Why eukaryotic chromosome replication finishes in a relatively short time: choose the best reason
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AEukaryotes have more DNA to replicate, so it must happen faster
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BEukaryotic polymerases are about 1000× faster than prokaryotic polymerases
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CEach eukaryotic chromosome contains multiple replicons (many origins of replication)
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DEukaryotic DNA is always single-stranded and thus quicker to copy
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EEukaryotic nucleosomes eliminate the need for RNA primers
Answer
Correct Answer: Each eukaryotic chromosome contains multiple replicons (many origins of replication)
Explanation
Introduction / Context:Eukaryotic genomes are large, yet S phase completes in hours. This is made possible not by super-fast polymerases but by orchestrating replication initiation at many sites simultaneously. Understanding “replicons” and “origins” is key.
Given Data / Assumptions:
- Eukaryotic chromosomes are linear and much larger than bacterial chromosomes.
- Replication speed per fork is limited by enzyme kinetics and chromatin context.
- Cells can fire many replication origins in parallel during S phase.
Concept / Approach:A “replicon” is the DNA segment replicated from a single origin. Eukaryotic chromosomes have thousands of licensed origins; many fire to create numerous bidirectional forks that replicate different regions concurrently, dramatically shortening total time. Polymerase rates (nucleotides per second) are not orders of magnitude faster than bacteria; parallelism is the solution.
Step-by-Step Solution:Identify constraints: polymerase speed is similar order of magnitude across life.Recognize strategy: increase fork number via multiple origins.Conclude: many replicons operating simultaneously compress overall replication time.
Verification / Alternative check:DNA fiber assays show multiple initiation events along eukaryotic DNA; origin licensing factors (for example, ORC, Cdc6, MCM) support this mechanism.
Why Other Options Are Wrong:
- More DNA (A) is a challenge, not a reason replication is fast.
- 1000× faster enzymes (B) is false; eukaryotic polymerases are not that much faster.
- Always single-stranded DNA (D) is incorrect; genomic DNA is double-stranded.
- Nucleosomes (E) actually impose structural hurdles; primers are still required.
Common Pitfalls:Confusing polymerase processivity with fork number; speedups primarily come from simultaneous initiation, not extreme catalytic acceleration.
Final Answer:Each eukaryotic chromosome contains multiple replicons (many origins of replication)