DNA Replication Overview
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Questions and Answers

What is the primary purpose of DNA replication in living organisms?

  • To ensure each daughter cell has a complete copy of genetic information (correct)
  • To increase the size of the DNA molecule
  • To modify the genetic information
  • To synthesize proteins for cell functions

During which phase of the cell cycle does DNA replication occur?

  • G2 phase
  • G1 phase
  • S phase (correct)
  • M phase

Which enzyme is responsible for unwinding the DNA helix during replication?

  • Ligase
  • Primase
  • DNA Polymerase I
  • Helicase (correct)

What bonding type stabilizes the DNA backbone?

<p>Covalent phosphodiester bonds (C)</p> Signup and view all the answers

What challenge arises due to the anti-parallel nature of DNA strands during replication?

<p>The direction of synthesis must differ for each strand (A)</p> Signup and view all the answers

Which enzyme replaces RNA primers with DNA nucleotides during replication?

<p>DNA Polymerase I (D)</p> Signup and view all the answers

Which of the following enzymes is involved in linking Okazaki fragments on the lagging strand?

<p>Ligase (D)</p> Signup and view all the answers

In the semi-conservative model of DNA replication, which statement is true?

<p>Each new DNA molecule consists of one old strand and one new strand (B)</p> Signup and view all the answers

What is the primary role of DNA topoisomerase during DNA replication?

<p>To prevent tangling of DNA strands (C)</p> Signup and view all the answers

Why does RNA primase add an RNA primer during DNA replication?

<p>To provide a starting point for DNA polymerase (B)</p> Signup and view all the answers

What do Okazaki fragments represent in DNA replication?

<p>Short segments of DNA built on the lagging strand (D)</p> Signup and view all the answers

What is the function of DNA polymerase I during DNA replication?

<p>To replace RNA primers with DNA nucleotides (B)</p> Signup and view all the answers

Which strand of DNA is synthesized continuously during replication?

<p>Leading strand (A)</p> Signup and view all the answers

Which enzyme is responsible for proofreading new DNA strands?

<p>DNA polymerase III (A)</p> Signup and view all the answers

What drives the formation of the phosphodiester linkage between nucleotides?

<p>The release of energy from nucleoside triphosphates (D)</p> Signup and view all the answers

What role does DNA ligase serve in DNA replication?

<p>To join Okazaki fragments together (C)</p> Signup and view all the answers

How do single-stranded binding proteins contribute to DNA replication?

<p>By preventing DNA strands from re-annealing (A)</p> Signup and view all the answers

What indicates the direction of DNA strand synthesis?

<p>5' to 3' direction of the new strand (B)</p> Signup and view all the answers

Which of the following is a characteristic of the lagging strand during DNA replication?

<p>It includes multiple RNA primers (D)</p> Signup and view all the answers

What is the basis for the discontinuous nature of lagging strand synthesis?

<p>DNA polymerase can only extend from a 3' end (B)</p> Signup and view all the answers

What happens to the RNA primers after DNA replication is completed?

<p>They are removed and replaced by DNA nucleotides (B)</p> Signup and view all the answers

Flashcards

DNA Replication

The process by which a cell creates an identical copy of its DNA before cell division.

Enzymes in DNA Replication

Special proteins that help carry out DNA replication by performing specific tasks.

Helicase

An enzyme that breaks the hydrogen bonds between the two strands of DNA, separating them.

Topoisomerase

An enzyme that relieves tension in the DNA molecule as it unwinds, preventing it from getting tangled.

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Primase

An enzyme that lays down a short RNA primer, which is needed for DNA Polymerase III to start building the new DNA strand.

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DNA Polymerase III

The main enzyme that adds new nucleotides to the existing DNA strand, building a new strand that is complementary to the template strand.

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DNA Polymerase I

An enzyme that replaces the RNA primer with DNA nucleotides.

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Ligase

An enzyme that connects the newly synthesized DNA fragments, forming a continuous DNA strand.

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DNA topoisomerase

An enzyme that prevents DNA strands from tangling during replication by breaking and rejoining DNA strands.

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RNA Primer

Short segments of RNA that are added to the 3' end of the template DNA strand to provide a starting point for DNA polymerase III.

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Single-stranded binding proteins (SSBs)

Proteins that bind to single-stranded DNA, preventing it from re-annealing and keeping it available for replication.

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Replication fork

The point where the DNA double helix is unwound during replication.

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5' to 3' direction

The direction in which DNA polymerase III can add nucleotides to a growing DNA strand (from 5' to 3').

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Leading strand

The continuous strand of DNA that is synthesized in the same direction as the replication fork (towards the fork).

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Okazaki fragments

Short DNA segments synthesized on the lagging strand in the opposite direction of the fork, later joined together.

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DNA ligase

An enzyme that joins the Okazaki fragments together on the lagging strand, creating a continuous DNA strand.

