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Question:

A genetic researcher is comparing the DNA replication process of prokaryotic and eukaryotic cells.  In an experiment, Escherichia coli and human cells are cultured in separate media containing tagged nucleotides and their rates of DNA replication are determined.  Although the eukaryotic genome is significantly larger and more complex than that of the prokaryote, eukaryotic DNA replication still occurs in a timely manner.  Which of the following features of eukaryotic replication best explains this observation?

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Explanation:

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The key steps of DNA replication are similar in eukaryotes and prokaryotes (eg, Escherichia coli) and are as follows:

  1. Unwinding of double-stranded DNA by helicase to produce single-stranded DNA
  2. Formation of a replication fork
  3. Synthesis of RNA primers by the enzyme primase
  4. Synthesis and concurrent proofreading of daughter DNA strands by DNA polymerases
  5. Removal and replacement of RNA primers with DNA
  6. Ligation of Okazaki fragments on lagging strands by the enzyme ligase

Despite the similarities of prokaryotic and eukaryotic DNA replication, there are important differences between these processes.  Prokaryotes possess 3 major DNA polymerases (I, II, and III), whereas eukaryotes have 5 major DNA polymerases (α, β, γ, δ, and ε).  The eukaryotic genome is also much larger and more complex than the prokaryotic genome, which can be partly explained by the abundance of noncoding DNA regions (introns) located between coding regions (exons).  In addition, prokaryotes typically have circular DNA with a single origin of replication, whereas eukaryotes have linear DNA with multiple origins of replication.  This feature allows the eukaryotic genome to be copied in a quick and effective manner despite its large size.

(Choice A)  Eukaryotic and prokaryotic DNA polymerases synthesize daughter strand DNA in the 5' to 3' direction.  The leading strand is formed continuously, whereas the lagging strand is formed discontinuously, creating Okazaki fragments.

(Choice B)  DNA unwinding is an energy-dependent process performed by the enzyme helicase in both prokaryotes and eukaryotes.

(Choice D)  Proofreading of daughter strands during DNA replication is necessary to preserve the genetic code and prevent potentially lethal mutations.  All 3 prokaryotic DNA polymerases and most eukaryotic DNA polymerases (eg, γ, δ, ε) possess 3' to 5' exonuclease ("proofreading") activity.

(Choice E)  Prokaryotic and eukaryotic DNA polymerases require an RNA primer before they can initiate synthesis of complementary DNA on a single-stranded template.  Primase (prokaryotes) and DNA polymerase α (eukaryotes) are the enzymes responsible for synthesizing this primer.

Educational objective:
Multiple origins of replication make eukaryotic DNA replication quick and effective despite the large size and complexity of the genome compared to that of prokaryotic organisms.