each replicated molecule
contain old and new segments.
What is the conservative model of replication?
The two strands of the original molecule serve as templates for the two strands of new DNA moleclel then, they rewind into an all “old” molecule.
What is the Dispersive replication model?
Neither parental strand is conserved and both chains of each replicated molecule contains old and new segments. They are broken off and combined together so it may require a lot of energy and coordination from the cell,
In the Meelson and Stahl experiment, what were bacterial cells grown in?
Heavy nitrogen 15N, thus DNA incorporated 15 N
Where was 15 N used in DNA?
In nitrogenous bases so 15N was incorporated 15 N after multiple rounds of replication.
What did meselson and Stahl do after DNA incorporated n15 in its nitrogenous bases?
They switched cells to media containing lighter 14N.
What was done after switching the cells to n14 from n15 in the MS experiment?
Small samples of DNA were extracted at
various time intervals.
0 min 0 Rounds of replication
20 mins. 1 round or replication
40 mins 2 rounds of explication
What were the three intervals where MS took DNA smples in n14 medium?
0 min 0 rounds
20 mins 1 round
40 mins 2 rounds
What was the DNA structure when the first DNA sample was taken in MS?
Takeen IMMEDIATLEY after switching E.coli from N15 to N14 at 0 mins.
We still had both strands of DNA at N15.
What was the DNA structure when the second. DNA sample was taken?
Taken after 20 mins, after 1st round o d replication.
DNA molecule had one strand of N15(parent) and one strand of N14(daughter).
What was the DNA structure when the third. DNA sample was taken?
Taken 40 mins after.
2 DNA molecules(bands)- One had N15(OG parent) /N14(new daughter)
And one lighter N14 (OG daughter) and another N14 new daughter)
What did they do with each sample in MS experimentally?
They extracted the DNA from bacterial cells and centrifuged it to create a Density Gradient.
What is a density gradient?
Shows heavy molecules on bottom and light molecules on top.
Created by Celsium Chloride.
How did the centrifuge the extracted DNA?
Once creating a density gradient using Celsium Chloride, they centrifuged the molecule for 48 hours.
At the end of centrigugation, the DNA positioned it self on the gradient according to its own density.
At 0mins and 0 rounds of replication in MS, what are the # of bands, location, and Compostion of all replication models?
In Semiconservative model: 15/15 bottom of time 1 band
Conservative: 15/15 bottom of tube, 1 band
Dispersive: 15/15, bottom of tube, 1 band
At 20 mins and 1 round of replication in MS, what are rhe # of, location and compostion bands in all replication models?
Semi: One band 15/14 center of the tube
Conservative: Two bands 14/14 top and 15/15 bottom [DOES NOT MATCH EXPERIMENTAAL RESULTS]
Dispersive: One band 15/14 center of tube
Because both Semi and DIspersive contain strands of both N15 qand N14 parental and Daughter DNA strands.
What model of replication is eliminated after one round of replication in MS?
Conservative: two bands 14/14 top and 15/15 bottom [DOES NOT MATCH EXPERIMENTAL RESULTS]
therefore eliminated
They are fixed and replicated
At 40 Mins and 2 round of replication what are the number of, location of, composition of the bands in replication models?
Semi: Two bands 14/14 top
14/15 bottom
Dis: one band more of n14 closer to top
Why only one strand in DISpersive model in 2nd replication in MS?
Only one strand because still have hybrid density but more n14, little bit more closer to the top of the tube [does not match experiment)
How does Semiconservative match at 2nd round of replication?
Two bands:
OG N15 + New Daughter N14 Middle
OG Daughter N14 + New Daughter N14 TOP
What was the conclusion of MS experiment?
DNA Replication is Semiconservative
DNA polymerases assemble complementary polynucleotide chains from what?
Individual deoxyribosenuecleotides: dATP, dGTP, dCTP, dTTP
What are the four different deoxyribosenucelotide triphosphates one for each base?
DATP, DGTP, dCTP, dTTP
To what end does DNA polymerase adds a nucleotides to?
The exploded 3’ OH end of an existing nucleotide chain
DNA polymerases only assemble nucleotide chains in what direction?
