BIO - DNA Replication

• DNA Replication


  The process by which the double-stranded DNA is copied to produce 2 identical DNA molecules

  
• DNA Replication


  1. DNA Replication

  2. Models of Replication

  3. Origin of Replication

  4. Replication Fork

  5. Different Enzymes

  6. Where to Start Synthesizing?

  7. Direction of Replication

  
• Conservative Model


  • Two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix (100% parental DNA is retained)
• Semi-Conservative Model


  • The two strands of the parental molecule separate and function as a template for synthesis of a new complementary strand (One strand is the parental molecule while the other is new/replaced)

  
• All living organisms use the Semi-Conservative Model
• Dispersive Model


  • Each strand of both daughter molecules contain a mixture of old and new synthesized DNA (Mixture of old and new DNA in a strand)

  
• Origin of Replication


  • Single points of separation in DNA, forming a "replication bubble"
  • Accumulation of A:T pairings (A:T are easier to separate because they only have 2 hydrogen bonds unlike G:C which has 3 hydrogen bonds)
  • The more origins of replications, the faster the speed of replication will be

  
• Replication Fork


  • When replication bubbles are zoomed in, it forms a Y-shaped region
• Helicase


  • Its primary function is to unwind/unzip the DNA double helix by breaking the hydrogen bonds between complementary base pairs creating a replication fork
  • It unzips to form new DNA
  • The helicase enzyme looks for the origin of replication (sign: lots of A:T pairings)

  
• Single Stranded Binding Proteins (SSB)


  • Binds and stabilizes single-stranded DNA (note: because DNA separated is unstable & prone to degradation)
  • The two separate bonds still have H bonds (magnet-like), SSB prevents the reannealing of DNA

  
• Topoisomerase


  • Lessens the stress caused by the unzipping & twisting of the DNA
  • Topoisomerase cuts the parts where there is strain and reconnects to lessen the pressure
• Primase


  • Can start an RNA chain called a primer from scratch and add it using the parental DNA as a template
• Primer


  • Short RNA (5-10 nucleotides long)
  • The 3' end serves as the starting point for the new DNA strand
  • Provides a starting point for DNA synthesis

  
• DNA Polymerase III


  • Primary enzyme responsible for synthesizing new DNA strands during replication
  • Requires a primer to add nucleotides
  • It gets free floating nucleotides and pairs its complementary with the template
  • Adds nucleotides ONLY in the 5' to 3' direction

  
• Leading Strand


  • Synthesizes continuously
  • The direction of replication is towards the replication fork

  
• Lagging Strand


  • Synthesized discontinuously in short fragments called Okazaki fragments
  • Direction of replication is away from the replication fork
  • Once an Okazaki fragment is synthesized, DNA polymerase I removes the RNA primer and replaces it with DNA, and DNA ligase joins the adjacent Okazaki fragments, forming a continuous complementary strand

  
• DNA Ligase


  • Binds Okazaki fragments together
• DNA can only elongate in the 5' to 3' direction
• DNA replication is bidirectional because of its antiparallel structure