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Lagging strand

The strand of DNA that is synthesized discontinuously in fragments (Okazaki fragments) in the opposite direction of the replication fork.

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RNA primase

The enzyme that adds RNA nucleotides to the template DNA strand, creating the RNA primer.

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Primer replacement

The process of removing the RNA primers and replacing them with DNA nucleotides before the lagging strand becomes continuous.

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Study Notes

DNA Replication Overview

  • DNA replication is crucial for living organisms, ensuring each daughter cell receives a complete copy of genetic information.
  • It occurs during the S phase of interphase.
  • Accurate replication is essential for cell growth, repair, and normal cell function.

Replication Models

  • Scientists proposed different models for how DNA replicates: semiconservative, conservative, and dispersive.
  • The semiconservative model was ultimately proven correct; this means each new DNA molecule consists of one original strand and one newly synthesized strand.

Meselson-Stahl Experiment

  • This experiment provided evidence supporting the semiconservative model.
  • Bacteria were grown in a medium containing "heavy" nitrogen.
  • Later, they were switched to a medium with "light" nitrogen.
  • DNA was extracted and analyzed using density gradient centrifugation.
  • The results confirmed the semiconservative model.

Semi-Conservative Model Details

  • Base pairing allows each strand to act as a template for the new strand.
  • Each new strand is half parent and half new.

Anti-Parallel Strands

  • Nucleotides form a backbone bonded from phosphate to sugar.
  • The bond is between the 3' and 5' carbons.
  • DNA strands run in opposite directions (antiparallel).
  • This creates a problem during replication.

Bonding in DNA

  • Covalent phosphodiester bonds hold the sugar-phosphate backbone together.
  • Hydrogen bonds hold the base pairs together.

DNA Replication Enzymes

  • Helicase: Unwinds the DNA double helix.
  • Topoisomerase: Prevents the DNA molecule from becoming tangled as it's unwound.
  • Primase: Adds short RNA primers to the template DNA to provide a starting point for DNA polymerase III.
  • DNA Polymerase III: Adds nucleotides in the 5' to 3' direction to build the new DNA strand. It also proofreads and corrects mistakes.
  • DNA Polymerase I: Removes the RNA primers and replaces them with DNA nucleotides.
  • Ligase: Forms phosphodiester bonds between Okazaki fragments, joining the DNA pieces on the lagging strand.

Step 1: DNA Unwinds

  • Helicase unwinds a portion of the DNA helix.
  • Single-stranded binding proteins stabilize the separated strands to prevent them from re-annealing.
  • Topoisomerase reduces tension ahead of the replication fork.

Step 2: Create a Primer

  • RNA primase adds short RNA primers to the template DNA strands.
  • DNA polymerase III needs a 3' end to add nucleotides.

Step 3: Build the New Strand

  • DNA polymerase III builds the daughter DNA strand, adding complementary bases to the RNA primer.
  • It proofreads and corrects errors in the new DNA sequence.

Building the New Strand Further

  • DNA polymerase III adds DNA nucleotides (which are actually in the form of nucleoside triphosphates) to the growing DNA strand.
  • This reaction releases energy from the removal of phosphate groups on the nucleotides, driving the formation of the phosphodiester bond between the nucleotides.

Step 4: Replace Primer with DNA

  • DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides.
  • Ligase joins together the Okazaki fragments on the lagging strand.

Leading & Lagging Strands

  • Leading strand: Built continuously toward the replication fork.
  • Lagging strand: Built discontinuously in small fragments called Okazaki fragments, away from the replication fork.

DNA Replication on the Lagging Strand

  • RNA primer is built by primase and serves as a starting sequence for DNA polymerase III
  • Short segments called Okazaki fragments are created because DNA polymerase III can only synthesize in a 5' to 3' direction.

Replacing RNA Primers with DNA

  • DNA polymerase I removes the RNA primers.
  • DNA ligase joins the Okazaki fragments together.

Replication Fork

  • The Y-shaped region where DNA is being replicated.
  • Multiple replication forks are typically active in eukaryotic DNA replication.
  • Enzymes involved in DNA replication are located near the replication fork.

Comparing Leading and Lagging Strands

  • Lagging: Built discontinuously, consisting of Okazaki fragments; synthesizes away from the replication fork using multiple primers.
  • Leading: Synthesized continuously toward the replication fork employing one primer.

Mind Map: DNA Replication

  • Visual representation of the different steps and enzymes involved in the process.
  • Includes labels for helicase, topoisomerase, nucleotides, DNA polymerase III, RNA primase, leading strand, lagging strand, Okazaki fragments, single-stranded binding proteins, DNA polymerase I, DNA ligase, and the replication origin.

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DNA Replication PDF

Description

Explore the fundamental concepts of DNA replication, including its importance in cell division and the different replication models. Learn about the groundbreaking Meselson-Stahl experiment, which provided evidence for the semiconservative model of DNA replication, ensuring genetic continuity in living organisms.

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