1. Helicase unwinds the parental double helix at the origin of replication→ forms a Y-shaped replication fork
  • Origin of replication: short, stretch of DNA with a specific sequence of nucleotides
  1. Single-stranded binding protein attaches to each strand of uncoiled DNA to keep separate
  2. As helicase unwinds the DNA → forces the double helix to twist → group of enzymes topoisomerases break and rejoin the double helix → allow twists to unravel and prevent knots
  3. Primase: an enzyme that initiates DNA replication at the origins of replication by placing an initial, short RNA nucleotide strand (RNA primer) using parental DNA as a template
  • Primase also slows replication fork
  • Need primase and primers bcuz DNA Polymerase can only attach to 3’ end of an already existing complementary strand → belongs elongation of new DNA at replication fork by assembling new (complementary) strand in the antiparallel 5’ → 3’ direction
  • Leading strand and every Okazaki fragment on lagging strand must begin with primer
  1. Since DNA consists of two opposing DNA strands, uncoiled DNA consists of 3’ → 5’ template strand and 5’ → 3’ template strand
  2. Leading strand: For the 3’ → 5’ strand, replication occurs continuously DNA polymerase moves towards the replication fork
  3. Lagging Strand: For the 5’ → 3’ template strand, replication occurs discontinuously as DNA polymerase moves away from the uncoiling replication fork

This is bcuz it can assemble nucleotides only as it moves in 5’ → 3’  direction; takes more time to assemble

  • After each complementary segment is assembled & DNA pol III reaches next RNA primer it must return back to the replication fork to begin assembling the next segments                                                                                                 
    • Okazaki fragments: Short segments of complementary DNA; have 5′ RNA nucleotides & DNA nucleotides 3′
  1. RNA nucleotides of RNA Primer are later replaced with DNA by DNA pol 1
  2. DNA ligase joins the sugar-phosphate backbones of Okazaki fragments and closes up gaps thru covalent bonds

Antiparallel Elongation

  • Energy for elongation provided by two additional phosphates that are attached to each nucleotide (total of 3 attached to nitrogen base)

                 Breaking the bonds that holds extra two provides chemical energy for process

Replication of Telomeres

  • Big problem when replication reaches the end of DNA strand
    • Eukaryotes can’t complete 5’ end of lagging strand bcuz last primer removed and no 3’ end for DNA pol to add DNA
  • Result: DNA loss → shorter and shorter DNA molecules with uneven ends → can trigger apoptosis
  • To solve this problem telomerase attaches to the end of the template strand and extends template by adding telomeres
  • Telomeres: noncoding, special DNA sequence, 5’-TTAGGG-3
  • Telomere Functions: Allows elongation of lagging strand to continue, stops staggered ends, and postpones loss of DNA in replicated       chromosomes
    • Telomerase activity declines as cells age → once stops, chromosome becomes shorter with each replication → eventually important DNA at end of the chromosome is lost → nonfunctional daughter cells
    • Telomerase activity high in cancer cells and tumors → prolongs life

DNA Proofreading and Repair

  • DNA replication error low bcuz of base pairings and DNA polymerase but not perfect so cells have mechanisms to repair errors
  • w/o mechanisms → accumulate cancer-causing errors                                                                                                 
  1. Proofreading: Polymerase proofreads newly made DNA, replacing any incorrect nucleotides with the correct ones                 
  2. Mismatch Repair Proteins: other enzymes correct errors in base pairing
  3. Excision repair proteins: nucleases identify & cut out damaged DNA → DNA pol 1 replaces → DNA ligase joins

Error rate after proofreading is low but not 0

  • Mutations may be passed onto next generations
  • Leads to genetic variation ⇒ natural selection