DNA Replication Process Overview
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Questions and Answers

What is the role of SSB protein in DNA replication?

  • To synthesize DNA continuously
  • To relieve overwinding strain
  • To prevent single strands from re-annealing (correct)
  • To facilitate the action of DNA polymerase

    • What distinguishes the leading strand from the lagging strand during DNA replication?

  • The leading strand is synthesized in pieces
  • The leading strand runs in the 3'-to-5' direction
  • The leading strand is synthesized continuously (correct)
  • The leading strand requires multiple primers

    • What is a key characteristic of the semiconservative model of DNA replication?

  • It produces molecules that are entirely hybrids of parental and new DNA.
  • Newly synthesized DNA strands consist entirely of new molecules.
  • Each daughter DNA strand consists of one parental strand and one new strand. (correct)
  • Both strands of the parental duplex remain completely intact.

    • What are Okazaki fragments?

    <p>Short segments of DNA synthesized on the lagging strand</p> Signup and view all the answers

    What is the function of DNA gyrase in replication?

    <p>To relieve supercoiling in DNA</p> Signup and view all the answers

    Which of the following accurately describes the dispersive model of DNA replication?

    <p>Daughter strands consist of a mix of parental and newly synthesized strands.</p> Signup and view all the answers

    Which of the following models of DNA replication was ultimately shown to be accurate by Meselson and Stahl?

    <p>Semiconservative model.</p> Signup and view all the answers

    Why can DNA polymerases synthesize DNA only in one direction?

    <p>They require a primer to initiate synthesis</p> Signup and view all the answers

    What is the primary outcome of the conservative model of DNA replication?

    <p>Both strands of the parental duplex remain intact, while new strands consist of entirely new molecules.</p> Signup and view all the answers

    What is the crucial function of DNA replication prior to cell division?

    <p>To ensure that the sequence of parental strands is accurately duplicated in daughter strands.</p> Signup and view all the answers

    What is the primary function of helicase in DNA replication?

    <p>Unwinds parental double helix</p> Signup and view all the answers

    What does single-strand binding protein do during replication?

    <p>Stabilizes single-stranded DNA</p> Signup and view all the answers

    What role does primase play in DNA replication?

    <p>Synthesizes an RNA primer</p> Signup and view all the answers

    Which enzyme is responsible for adding nucleotides to a new DNA strand during replication?

    <p>DNA pol III</p> Signup and view all the answers

    What is the function of DNA pol I in DNA replication?

    <p>Removes RNA nucleotides from primer</p> Signup and view all the answers

    How does topoisomerase assist during DNA replication?

    <p>Relieves overwinding strain</p> Signup and view all the answers

    What is the role of DNA ligase in DNA replication?

    <p>Joins Okazaki fragments and other DNA ends</p> Signup and view all the answers

    What is the structure formed during the unwinding of the DNA helix known as?

    <p>Replication fork</p> Signup and view all the answers

    Which strand of DNA is synthesized continuously during replication?

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

    What happens to the RNA primer after it has served its purpose in DNA replication?

    <p>It is removed and replaced with DNA nucleotides</p> Signup and view all the answers

    In the conservative model of DNA replication, both strands of the parental duplex remain intact.

    <p>True</p> Signup and view all the answers

    The dispersive model of DNA replication produces entirely new DNA strands without any parental components.

    <p>False</p> Signup and view all the answers

    Meselson and Stahl demonstrated that the basic mechanism of DNA replication is dispersive.

    <p>False</p> Signup and view all the answers

    The semiconservative model of DNA replication results in daughter strands that consist of both new and parental DNA.

    <p>True</p> Signup and view all the answers

    All three models of DNA replication were evaluated by Matthew Meselson and Franklin Stahl in 1968.

    <p>False</p> Signup and view all the answers

    DNA polymerase can synthesize DNA in both 5′-to-3′ and 3′-to-5′ directions.

    <p>False</p> Signup and view all the answers

    Replication in E.coli starts at the origin known as oriC and ends at a site called terminus.

