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Questions and Answers
What is the primary function of DNA helicase during DNA replication?
What is the primary function of DNA helicase during DNA replication?
Eukaryotic DNA has multiple replication origins, which accelerates the rate of replication.
Eukaryotic DNA has multiple replication origins, which accelerates the rate of replication.
True (A)
What enzyme relieves tension caused by the unwinding of parent DNA?
What enzyme relieves tension caused by the unwinding of parent DNA?
Topoisomerase
New DNA is made at a rate of about 50 base pairs per ________ at each replication fork in Eukaryotes.
New DNA is made at a rate of about 50 base pairs per ________ at each replication fork in Eukaryotes.
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Match the following enzymes with their functions:
Match the following enzymes with their functions:
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Which enzyme is responsible for filling gaps between Okazaki fragments on the lagging strand?
Which enzyme is responsible for filling gaps between Okazaki fragments on the lagging strand?
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The lagging strand is copied in the direction towards the replication fork.
The lagging strand is copied in the direction towards the replication fork.
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What is the function of DNA ligase in DNA replication?
What is the function of DNA ligase in DNA replication?
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Telomeres are repetitive sequences of DNA that help slow down the loss of important information in the _______ DNA.
Telomeres are repetitive sequences of DNA that help slow down the loss of important information in the _______ DNA.
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Match each enzyme with its primary function:
Match each enzyme with its primary function:
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Which of the following best describes an Okazaki fragment?
Which of the following best describes an Okazaki fragment?
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Prokaryotic DNA replication uses multiple origins of replication.
Prokaryotic DNA replication uses multiple origins of replication.
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What is mismatch repair in DNA replication and what does it do?
What is mismatch repair in DNA replication and what does it do?
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Study Notes
DNA Replication Overview
- DNA replication is the process of creating two identical copies of a DNA molecule from a single original DNA molecule.
- This process ensures genetic information is passed on correctly during cell division.
- The semi-conservative model, proven by the Meselson-Stahl experiment, shows each new DNA molecule consists of one original strand and one new strand.
Replication Models
- Semiconservative model: Each new DNA molecule contains one original strand and one newly synthesized strand.
- Conservative model: One DNA molecule retains both original strands, and the other consists of two newly synthesized strands.
- Dispersive model: Original and new DNA segments are interspersed in both new DNA molecules.
Meselson-Stahl Experiment
- This experiment, using isotopes of nitrogen (15N and 14N), confirmed the semiconservative model.
- E. coli were grown in a medium containing 15N, then transferred to a 14N medium.
- DNA extracted at various time points was analyzed using density gradient centrifugation.
- The results showed DNA progressively shifted to a lower density, supporting the semiconservative model.
Steps of DNA Replication
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Step 1: Strand Separation
- Helicase unwinds the DNA double helix.
- Topoisomerases relieve the tension created by unwinding.
- Single-strand binding proteins prevent the separated strands from reannealing.
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Step 2: Building Complementary Strands
- RNA primase synthesizes RNA primers, providing a starting point for DNA polymerase.
- DNA polymerase III builds new DNA strands using complementary base pairing.
- The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments (Okazaki fragments).
- DNA polymerase I removes RNA primers and replaces them with DNA.
- DNA ligase joins Okazaki fragments.
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Step 3: Dealing with Errors
- DNA polymerase proofreads the newly synthesized DNA.
- Mismatch repair systems identify and correct errors.
- Recognition, excision of the error, gap-filling and ligation are some of the steps
DNA Replication in Prokaryotes and Eukaryotes
- Prokaryotes (bacteria) have a single replication origin, forming a replication bubble that expands bidirectionally.
- Eukaryotes have multiple replication origins, resulting in numerous replication bubbles that eventually merge.
- Eukaryotes use telomeres (repetitive sequences at chromosome ends) to prevent loss of genetic material during replication. Telomerase re-extends the telomeres.
- Eukaryotes have more variations of DNA polymerases compared to prokaryotes.
Key Enzymes
- Helicase: Unwinds the DNA double helix.
- Topoisomerases: Relieve tension caused by unwinding.
- Single-strand binding proteins (SSBs): Prevent reannealing of separated strands.
- RNA primase: Synthesizes RNA primers.
- DNA polymerase III: Builds new DNA strands.
- DNA polymerase I: Removes RNA primers and replaces them with DNA.
- DNA ligase: Joins Okazaki fragments.
- DNA polymerase II: Repairs DNA damage.
Rate of Replication
- In eukaryotes, new DNA is synthesized at a rate of approximately 50 base pairs per second at each replication fork.
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Description
This quiz covers the fundamental concepts of DNA replication, including the different models such as semiconservative, conservative, and dispersive. It also highlights the significance of the Meselson-Stahl experiment in confirming the semiconservative model of DNA replication.
