DNA Replication Process

Unwinding of DNA

• DNA Helicase

  • Binds to the origin of replication on double-stranded DNA.
  • Unwinds and unzips DNA by breaking hydrogen bonds between nitrogenous bases.
  • Exposes single-stranded DNA (ssDNA).

• Single-Stranded DNA Binding Proteins (SSBs)

  • Bind to exposed ssDNA regions.
  • Prevent reassociation of the DNA strands.

• DNA Gyrase (Topoisomerase)

  • Introduces negative supercoils.
  • Reduces stress from unwinding.

Synthesis of New DNA Strands

RNA Primers and Primase

• Primase
  • An RNA polymerase that creates RNA primers.
  • RNA primers initiate DNA synthesis.

Role of DNA Polymerase

• DNA Polymerase
  • Catalyzes formation of phosphodiester bonds between nucleotides.
  • Reads parent strand in 3’ to 5’ direction.
  • Synthesizes new DNA strand in 5’ to 3’ direction.
  • Releases pyrophosphate to drive replication.

Replication Fork and Directionality

• Replication Fork
  • Location of DNA helicase.
  • Movement of helicase and replication proceeds in the same direction.

Leading and Lagging Strands

• Leading Strand

  • Synthesized continuously.
  • DNA polymerase moves in the same direction as replication fork.

• Lagging Strand

  • Synthesized discontinuously in Okazaki fragments.
  • Multiple RNA primers laid down by primase.
  • DNA polymerase works backwards relative to replication fork movement.

Okazaki Fragments and DNA Ligase

• Okazaki Fragments

  • Short DNA fragments on the lagging strand.

• DNA Ligase

  • Connects Okazaki fragments by forming phosphodiester bonds.
  • Removes RNA primers and replaces them with DNA nucleotides.

Key Concepts

• DNA polymerase synthesizes only in 5’ to 3’ direction.
• Both leading and lagging strands are synthesized simultaneously.
• DNA replication involves several enzymes and steps to ensure accurate duplication of the genetic material.