41. In the diagram below, the dotted line marks the point of initiation of bidirectional replication
c,
A. On the right side of the dotted line, leading strand synthesis occurs using the upper strand as the template.
B. On the right side of the dotted line, leading strand synthesis occurs using the lower strand as the template.
C. A ligase deficient (lig–) mutant would affect replication of the upper strand on the left side of the dotted line.
D. A ligase deficient (lig–) mutant would affect replication of the lower strand on the left side of the dotted line.
Which one of the following options represents the combinations of the correct statements?
(1) A and D (2) B and C
(3) B and D (4) A and C
Analysis of the Statements
A. On the right side of the dotted line, leading strand synthesis occurs using the upper strand as the template.
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In bidirectional replication, each replication fork synthesizes a leading strand continuously using the strand oriented 3′→5′ as template.
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On the right side of the origin, the replication fork moves rightward, so the leading strand is synthesized continuously on the strand whose polarity is 3′→5′ in that direction.
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If the upper strand on the right side runs 3′→5′, then A is correct.
B. On the right side of the dotted line, leading strand synthesis occurs using the lower strand as the template.
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This contradicts A; only one strand can serve as the leading strand template at a given fork.
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If A is correct, B is incorrect.
C. A ligase deficient (lig-) mutant would affect replication of the upper strand on the left side of the dotted line.
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Ligase seals nicks between Okazaki fragments on the lagging strand.
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On the left side, the lagging strand template is the strand opposite to the leading strand template.
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If leading strand synthesis on the left uses the lower strand as template, then lagging strand synthesis occurs on the upper strand.
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Therefore, ligase deficiency would affect the newly synthesized upper strand on the left side, making C correct.
D. A ligase deficient (lig-) mutant would affect replication of the lower strand on the left side of the dotted line.
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This is incorrect because the lower strand on the left side serves as the leading strand template and is synthesized continuously without the need for ligase to join fragments.
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So, D is incorrect.
Correct Combination
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A and C are correct.
Final Answer
(4) A and C
Summary Table
| Statement | Correctness | Explanation |
|---|---|---|
| A | Correct | Leading strand on right side uses upper strand as template if its polarity matches 3′→5′ |
| B | Incorrect | Contradicts A; only one strand serves as leading strand template per fork |
| C | Correct | Ligase deficiency affects lagging strand synthesis on upper strand on left side |
| D | Incorrect | Lower strand on left side is leading strand; ligase not required for continuous synthesis |
Conclusion
In bidirectional DNA replication, the leading strand synthesis on the right side uses the upper strand as template, while ligase deficiency affects lagging strand synthesis on the upper strand on the left side. Therefore, statements A and C correctly describe the replication dynamics and effects of ligase deficiency.
15 Comments
Manisha choudhary
July 29, 2025Right side lower strand kyo nhi h sir leading strand
Jab dotted line s replication start ho rha h too
Priya Khandal
July 29, 2025Sir yw nahi aa raha ga
Priti Khandal
July 29, 2025Sir ye nahi aa raha hai
Pratibha
July 30, 2025Correct option is option (4)
the leading strand is synthesized continuously on the strand whose polarity is 3′→5′ in that direction. So the leading strand synthesis on the right side uses the upper strand as template,
while ligase deficiency affects lagging strand synthesis on the upper strand on the left side.
Swati Choudhary
July 30, 2025A AND C are correct
Divya rani
July 30, 2025A and C is right answers
Khushi Vaishnav
July 30, 2025A and C are correct statement
shruti sharma
July 30, 2025READ IT SIR
Ankita pareek
July 31, 2025Here replication starts from dotted line and for leading strand synthesis polarity should be 3′-5′ and this polarity is in upper right side and lower left side that’s why leading strand is form in upper right side and lower left side…..and mutant ligase affects lagging strand on the left upper side because at lower left side leading strand is formed
Aafreen
July 31, 2025Ans-4. right side of the dotted line the upper strand 5′ to 3′ in the direction of replication serves as the template for leading strand synthesis, while ligase deficiency affects lagging strand synthesis on the upper strand on the left side
Soniya Shekhawat
July 31, 2025According to dotted lines marks the point of initiation of bidirectional replication toh right side ka jo lower stand hai woh as a temple in work Karega for the leading stand ke liye but option me B and C ko correct bataya h but kaise hoga sir yeh plz sir this question is little bit confusing so this question is explain on the class🙏
Kajal
July 31, 2025A and C is correct
Mahima Sharma
August 2, 2025Nahi aa rha sir
Varsha Tatla
August 3, 2025A,c will be correct answer
Aman Choudhary
August 28, 2025A and C is correct