DNA Replication

• Okazaki fragments are joined together by ligases to form the complete lagging strand.
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as short fragments called Okazaki fragments.
• DNA polymerase III is the primary enzyme responsible for replicating most of the genome.
• The DNA helicase enzyme unwinds the double-stranded DNA at the origin of replication.
• The DNA replication process occurs during the S phase of the cell cycle.
• RNA primers are added at the origin of replication and serve as starting points for DNA synthesis.
• DNA polymerase III requires a primer to initiate DNA synthesis.
• Single-stranded binding proteins (SSB) bind to single-stranded regions of DNA during replication.
• Topoisomerase relaxes supercoiled DNA ahead of the replication fork.
• Replication begins at specific sites on the chromosomes known as origins of replication.
• DNA ligase joins adjacent segments of DNA together by forming phosphodiester bonds between them.
• DNA replication


  The process in which DNA makes a duplicate copy of itself from a parent DNA molecule

  
• When does DNA replication occur?

  S phase of interphase

  
• 3 models of DNA replication

  semiconservative, conservative, dispersive

  
• dispersive replication


  replication results in both original and new DNA dispersed among the daughter strands

  
• semi-conservative replication


  Method of DNA replication in which parental strands separate and produce molecules of DNA with one parental and one new DNA strand, each newly synthesized DNA is composed of a parent strand and a new strand

  
• conservative replication


  the parental DNA produces one exact copy of itself and the rest are completely new strands

  
• Meselson-Stahl Experiment

  Using isotope of nitrogen to change the weight of DNA N15 & N14, demonstrated that the semi-conservative model is the best description of replication.

  
• Step 1 of Meselson and Stahl's experiment

  DNA from bacteria grows in an N15 medium and spins in a centrifuge, leaving a heavy strand of DNA at the bottom of the vial as a single band
• Step 2 of Meselson and Stahl's experiment

  The DNA is transferred to an N14 medium to replicate, and the DNA appears as a single band of intermediate DNA in the middle of the vial after it is spun by the centrifuge
• Step 3 of Meselson and Stahl's experiment

  The DNA is replicated again in the N14 medium and spun in a centrifuge, and 1 band of intermediate DNA appears in the middle of the vial and 1 band of light DNA appears at the top of the vial
• intermediate strand of DNA


  a strand of DNA including one strand from the parent and another new daughter strand
• light strand of DNA


  DNA strand that only consists of the new daughter strands
• 3 phases of DNA replication

  1. Initiation2. Elongation3. Termination

  
• Initiation of DNA replication


  a portion of DNA is unwound at the origin of replication

  
• Elongation of DNA replication


  2 new strands of DNA are assembled using the parent DNA as a template

  
• Termination of DNA replication


  replication ends and DNA is separated; replication machine dismantled

  
• Where does replication start?

  origin of replication (a specific nucleotide sequence)

  
• What happens at the origin of replication?

  DNA strands unwind and form a replication bubble and replication occurs from 2 replication forks

  
• DNA helicase


  an enzyme that unwinds DNA by breaking their hydrogen bonds

  
• Replication fork


  a Y-shaped point that results when the two strands of a DNA double helix separate so that the DNA molecule can be replicated (point of separation)

  
• Single-strand binding proteins


  bind to the unpaired DNA strands and stabilize them, keeping them from re-pairing

  
• Topoisomerase


  A protein that functions in DNA replication, helping to relieve strain in the double helix ahead of the replication fork. without it, the DNA could snap

  
• Why are there multiple origins of replication?


  So more DNA can be replicated at once, which speeds up the process instead of waiting for the entire strand to break with only one origin
• RNA primers


  strands of RNA made by primase that are attached to DNA to let the DNA polymerase III know when to begin adding nucleotides

  
• Why can't DNA polymerase III initiate DNA synthesis on a bare template of single-stranded DNA?

  because it can only add a nucleotide onto a pre-existing 3' hydroxyl, so a primer is added at the beginning of the strand
• DNA polymerase III


  adds bases from nucleoplasm to the new strand in a 5' to 3' direction

  
• In which direction is DNA synthesized?

  5' to 3' direction

  
• Leading strand


  The new continuous complementary DNA strand synthesized towards the replication fork in the mandatory 5' to 3' direction.

  
• Lagging strand


  A DNA strand that is discontinuously synthesized in a 5' to 3' direction away from the replication fork.