Q.81 During replication of the E. coli chromosome, Okazaki fragments are produced from
(A) only one of the strands of the circular genome
(B) both the strands of the circular genome
(C) one of the strands in one generation and the other strand in the next generation
(D) both the strands of the circular genome provided that the heavy nitrogen (15N) is present in the medium
Okazaki fragments form on both strands of the E. coli circular chromosome during replication due to bidirectional replication forks from oriC. The correct answer is (B) both the strands, as each fork creates one continuous leading strand and one discontinuous lagging strand requiring ~1-2 kb Okazaki fragments.
Replication Fork Mechanics
E. coli‘s 4.6 Mb circular chromosome initiates at oriC, forming two bidirectional forks moving ~600 bp/sec in opposite directions. At each fork:
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Leading strand: Continuous 5’→3′ synthesis toward fork.
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Lagging strand: Discontinuous synthesis away from fork → Okazaki fragments (1000-2000 nt), primed by DnaG primase every ~1 kb.
Two forks = two lagging strands = Okazaki fragments on both template strands simultaneously.
Option Analysis
| Option | Statement | Correct? | Explanation |
|---|---|---|---|
| (A) | Only one strand | FALSE | Ignores bidirectional replication; each fork needs lagging strand synthesis. |
| (B) | Both strands | TRUE | Two forks → 2 leading + 2 lagging strands → Okazaki on both templates. |
| (C) | Alternating generations | FALSE | Replication is synchronous; both strands processed every cycle. No generational switch. |
| (D) | Both strands + 15N | FALSE | 15N labels DNA (Meselson-Stahl) but doesn’t trigger Okazaki formation—mechanical necessity. |
E coli Okazaki fragments strands questions test bidirectional replication understanding—crucial for CSIR NET Life Sciences, GATE Biotechnology, and microbiology/biochemical engineering students. This guide proves Okazaki fragments form on both strands of the circular chromosome due to two replication forks from oriC.
Bidirectional Replication Reality
Unlike linear eukaryotes (one leading + one lagging per fork), E. coli‘s circular chromosome fires from single oriC origin → two forks travel opposite directions → two leading strands + two lagging strands requiring Okazaki fragments simultaneously on both template strands.
Keywords: E coli Okazaki fragments strands, bidirectional replication forks, leading lagging strand synthesis, circular chromosome replication.
Why “Both Strands” Beats Other Options
Myth: "Only lagging template gets Okazaki"
Reality: Both templates serve as lagging for one fork each
Maturation: DNA Pol I removes RNA primers, fills gaps → DNA ligase seals → continuous strands.
Exam Traps Exposed
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Option A: Thinks unidirectional replication (eukaryote mistake).
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Option C: Confuses replication with cell division cycles.
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Option D: Mixes Meselson-Stahl isotope labeling with replication mechanics.
Key Numbers
| Parameter | Value |
|---|---|
| Fork speed | 600 bp/sec |
| Okazaki size | 1-2 kb |
| Primers/fork | ~1000 |
| Total fragments | ~4000/round |
Related Concepts
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DnaB helicase unwinds at forks
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SSB coats single strands
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β-clamp recycles for processivity
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Pol III holoenzyme synthesizes both strands
Practice: T4 phage (linear, still semidiscontinuous); eukaryotes (multiple origins).
Related Searches: oriC bidirectional replication, E coli replisome structure, Okazaki fragment maturation, CSIR NET DNA replication MCQs.
This E coli Okazaki fragments strands guide equips biotechnology grad students for molecular biology entrance exams and microbial genetics research. Master bidirectional replication—ace your exam!
