المحاضرة  الرابعة جينتكس (بعد التعديل)
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Questions and Answers

What is the primary purpose of DNA replication?

  • To initiate cell division directly
  • To form identical copies of DNA (correct)
  • To create a unique genetic sequence
  • To synthesize proteins from DNA templates
  • Which statement best describes the nature of DNA replication?

  • It requires only one parent strand.
  • It is a conservative process.
  • It involves the replication of RNA.
  • It is a semiconservative process. (correct)
  • In which direction does DNA synthesis occur?

  • 3' to 5'
  • 5' to 3' (correct)
  • Bidirectional from a central point
  • In reverse complementary direction
  • Which enzyme is primarily involved in unwinding double-stranded DNA prior to replication?

    <p>DNA helicase</p> Signup and view all the answers

    What is required for DNA polymerase to initiate DNA synthesis?

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

    Where does DNA replication typically initiate?

    <p>At multiple origins of replication</p> Signup and view all the answers

    What components serve as precursors for DNA synthesis?

    <p>Desoxyribonucleotides triphosphate</p> Signup and view all the answers

    What role does the primase enzyme play in DNA replication?

    <p>It synthesizes RNA primers from DNA templates.</p> Signup and view all the answers

    What distinguishes the replication of the leading strand from the lagging strand?

    <p>The leading strand is synthesized continuously.</p> Signup and view all the answers

    Which enzyme is responsible for synthesizing the final connections between DNA fragments?

    <p>DNA ligase</p> Signup and view all the answers

    What initiates the formation of the replication fork during DNA replication?

    <p>DNA helicase</p> Signup and view all the answers

    Which DNA polymerase is specifically involved in synthesizing the lagging strand?

    <p>DNA polymerase δ</p> Signup and view all the answers

    Which of the following describes single strand binding proteins (ssBP) in DNA replication?

    <p>They prevent the DNA strands from re-annealing.</p> Signup and view all the answers

    What is the directionality in which DNA polymerases synthesize a new strand?

    <p>5' to 3'</p> Signup and view all the answers

    What is the function of exonucleases during DNA replication?

    <p>They remove RNA primers.</p> Signup and view all the answers

    What is one essential requirement for DNA polymerases to function in DNA replication?

    <p>They must have an RNA primer for initiating new strands.</p> Signup and view all the answers

    Which of the following components is NOT involved in the process of DNA replication?

    <p>RNA replicase</p> Signup and view all the answers

    Which process ensures that each daughter DNA molecule contains one old strand and one new strand?

    <p>Semiconservative replication</p> Signup and view all the answers

    What is the significance of the origin of replication (ori) in DNA replication?

    <p>It is the site where replication begins.</p> Signup and view all the answers

    What type of molecular structure is primarily recognized by DNA helicases during the unwinding of DNA?

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

    In what manner does DNA synthesis occur on each of the two strands during replication?

    <p>Simultaneously and bidirectional.</p> Signup and view all the answers

    Which of the following statements about the direction of DNA polymerization is accurate?

    <p>DNA synthesis must occur in 5' to 3' direction.</p> Signup and view all the answers

    What are the characteristics of RNA primers used in DNA replication?

    <p>Short sequences synthesized from DNA templates.</p> Signup and view all the answers

    How do DNA polymerases assist in the DNA replication process?

    <p>They are responsible for synthesizing both the leading and lagging strands.</p> Signup and view all the answers

    What is the role of DNA ligase in DNA replication?

    <p>To form the final phosphodiester bonds between DNA fragments.</p> Signup and view all the answers

    What distinguishes the synthesis of the lagging strand during DNA replication?

    <p>It results in the formation of fragments known as Okazaki fragments.</p> Signup and view all the answers

    Which component is essential in maintaining the separation of DNA strands during replication?

    <p>Single strand binding proteins (ssBP).</p> Signup and view all the answers

    What is the primary function of exonucleases in DNA replication?

    <p>To remove RNA primers from DNA template.</p> Signup and view all the answers

    Which of the following statements about DNA replication forks is true?

    <p>DNA helicase is responsible for forming the replication forks by unwinding the DNA.</p> Signup and view all the answers

    What is the significance of the 3' to 5' reading direction of the template strand?

    <p>It enables DNA polymerase to synthesize a new strand in the 5' to 3' direction.</p> Signup and view all the answers

    Study Notes

    DNA Replication

    • DNA replication is the process of copying a DNA molecule into an identical copy.
    • DNA replication occurs before cell division, so that the two daughter cells have identical DNA copies.

    Semiconservative Replication

    • Semiconservative replication refers to the mechanism by which each of the two resulting double-stranded DNA molecules contains one old strand and one newly synthesized strand from the original molecule.
    • One old strand (parent strand) is conserved.
    • One new strand is synthesized using free nucleotides found in the nucleus.
    • Both strands serve as a template for DNA replication simultaneously.

    Criteria for DNA Replication

    • Replication is bidirectional.
    • DNA synthesis occurs in the 5' to 3' direction.
    • DNA replication requires a complex of enzymes and proteins.

    Components of DNA Replication

    • Template: Double-stranded DNA.
    • Precursors: dATP, dGTP, dCTP, dTTP (deoxyribonucleotide triphosphates)
    • Cofactors: Mg++, Mn++, ATP.
    • Enzymes: DNA Polymerases, DNA helicases, DNA primase (a subunit of DNA polymerase α), Exonucleases.
    • Proteins: Single-stranded DNA binding proteins (ssDB Proteins), Nick sealing enzyme DNA ligase.

