DNA Structure and Replication

DNA Structure

• Double Helix: DNA is structured as two strands twisted into a double helix.
• Chemical Bases: Composed of four bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
  • Complementary Pairs:
    • A pairs with T
    • C pairs with G
• Strand Orientation:
  • Each strand has a 5' end and a 3' end.
  • Strands run in opposite directions (antiparallel).

DNA Replication Process

Step 1: Unzipping of DNA

• Helicase Enzyme: Unzips the DNA strands, forming a replication fork.

Step 2: Primer Formation

• Primase Enzyme: Creates a short RNA primer to initiate new strand synthesis.

Step 3: New Strand Synthesis

• DNA Polymerase: Binds to the primer and adds DNA bases in the 5' to 3' direction.
• Leading Strand:
  • Synthesized continuously in the 5' to 3' direction.
• Lagging Strand:
  • Synthesized in fragments (Okazaki fragments) due to reverse orientation.
  • Each fragment begins with an RNA primer.

Step 4: Primer Removal and Strand Completion

• Exonuclease Enzyme: Removes RNA primers from DNA.
• DNA Polymerase: Fills in gaps with DNA.
• DNA Ligase: Seals the fragments to create a continuous strand.

Semi-Conservative Replication

• Description: Each new DNA molecule includes one old strand and one new strand, thus conserving half of the original DNA molecule.