DNA Structure and Replication

Structure of DNA

• DNA is a molecule consisting of two strands twisted into a double helix shape.
• Each strand consists of a sequence of four chemical bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
• The strands are complementary:
  • T pairs with A
  • C pairs with G
• Strands have directional ends:
  • 5' end
  • 3' end
• Strands run in opposite directions.

DNA Replication Process

1. Separation of Strands

   • Unzipping of DNA strands is facilitated by the enzyme helicase.
   • Results in the formation of a replication fork.

2. Formation of New Strands

   • Each of the separated strands serves as a template for a new strand.
   • Initiation
     • Enzyme primase begins the process, creating an RNA primer.
     • The primer serves as a starting point for DNA synthesis.
   • Elongation
     • DNA polymerase binds to the primer.
     • Adds DNA bases from the 5’ end to the 3’ end.

3. Differences in Strand Synthesis

   • Leading Strand
     • Synthesized continuously in the 5’ to 3’ direction.
   • Lagging Strand
     • Synthesized in segments called Okazaki fragments.
     • Requires multiple RNA primers, each initiating a new fragment.

4. Completion of Replication

   • Enzyme exonuclease removes RNA primers.
   • DNA polymerase fills in the gaps with DNA.
   • DNA ligase seals all fragments into a continuous double strand.

Nature of DNA Replication

• Described as semi-conservative.
• Each new DNA molecule consists of one old strand and one new strand.