    DNA Polymerase

    • DNA polymerase synthesizes a new DNA strand using the template strand.
    • It reads the template strand in the 3' to 5' direction and synthesizes the new strand in the 5' to 3' direction.
    • Deoxyribonucleotide triphosphates (dATP, dGTP, dCTP & dTMP) serve as precursors for the new strand.
    • DNA polymerase cannot initiate DNA synthesis and it needs an RNA primer.
    • There are five types of DNA polymerases in eukaryotes: α, ε, β, γ and δ.

    Initiation of DNA Replication

    • DNA replication begins at multiple DNA sites called origins of replication (ori).
    • Origins of replication are rich in AT base pairs (consensus sequence).
    • Multiple origins of replication begin simultaneously so replication occurs bidirectionally.
    • DNA helicases break the hydrogen bonds between the two DNA strands, unwinding the double helix and creating a "replication bubble."
    • Replication forks - The V-shaped structure at each end of the replication bubble.
    • Single-stranded binding proteins (ssBP) hold the separated DNA strands apart to prevent them from rejoining.

    Priming by Primase

    • Primase is an RNA polymerase.
    • Primase synthesizes a short RNA primer that provides a starting point for DNA polymerase.
    • One RNA primer is required for the leading strand.
    • Multiple primers are needed for the lagging strand.

    Synthesis of Leading and Lagging Strands

    • DNA polymerases are responsible for synthesizing both strands of DNA.
    • The presence of many DNA polymerases (more than 20,000) reduces the time needed for replication.

    Leading Strand

    • Synthesized continuously.
    • Replicated by DNA polymerase ε in the direction of the advancing replication fork.

    Lagging Strand

    • Synthesized discontinuously.
    • Replicated by DNA polymerase δ in the opposite direction of the advancing replication fork.
    • Synthesized in fragments called Okazaki fragments.

    Excision of RNA Primers

    • RNA primers are removed by exonucleases.

    Ligation of DNA Fragments

    • The final phosphodiester linkage between the 5'-phosphate group on one DNA chain and the 3'-hydroxyl group on the adjacent chain is catalyzed by DNA ligase.
    • This reaction requires ATP.

    Proofreading of Newly Synthesized DNA Strands

    • DNA polymerases δ and ε have proofreading activity.
    • They check for errors in base pairing and correct them, ensuring the accuracy of DNA replication.

    DNA Replication

    • DNA replication is the process of creating an identical copy of a DNA molecule.
    • It occurs before cell division (mitosis) ensuring each daughter cell receives a complete set of DNA.
    • DNA replication is semiconservative, meaning each new DNA molecule contains one original strand (parent strand) and one newly synthesized strand.
    • Both DNA strands serve as templates for replication simultaneously.
    • Replication proceeds bidirectionally, meaning it happens in both directions from the origin of replication.
    • The direction of new DNA synthesis is always 5' to 3'.

    Key Components of DNA Replication

    • DNA Polymerases: The main enzymes responsible for synthesizing new DNA strands.
      • They require an existing RNA primer to begin replication.
      • In eukaryotes, there are five DNA polymerases: α, ε, β, γ, and δ.
    • DNA Helicase: Unwinds the double helix structure of DNA, separating the two strands.
    • Single-stranded DNA binding proteins (ssBPs): Prevent the separated strands from re-annealing, keeping them accessible for replication.
    • DNA Primase: Synthesizes short RNA primers that provide a starting point for DNA polymerase to attach to.
    • Exonucleases: Remove RNA primers from the newly synthesized DNA strands.
    • DNA Ligase: Seals the gaps between newly synthesized DNA fragments, creating a continuous strand.

    Stages of DNA Replication

    • Initiation: Begins at specific sites called "origins of replication" (ori)
      • Ori regions are rich in AT base pairs (adenine and thymine) due to their weaker bonding, making them easier to separate.
    • Unwinding and Separation:
      • DNA helicase unwinds the double helix at the origin of replication, creating a "replication bubble".
      • Two replication forks form at the ends of the bubble, and replication proceeds in opposite directions.
      • Single-stranded binding proteins (ssBPs) prevent the separated strands from re-annealing.
    • Priming:
      • Primase creates short RNA primers on both strands, providing starting points for DNA polymerase.
      • The leading strand only needs one primer, while the lagging strand needs multiple primers.
    • Synthesis of Leading and Lagging Strands:
      • Leading strand: Synthesized continuously in the 5' to 3' direction, following the replication fork movement.
      • Lagging strand: Synthesized discontinuously in fragments (Okazaki fragments) in the opposite direction of fork movement.
    • Excision of RNA Primers:
      • Exonucleases remove the RNA primers.
    • Ligation:
      • DNA ligase joins the Okazaki fragments on the lagging strand and fills in any remaining gaps to make a continuous strand.

    Proofreading Mechanism

    • DNA polymerases (δ and ε) have a proofreading function.
    • This mechanism helps ensure that the newly synthesized DNA strand is accurate and free of errors.
    • DNA polymerase can backtrack (3' to 5' direction) and remove mismatched nucleotides.

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