DNA replication - 3D

Structure of DNA

  • Double Helix:

    • DNA consists of two strands twisted around each other, forming a double helix shape.

  • Chemical Bases:

    • Each strand comprises a sequence of four chemical bases represented by the letters A (adenine), C (cytosine), G (guanine), and T (thymine).

    • Complementary Strands:

      • A pairs with T and C pairs with G.

  • Ends of Strands:

    • Each strand has a five-prime (5') end and a three-prime (3') end.

Directionality of Strands

  • Opposite Directions:

    • The two strands run in opposite directions.

    • This arrangement is crucial for the process of DNA replication.

DNA Replication Process

  • Separation of Strands:

    • The process begins with the separation of the two strands, known as unzipping, facilitated by the enzyme helicase.

    • This creates a replication fork where the strands diverge.

  • Template Utilization:

    • Each separated strand serves as a template for creating a new strand of DNA.

Initiation of DNA Synthesis

  • Role of Primase:

    • The enzyme primase synthesizes a short RNA primer, which serves as the starting point for new DNA strand construction.

  • DNA Polymerase Activation:

    • DNA polymerase binds to the primer to begin adding DNA bases.

    • It can only add bases in a direction from the five-prime end to the three-prime end.

Leading and Lagging Strands

  • Leading Strand:

    • Synthesized continuously as DNA polymerase adds bases one by one in the 5' to 3' direction.

  • Lagging Strand:

    • Synthesized discontinuously due to its opposite orientation, resulting in short segments known as Okazaki fragments.

    • Each Okazaki fragment is initiated with an RNA primer.

  • Fragmentation Process:

    • As DNA polymerase synthesizes each Okazaki fragment, further RNA primers are added further along the lagging strand, and the process repeats.

Finalization of DNA Replication

  • RNA Primer Removal:

    • The enzyme exonuclease removes all RNA primers from both newly synthesized strands.

  • Gap Filling:

    • Another DNA polymerase enzyme fills in the resulting gaps left by the removed primers.

  • Sealing of Strands:

    • The enzyme DNA ligase seals the fragments of DNA on both strands, creating a continuous double strand.

Semi-Conservative Nature

  • Definition:

    • DNA replication is termed semi-conservative because each resulting DNA molecule consists of one old (conserved) strand and one newly synthesized strand.