DNA Replication

Overview of DNA Structure and Replication

• DNA Structure
  • DNA is a molecule composed of two strands twisted around each other in a double helix shape.
  • Each strand consists of a sequence of four chemical bases represented by the letters:
  • A (Adenine)
  • C (Cytosine)
  • G (Guanine)
  • T (Thymine)
  • Complementarity of Strands
  • The two strands of DNA are complementary:
    • Wherever there is a T in one strand, there is an A in the opposite strand.
    • Wherever there is a C, there is a G in the other strand.
  • Each strand has a 5' end and a 3' end.
  • The two strands run in opposite directions (antiparallel orientation).
  • This orientation is crucial for DNA replication.

DNA Replication Process

• Initiation of Replication

  • The first step in DNA replication is the separation of the two strands.
  • This process, referred to as unzipping, is facilitated by an enzyme called helicase.
  • The separation creates a replication fork, which serves as the site for the synthesis of new strands.

• Template Creation

  • The separated strands of DNA serve as templates for creating new strands of DNA.
  • An enzyme called primase initiates the process by synthesizing a small piece of RNA known as a primer.
  • The primer acts as a starting point for the construction of a new strand of DNA.

• Synthesis of New DNA Strands

  • DNA Polymerase
  • An enzyme called DNA polymerase binds to the primer and synthesizes a new strand of DNA.
  • DNA polymerase can add DNA bases only in the 5' to 3' direction.
  • Leading Strand vs. Lagging Strand
  • Leading Strand:
    • Synthesized continuously by DNA polymerase adding bases one after another in the 5' to 3' direction.
  • Lagging Strand:
    • Cannot be synthesized continuously due to its opposite orientation to the leading strand.
    • Synthesized in short sections called Okazaki fragments.
    • Each Okazaki fragment starts with its own RNA primer, followed by DNA polymerase adding bases in the 5' to 3' direction.
    • The process continues as the next primer is added further down the lagging strand, leading to the formation of additional Okazaki fragments.

• Completion of DNA Replication

  • Once the new strands of DNA are completed, an enzyme called exonuclease removes all RNA primers from both strands.
  • Another DNA polymerase enzyme fills in the gaps left by the removed primers with DNA.
  • Finally, the enzyme DNA ligase seals the fragments of DNA in both strands, resulting in the formation of a continuous double strand.

• Semi-Conservative Nature of DNA Replication

  • DNA replication is termed semi-conservative because each new DNA molecule comprises one original (old) strand and one newly synthesized strand as part of the replication products.