W3L5 - DNA replication and recombination

• Summary

  DNA replication:

  • Copying of DNA
  • Occurs during meiosis and mitosis

  DNA recombination:

  • Exchange of genetic material
  • Helps drive evolution
  • Typically occurs in meiosis
  • Can occur as part of DNA repair
• DNA replication:

  • Universal feature of cells
  • All cells maintain their DNA material from one generation to the next
  • Replication = make new copy
  • Repair = repairing damage
  • To have evolution, DNA must be 'mixed'
  • Recombination and mutation
  • Must be accurate, process is simple:
  • 2 DNA strands of double helix are separated
  • Each strand used as template to make a new copy of the 'missing' strand
  • This process is semi conservative
  • Each daughter strand has one parental strand and one new
  • Accuracy of copying is achieved by matching of base pairs
• Semi Conservative replication:

  A) template

  B) complementary

  
• 1. DNA replication - dNTPs

  A) glycosidic

  B) purines

  C) pyrimidines

  D) nucleoside

  
• 2. DNA helicase

  • DNA is a double helix
  • Needs to be unwound and separated to allow DNA polymerase to gain access to make a copy
  • Process is carried out by an enzyme called DNA helicase

  
• Summary of key components of DNA replication

  1. The four different deoxynucleoside triphosphates (nucleoside triphosphate) - dNTPs [Deoxyadenosine triphosphate (dATP), deoxyguanosine triphosphate (dGTP), deoxycytidine triphosphate (dCTP), deoxythymidine triphosphate (dTTP) ]
  2. DNA helicase - the enzyme that unwinds DNA
  3. Single-stranded DNA binding proteins
  4. DNA polymerase - makes the copy
  5. DNA topoisomerase - helps with unwinding
  6. DNA primase - helps with the copying
  7. DNA ligase - repairs DNA single strand nick
• 3. Single-stranded DNA binding proteins

  • Bind to the lagging strand to prevent DNA from sticking to itself and blocking copying

  A) polymerase

  B) binding

  
•  4. DNA polymerase

  • DNA synthesis is carried out (5’ to 3’) by a nucleotide polymerising enzyme
  • Uses deoxynucleoside triphosphates
  • DNA polymerase adds about 500 to 1,000 bases per second
  • As DNA polymerase creates the new strand it checks that it has added the correct base (proofreading)
  • If error is found, replication stops and the error is corrected
  • If the polymerase is adding 500 - 1,000 bases per second, the helicase must be unwinding DNA at the same rate

  
• 5. DNA topoisomerase I

  • DNA supercoiling and DNA topoisomerase solve the DNA winding problem
  •  BUT topoisomerase introduces new problem
  • topoisomerase cuts and repairs the DNA phosphodiesterase bond

  
• 4. DNA polymerase - another problem

  • DNA synthesis is carried out in the 5’ to 3’ direction
  • DNA anti-parallel
  • Other DNA strand is going the wrong way for replication

  
• 4. DNA polymerase - and another problem

  • DNA polymerase can't start making a copy of DNA
  • Needs a 3' OH group to attach the nucleoside triphosphate
  • DNA polymerase needs a primer

  
• 6. DNA primase

  • builds a short RNA primer on the DNA template
  • RNA primer provides the 3’ OH group for the DNA polymerase to add the nucleoside triphosphate
  • RNA is later corrected

  
• 4. DNA polymerase - the final problem

  A) leading

  B) lagging

  
• 7. DNA ligase - seals the nicks

  A) leading

  B) lagging

  
• DNA replication fork

  • the lagging strand also needs
  • single-strand DNA-binding proteins to prevent base-pairing
  • DNA primase to add RNA as a start point for DNA polymerase

  
• Summary for DNA replication:

  • is the copying of DNA
  • is a complex process that requires seven key components
  • dNTPs, DNA helicase, single-stranded DNA binding proteins, DNA topoisomerase, DNA polymerase, DNA primase and DNA ligase
  • multiple copy sites
  • leading strand, lagging strand and okazaki fragments