    <p>True</p> Signup and view all the answers

    All DNA polymerases require a primer to stop synthesis.

    <p>False</p> Signup and view all the answers

    Nucleoside triphosphates serve as the building blocks necessary for DNA replication.

    <p>True</p> Signup and view all the answers

    The process of elongation in DNA replication involves the addition of new bases at the 5' end of existing strands.

    <p>False</p> Signup and view all the answers

    Study Notes

    DNA Replication

    • DNA replication is essential for cell division.
    • The process requires multiple cellular proteins.

    Models of DNA Replication

    • Three models of DNA replication were proposed in 1958:
      • Conservative model: Both parental strands remain intact.
      • Semiconservative model: One parental strand remains intact.
      • Dispersive model: Copies of DNA contain mixtures of parental and newly synthesized strands.
    • Meselson and Stahl proved that DNA replication is semiconservative.

    Replication Process

    • The DNA helix unwinds to form a replication fork.
    • Topoisomerase (DNA gyrase) relieves overwinding strain.
    • Single-strand binding protein (SSB) stabilizes single-stranded DNA.

    Leading and Lagging Strands

    • The leading strand is synthesized continuously in the 5'-to-3' direction.
    • The lagging strand is synthesized discontinuously in Okazaki fragments.

    Replication Proteins

    • Helicase: Unwinds parental double helix at the replication fork.
    • Single-strand binding protein: Binds to and stabilizes single-stranded DNA.
    • Topoisomerase (DNA gyrase): Relieves overwinding strain.
    • Primase: Synthesizes RNA primers.
    • DNA polymerase III: Synthesizes new DNA strands using parental DNA as a template.
    • DNA polymerase I: Removes RNA primers and replaces them with DNA nucleotides.
    • DNA ligase: Joins Okazaki fragments and seals nicks in the DNA backbone.

    DNA Primase

    • Synthesizes short RNA primers (10-20 bp) that initiate DNA synthesis.

    Leading Strand

    • DNA polymerase III synthesizes the leading strand continuously using its high processivity.

    Lagging Strand

    • DNA polymerase III synthesizes Okazaki fragments.
    • DNA polymerase I removes RNA primers and replaces them with DNA.
    • DNA ligase joins Okazaki fragments.

    Replisome

    • The enzymes involved in DNA replication form a molecular assembly called the replisome.
    • The replisome contains two main subcomponents:
      • Primosome: Includes primase and helicase.
      • Complex of two DNA Pol III enzymes: One for each strand.

    Replisome Components

    • The two Pol III complexes include two synthetic core subunits, each with its own β sliding clamp subunit.
    • The entire replisome complex is held together by multiple proteins, including the clamp loader.

    Overall Process

    • The replisome moves along the DNA, unwinding the helix, synthesizing new strands, and replacing primers.
    • The process continues until the entire DNA molecule is replicated.

    DNA Replication

    • DNA replication is a fundamental process that occurs before cell division, ensuring each daughter cell receives a complete copy of the parental DNA.
    • Replication requires multiple proteins to work in concert.

    Models of DNA replication

    • There were three models proposed for DNA replication: Conservative, Semiconservative, and Dispersive.
    • The Conservative model proposed the parental DNA duplex remains intact, while entirely new DNA molecules are synthesized.
    • The Semiconservative model suggested one parental strand remains intact in each daughter strand, and a new complementary strand is synthesized for each parental strand.
    • The Dispersive model proposed a mixture of parental and newly synthesized strands in each copy of DNA.
    • Experiments conducted by Meselson and Stahl demonstrated that DNA replication follows the Semiconservative model, with each new DNA helix comprising one old strand and one new strand.

    Requirements of DNA Replication

    • DNA replication requires a template (parental DNA molecule), enzymes to perform the copying process, and building blocks (nucleoside triphosphates) to construct the new DNA strand.
    • The replication process involves three main steps: initiation (starting point), elongation (adding building blocks), and termination (ending point).
    • These steps can be referred to as initiation, elongation, and termination.

