1) To confirm the semiconservative model of replication, it was important for Me
ID: 65236 • Letter: 1
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1) To confirm the semiconservative model of replication, it was important for Meselson and Stahl to quantify the amount of DNA in each band produced by density-gradient centrifugation. To accomplish this, they took advantage of the fact that DNA absorbs ultraviolet light, and used UV light to photograph each tube. By scanning the UV photographs with a microdensitometer, graphs like the ones below were produced. The height of each peak in the graph is directly proportional to the concentration of DNA in the corresponding band. Also, the position of each peak reflects the 14N and 15N content of the band. Suppose that the scientists analyzed the same amount of DNA (10 units) by density-gradient centrifugation after two, three, and four rounds of replication in 14N medium. What would you predict the microdensitometer graph would look like after each round? The beginning of the experiment as well as Replication Cycle 1 is given to you. Match up replication rounds “A”, “B” and “C” in the correct order with Replication Cycles 2, 3 and 4.
Match up replication rounds “A”, “B” and “C” in the correct order with Replication Cycles 2, 3 and 4.
a) replication cycle 2 =
b) replication cycle 3 =
c) replication cycle 4 =
2) DNA polymerase I has multiple functions during replication. Correctly identify what would happen if there was a mutation in each of it's functions below.
a) more errors incorporated into the newly synthesized DNA strand
b) RNA primer would not be removed
c) A removed primer would not be replaced
the answer choices to choose from are:
3' - 5' exonuclease activity
5' - 3' polymerase activity
5' - 3' exonuclease activity
Explanation / Answer
1.
a) replication cycle 2 =B
b) replication cycle 3 =A
c) replication cycle 4 =C
Explanation: Important point to remember:
The density of double stranded DNA is in the following order-
N15-N15 double stranded DNA> N15-N14 double stranded DNA> N14-N14 double stranded DNA
The DNA was labelled with N15 so both the DNA strands in double stranded DNA are N15. Therefore, the density of the DNA stranded DNA is high as given in the figure labelled as beginning of experiment.
In round 1: the N15 DNA replicated in N14 media, hence the double stranded DNA synthesised will contain one strand as N15 and other strand as N14. Therefore, the density of DNA is intermediate as depicted in figure labelled as first round of replication.
Round 2 of replication (corresponding to Fig. B)- Double stranded DNA from round 1 (N15-N14) separates so that newly synthesised daughter DNA strand is N14. Therefore, the double stranded DNA formed after round 2 of replication contains N14- N14: N15-N14 in the ratio of 1:1 and that is what we observe in Fig. B.
Round 3: (corresponding to Fig. A) - Double stranded DNA from round 2 (N15-N14, N14-N14) separates so that newly synthesised daughter DNA strand is N14. Therefore, the double stranded DNA formed after round 3 of replication contains N14- N14: N15-N14 in the ratio of 3:1 and that is what we observe in Fig. A.
Round 4: (corresponding to Fig. C) - Double stranded DNA from round 3 (N14- N14: N15-N14 in the ratio of 3:1) separates so that newly synthesised daughter DNA strand is N14. Therefore, the double stranded DNA formed after round 4 of replication contains N14- N14: N15-N14 in the ratio of 7:1 and that is what we observe in Fig. C.
2.
DNA polymerase I is concerned with proof reading activity also. If a wrong base in incorporated, its 3' ----> 5' exonuclease activity is used to remove the mismatched base and incorporate a correct one.
Removal of primer is not the function of pol I. DNA pol I cannot add nucleotides de nove, without a primer.
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