-DNA REPLICATION-KIM

1. Functions of DNA as Genetic Material

  • Key Functions:

    • Storage of Genetic Information: Encodes instructions for cellular function.

    • Self-Duplication and Inheritance: Ensures genetic continuity during cell division.

    • Expression of Genetic Information: Facilitates the production of proteins via transcription and translation.

2. Models of DNA Replication

  • Conservative: Entire DNA molecule is conserved, producing a completely new copy.

  • Dispersive: New DNA interspersed with sections of old DNA.

  • Semiconservative (Biologically Significant): Each daughter DNA molecule consists of one old strand and one new strand.

    • Supported by the Meselson and Stahl Experiment using isotopic nitrogen (15N and 14N).

3. DNA Replication Process

  • Definition: The duplication of a DNA molecule to produce two identical copies, one for each daughter cell.

  • Phases of the Cell Cycle:

    • Occurs during the S-phase in eukaryotes and cytoplasmic replication in prokaryotes.

4. Mechanisms of DNA Replication

  1. Initiation:

    • Origin of Replication (Ori):

      • Prokaryotes: Single Ori (AT-rich sequence).

      • Eukaryotes: Multiple Ori sites.

    • Enzymes:

      • Helicase: Unwinds DNA by breaking hydrogen bonds.

      • Topoisomerase/DNA Gyrase: Relieves supercoiling.

      • Single-Strand Binding Proteins (SSBP): Prevent reannealing of separated strands.

  2. Elongation:

    • Leading Strand: Synthesized continuously in the 5′ → 3′ direction.

    • Lagging Strand: Synthesized discontinuously in fragments (Okazaki fragments), requiring multiple primers.

    • Enzymes:

      • Primase: Synthesizes RNA primers.

      • DNA Polymerase: Adds nucleotides to the growing chain.

  3. Termination:

    • Prokaryotes: Concludes at termination sequences.

    • Eukaryotes: Telomeres ensure stability of chromosome ends.

5. Key Enzymes and Molecules

  • DNA Polymerases:

    • Prokaryotes: DNA Pol I, II, III.

    • Eukaryotes: DNA Pol α, β, δ, ε, γ.

    • Functions: Synthesize DNA, proofread, and repair errors.

  • Telomerase: Adds repetitive sequences (e.g., TTAGGG) to the ends of chromosomes to prevent shortening during replication.

6. Inhibitors of DNA Replication

  • Antibacterial Agents:

    • Ciprofloxacin, Nalidixic Acid: Inhibit DNA gyrase.

  • Anticancer Agents:

    • Etoposide, Doxorubicin: Inhibit topoisomerase.

7. Polymerase Chain Reaction (PCR)

  • Definition: In vitro technique to amplify specific DNA sequences.

  • Steps:

    • Denaturation: Separation of DNA strands.

    • Annealing: Binding of primers to template DNA.

    • Extension: DNA polymerase adds nucleotides to synthesize complementary strands.