Q1. Source: CSIR NET June 2017
Two experiments were performed. in the first one, okazaki fragments were prepared from a replicating cell of E.coli grown in the presence of 32P. In the other, the two strands of E.coli chromosome were separated into a H strand and L strand, immobilized onto a nitrocellulose membrane and hybridized with the okazaki fragments prepared in the first experiment. which one of the following options correctly describes the observation?
A. Okazaki fragments will hybridize only to H strand
B. Okazaki fragments will hybridize only to L strand
C. Okazaki fragments will hybridize to both H and L strand
D. Because the H and L strnads have been prepared from different cultures of Ecoli, the okazaki fragments will hybridize to neither
Okazaki fragments are short DNA segments synthesized on the lagging strand during DNA replication. These fragments are complementary to their template strands.
In E. coli, DNA replication is Semiconservative and Bidirectional.
This means:
- Both parental strands act as templates
- At each replication fork, one strand is synthesized continuously (leading strand) and the other discontinuously (lagging strand, forming Okazaki fragments)
- Due to bidirectional replication, each parental strand serves as a lagging strand template in different regions of the genome
As a result, Okazaki fragments originate from both strands of DNA
When these labeled fragments are used as probes, they will hybridize with complementary sequences present on both H and L strands.
Statement Analysis:
Option A: Incorrect
This assumes Okazaki fragments are derived from only one strand.
However, due to bidirectional replication, both strands contribute to Okazaki fragment formation in different regions.
Option B: Incorrect
Same reasoning as Option A.
Both strands act as lagging templates at different replication forks, so fragments are not limited to one strand.
Option D: Incorrect
Hybridization depends on sequence complementarity, not on whether DNA samples come from different cultures. As long as the sequences are complementary, binding will occur.
Because DNA replication is bidirectional, Okazaki fragments arise from both strands allowing them to hybridize with both H and L DNA.
Correct Answer: C – Okazaki fragments will hybridize to both H and L strands
Q2. Source: CSIR NET June 2019
On sequence analysis of a double stranded DNA, the results showed the content of cytosine was 20%. What is the amount of A and T put together?
A. 20%
B. 30%
C. 50%
D. 60%
According to Chargaff’s rules for double-stranded DNA:
%A = %T
%G = %C
Total = 100%
Cytosine= 20%, so Guanine will be = 20%
So, % (A+T)=100-40= 60%
Correct answer: D. 60%
In double-stranded DNA, once you know one base percentage, you can calculate all others using base-pairing rules.