    Action of DNA Polymerase

    • DNA polymerase is the enzyme responsible for matching existing DNA bases with complementary nucleotides and linking them together to create the new strand.
    • DNA polymerase has these key features:
      • Adds new bases to the 3' end of existing strands.
      • Synthesizes DNA in a 5' to 3' direction, extending a strand base-paired to the template.
      • Requires a primer to initiate synthesis.
    • RNA polymerases often synthesize the primers.

    Prokaryotic Replication

    • Focusing on E.coli as a model, prokaryotic replication initiates at a unique origin called oriC and terminates at a specific terminus.
    • Replication proceeds bidirectionally from oriC, meaning two replisomes initiate synthesis in opposite directions until they meet at the terminus.
    • The DNA segment controlled by an origin is referred to as a replicon.

    Types of DNA Polymerase

    • There are three major DNA polymerases in prokaryotes: DNA polymerase I, II, and III.
    • All three polymerases synthesize DNA in the 5' to 3' direction and require a primer.
    • Their functions include:
      • DNA Polymerase I: Removes RNA primers and replaces them with DNA.
      • DNA Polymerase II: Involved in DNA repair processes, but not primarily in replication.
      • DNA Polymerase III: The primary replication enzyme responsible for most DNA synthesis.
    • Nuclease enzymes, either endonucleases (cut DNA internally) or exonucleases (remove nucleotides from the end of DNA), play a role in DNA replication.
    • DNA polymerases possess exonuclease activity, serving as a proofreading function to remove mismatched bases.
    • DNA Polymerase I has 5' to 3' exonuclease activity, which removes RNA primers.

    Unwinding of DNA

    • Helicase is the enzyme that unwinds the DNA helix, requiring energy in the form of ATP.
    • Topoisomerase enzymes alleviate the torsional strain caused by unwinding, preventing supercoiling in the double helix.

    DNA Primase

    • DNA primase, an RNA polymerase, synthesizes short RNA primers (10 to 20 bp) that serve as starting points for DNA polymerase to begin DNA synthesis.

    Leading Strand

    • After RNA primer synthesis, the leading strand can be elongated continuously by DNA polymerase III.
    • DNA polymerase III, a large enzyme with high processitivy, uses a subunit called the β subunit, a "sliding clamp", to attach to the DNA strand and efficiently elongate DNA.
    • The clamp loader, a complex protein, opens and closes the β subunit around the DNA.

    Lagging Strand

    • Replication on the lagging strand is discontinuous due to the 5' to 3' directionality of DNA synthesis, requiring a series of steps.

    • Primase synthesizes RNA primers for each Okazaki fragment, which are then removed and replaced with DNA.

    • DNA polymerase III synthesizes the Okazaki fragments.

    • Okazaki fragments are stitched together through several steps: - DNA polymerase I removes the RNA primer and replaces it with DNA. - DNA Ligase seals the nick between the Okazaki fragments after primer removal.

    DNA Replication by Replisome

    • The enzymes involved in DNA replication form a molecular assembly called the replisome.
    • The replisome has two main sub-components:
      • The primosome, which consists of primase and helicase, with various accessory proteins.
      • A complex of two DNA polymerase III enzymes, one for each strand.
    • The replisome is held together by multiple proteins, including the clamp loader.
    • As DNA polymerase III finishes an Okazaki fragment, the clamp loader transfers it to the next fragment and loads a β subunit onto the next fragment, effectively transferring the DNA polymerase III to this new β subunit, allowing replication to proceed.

    Replication is Semi-Discontinuous

    • The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as a series of Okazaki fragments.
    • This semi-discontinuous nature ensures efficient replication of both strands, despite the 5' to 3' directionality of DNA polymerase.

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    Description

    This quiz covers the essential aspects of DNA replication including models, processes, and key proteins involved. You'll learn about the semiconservative model and the roles of various enzymes during replication. Test your understanding of leading and lagging strands and their synthesis.

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