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Transcript DNA Replication

DNA Structure

  • DNA (Deoxyribonucleic Acid) is a complex molecule composed of two strands.

  • The strands are twisted together to form a double helix shape.

  • Each strand consists of a sequence of four chemical bases: A (adenine), C (cytosine), G (guanine), and T (thymine).

  • The strands are complementary:

    • A pairs with T

    • C pairs with G

  • Each strand has two ends: a 5' end and a 3' end.

  • Strands run in opposite directions, impacting the replication process.

DNA Replication Process

  • Step 1: Separation of Strands

    • The first step is to separate the two strands of DNA.

    • This unzipping is performed by the enzyme helicase.

    • Results in a formation known as a replication fork.

Primer Formation

  • The separated strands serve as templates for new DNA strand creation.

  • The enzyme primase initiates the process by creating a small RNA molecule called a primer.

    • The primer marks the starting point for constructing the new DNA strand.

Role of DNA Polymerase

  • The enzyme DNA polymerase binds to the primer and synthesizes the new DNA strand.

  • DNA polymerase adds bases only in one direction: from the 5' end to the 3' end.

  • Leading Strand:

    • Synthesized continuously in the 5' to 3' direction.

  • Lagging Strand:

    • Made in short segments called Okazaki fragments due to the opposing direction.

    • Each Okazaki fragment is initiated by an RNA primer followed by the addition of DNA bases.

Lagging Strand Synthesis

  • After each Okazaki fragment, a new primer is positioned further down the lagging strand.

  • The process of fragment synthesis is repeated multiple times.

Final Steps of DNA Replication

  • After DNA strands are synthesized, the enzyme exonuclease removes all RNA primers from both strands.

  • Another DNA polymerase fills in the gaps left by the removed RNA primers with DNA.

  • The enzyme DNA ligase seals the fragments on both strands, resulting in a continuous double-strand formation.

Semi-Conservative Nature of DNA Replication

  • DNA replication is termed semi-conservative.

  • Each new DNA molecule consists of one old (conserved) strand and one newly synthesized strand.