Q.46 For DNA replication, which of the following statements are true?

1. Circular chromosome of E. coli replicates through multiple origin of replication.
2. In each linear chromosome of eukaryote, DNA replication begins when replication bubbles form at single site along giant DNA molecule.
3. In each linear chromosome of eukaryote, DNA replication begins when replication bubbles form at multiple sites along giant DNA molecule.
4. In circular chromosome of E. coli, one origin of replication is present.

Choose the correct answer from the options given below:

1. A and C only
2. B and D only
3. C and D only
4. A and B only

   DNA replication MCQ often trips students on prokaryotic vs. eukaryotic differences, especially E. coli’s circular chromosome and eukaryotic linear giants. This question tests origin sites and bubble formation precisely.

   Correct Answer: C (C and D only)

   E. coli uses one origin (oriC); eukaryotes need many due to DNA length—making A and B false.

   Option A: E. coli Multiple Origins – Incorrect

   E. coli’s circular chromosome replicates from one origin (oriC) via DnaA protein, proceeding bidirectionally to terminus. Multiple origins occur only in mutants or fast-growth under special conditions, not standard replication.

   Option B: Eukaryote Single Site – Incorrect

   Eukaryotic linear chromosomes (millions of base pairs) can’t replicate from one site in time for cell division. A single origin would take too long; instead, replication starts at multiple origins (origins of replication) forming theta structures and bubbles.

   Option C: Eukaryote Multiple Sites – Correct

   In eukaryotes like humans, thousands of replication bubbles form simultaneously at ARS (autonomously replicating sequences). This parallel replication finishes S-phase efficiently, with forks merging later. ORC, Cdc6, and MCM helicase initiate at these sites.

   Option D: E. coli Single Origin – Correct

   E. coli has exactly one oriC (245 bp sequence with DnaA boxes), ensuring controlled, once-per-cycle replication. Bidirectional forks meet opposite at Ter sites, regulated by Tus protein.

   Exam Strategy Tip

   For DNA replication MCQ, remember size logic: prokaryotes (small, circular) = 1 origin; eukaryotes (huge, linear) = multiple origins. Links to previous echinoderm/steroid questions build deuterostome molecular mastery for NEET.