DNA Structure and Replication

DNA Structure

• Molecule Shape: Double helix
• Strands: Two strands twisted around each other
  • Made of sequences of four chemical bases: A (adenine), C (cytosine), G (guanine), T (thymine)
• Complementarity:
  • T on one strand pairs with A on the opposite strand
  • C pairs with G
• Directionality:
  • Each strand has a 5' end and a 3' end
  • Strands run in opposite directions

DNA Replication Process

1. Strand Separation

• Enzyme: Helicase
• Result: Formation of a replication fork

2. Template for New Strands

• Initiation:
  • Enzyme: Primase
  • Produces a small RNA piece called a primer

3. New Strand Synthesis

• Enzyme: DNA Polymerase
• Directionality: Adds DNA bases from 5' to 3'

Leading Strand

• Made continuously
• DNA polymerase adds bases one by one

Lagging Strand

• Made in Okazaki fragments
• Process:
  • Each fragment starts with an RNA primer
  • DNA polymerase adds a short row of DNA bases
  • A new primer is added further down the strand
  • Process repeats

4. Primer Removal and Gap Filling

• Enzyme: Exonuclease removes RNA primers
• Gap Filling: Another DNA polymerase fills gaps with DNA

5. Sealing DNA Fragments

• Enzyme: DNA Ligase
• Function: Seals DNA fragments to form a continuous double-strand

Semi-Conservative Replication

• Each new DNA molecule consists of one old strand and one new strand
• Described as semi-conservative because of the conservation of one original strand in each new DNA molecule.