DNA Structure and Replication

DNA Structure

• DNA is a molecule composed of two strands.
• Strands are twisted in a double helix shape.
• Comprised of four chemical bases: A, C, G, T.
• Strands are complementary:
  • T pairs with A
  • C pairs with G
• Each strand has a 5' end and a 3' end.
• Strands run in opposite directions.

DNA Replication Process

1. Separation of Strands
   • Done by the enzyme helicase.
   • Results in a replication fork.
2. Template Creation
   • Separated strands serve as templates for new DNA.
3. Initiation by Primase
   • Enzyme primase synthesizes a small piece of RNA called a primer.
   • Primer marks the starting point for new strand construction.
4. DNA Synthesis by Polymerase
   • DNA polymerase binds to the primer to create the new DNA strand.
   • Can only add bases in the 5' to 3' direction.

Leading and Lagging Strands

• Leading Strand
  • Synthesized continuously.
  • DNA polymerase adds bases one by one in the 5' to 3' direction.
• Lagging Strand
  • Synthesized in small chunks called Okazaki fragments.
  • RNA primers are added to start each fragment.
  • DNA polymerase adds bases in the 5' to 3' direction for each fragment.

Finalization of DNA Replication

1. Removal of RNA Primers
   • Exonuclease removes all RNA primers from both DNA strands.
2. Filling in Gaps
   • Another DNA polymerase fills in gaps left by removed primers.
3. Sealing Fragments
   • The enzyme DNA ligase seals all fragments to form a continuous double strand.

Semi-Conservative Nature

• DNA replication is described as semi-conservative.
• Each new DNA molecule consists of one old (conserved) strand and one new strand.