Why eukaryotic chromosome replication finishes in a relatively short time: choose the best reason

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)