5’ to 3’ direction
What is exposed at the oldest and newest end of ta new DNA strand?
oldest- 5’ triphosphate
newest- 3’ OH hydroxyl
Why is the template strand “read” in the 3’ to 5’ direction?
Because DNA strands run anti parallel to each other
What should the incoming nucleotide be when adding onto the 3’ OH end when the complementary pair is Thymine? What molecule checks this is correct?
DATP. DNA polymerase.
What molecule checks if the incoming nucleotide is correct or not according to complementary base pairing?
DNA polymerase
Explain what happens to the triphosphates in deoxyribosenuceleotides?
One of the phosphates is forms a bond with the 3’ OH group of the previous nucleotide and for the other 2 hydrolosis provides energy for DNA chain elengoation reaction.
What 3 things does DNA replication require?
Something to copy- parental DNA molecule(Template)
Something to do the copying- enzymes(DNA polymerases)
Building blocks to make a copy- Nucleotide Triphosphates- dATP, dTTP, dGTP,dCTP
DNA polymerase can only add a nucleotide to what end?
only to the 3’ end with the exposed OH group of an existing nucleotide chain
DNA polymerase adds whar?
Adds a single nucleotide derived from the nucleotide triphosphates
Accordance to the HAND model of DNA polymerase+ Sliding clamp what is the palm represent?
Polymeraziation reaction active site
Why was the sliding clamp created?
Because DNA is so long and needs to be meticously copied in 9 hours we need to maximize effiency.
What does the sliding clamp do?
Attached to the rear DNA polymerase, making sure DNA polymerase does not fall off the template- increasing rate of DNA synthesis.
What is the DNA sliding clamp?
protein than encircles DNA and attaches to the rear of DNA polymerase (relative to forward movement)- tethers DNA polymerase to the template strand.
Research question: How is the sliding clamp loaded and unloaded onto replicating DNA in humans?
The efficnet loading and unloading of sliding clamps by CLAMP LOADERS once DNA poluymerase has disassoicated with DNA is probably important for the overall efficiency of DNA replication.
How does a new strand begin?
begins with a short stretch of nucleotides: RNA primer synthesized by primate
what is RNA primer?
A short stretch of RNA nucleotides
What synthesized RNA primer?
The enzyme primase
What happens after primase beings a new strand?
IT leaves, DNA polymerase takes over extending the RNA primer with DNA nucleotides as it synthesized the new DNA chain RNA primers are later replaced with DNA later in replication.
Which molecule unwinds(separate/ H-bonds broken) DNA strands producing a Y-shaped fork?
DNA helicase- uses ATP hydrolysis to finish the job
Once separated, what keeps single stranded DNA segments from pairing?
SSBS- single stranded binding proteins
What does to toposiomerse do?
When separating DNA strands, torsional strain is possible with twisting- preventing DNA polymerase from performing,.
Topoisomerase cuts the DNA ahaed of the point where unwinding is taking place- relieving torsional strain
Then rejoins DNA
Mainly circular bacterial chromosome but also in long linear DNA.
Helicase does what?
Using ATP hydrolysis binding to the ori and unwinding the DNA strand to produce the replication fork
Two main criteria of leading strand template
- Single primer
- Synthesized 5’ to 3’ direction towards the fork or direction of DNA unwinding
How does replication occur in lagging strand?
DNA polymerase happens in the opposite direction of unwinding.
Discontinuous synthesis with a number of primers to keep adding nucleotides from 5’ to 3’ end.
Criteria of lagging strands>
- Multiple primers
- Synthesized 5’ to 3’ away from the fork or opposite to the direction of unwinding
Why does the leading strand have continuous synthesis?
Because it only has one primer and does not break when replicating from 5’ to 3’ end reading Template from 3’ to 5’
In the lagging strand, the new DNA strand is synthesized ?
in the direction opposite of DNA unwinding require multiple primers on the laggin strand template
The discountnous synthesis of DNA in the lagging strand template creates fragments of DNA known as?
Okazaki fragments which are 100 to 200 base pairs long.
