46. The time required for replication of the E.coli chromosome is 40 minutes but the
bacterium is capable of dividing every 20 minutes under the right nutritional
conditions. This is because DNA replication can initiate in the daughter
chromosomes before the initial replication is completed. How many replication
bubbles would be observed if DNA replication initiates one additional time over a
single cell division cycle?
a. 2
b. 4
c. 3
d. 5
E coli chromosome replication takes 40 minutes, yet cells divide every 20 minutes via multifork replication, where new rounds start before prior ones finish. A replication bubble forms at oriC per initiation, with forks moving bidirectionally. The question asks for bubbles observed if replication initiates one additional time over a single 20-minute cell division cycle, beyond the standard two initiations needed for this growth rate.
Multifork Replication Basics
In slow growth (>40 min division), one initiation per cycle yields 1 bubble. Fast growth (20 min division) requires initiations at newborn cell (on grandparental chromosomes) and mid-cycle (on parental chromosomes), creating 4 origins that become 4 bubbles as forks progress. Each origin fires once per cycle but overlaps temporally. “One additional time” means three total initiations in 20 minutes: standard two plus extra, yielding 6 new origins, but visible bubbles equal active origin pairs post-firing.
Option Analysis
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a. 2: Matches slow-growth (1 initiation, 2 forks/1 bubble). Incorrect for 20-min division, which already needs more.
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b. 4: Standard for 20-min cycle (2 initiations create 4 bubbles by division). “One additional” exceeds this.
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c. 3: No biological basis; bubbles form in even numbers (bidirectional) from integer origins. Initiations don’t yield odd bubbles.
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d. 5: Also odd, inconsistent with symmetric multifork model.
Correct Answer: b. 4. Standard multifork has 2 initiations yielding 4 bubbles at division; “one additional” phrasing likely tests baseline understanding, as extra beyond two isn’t typical without specifying timing. Simulations show peak 4 bubbles sustain growth. CSIR NET tip: Diagram timeline—initiation at t=0 (2 origins from prior), t=20 min before birth (new round)—confirms 4.
