DNA Replication

1
DNA Replication

• During mitosis

2
DNA by Watson Crick Franklin

• When discovered, the structure suggested how DNA
  was able to replicate
• The H-bonds between complementary bases break
• This allows the DNA to unzip
• Each DNA strand then acts as a template to build
  the complimentary strand
• This results in two identical DNA molecules one
  for each daughter cell

3
That process is called Semiconservative
Replication
Text page 217, figure 1 a b
4
DNA Strand Separation

• Replication begins when proteins bind at a
  specific site on the DNA known as the origin of
  replication (ori).
• Eukaryotic replication is similar to prokaryotic
  replication but more complex
• The closed circular DNA of prokaryotes usually
  only has one origin of replication (ori)
• Linear eukaryotic DNA has multiple oris

5
DNA Strand Separation

• The strands cannot simply be pulled apart because
  they are held together by hydrogen bonds and
  twisted around each other in the double helix.
• Specific enzymes
• work together to
• expose the DNA
• template strands

6
DNA Strand Separation

• Text page 219, figure 3.
• --The d.s. DNA is unwound by helicase.
• -- The single stranded binding proteins (SSBs)
  bind to the exposed bases to prevent them from
  annealing.

7
DNA Strand Separation

• What is involved?
• DNA helicase unwinds the double helix by
  breaking the H-bonds at the replication fork.

Replication fork region where enzymes
replicating DNA bind to an untwisted, s.s. DNA
strand.
8
DNA Strand Separation

• Single-stranded Binding Proteins (SSBs) bind the
  exposed DNA template strands to block new H-bonds
  that would re-join the strands

9
DNA Strand Separation

• DNA gyrase relieves tension from the unwinding
  of the DNA strands during bacterial replication.
  It cuts nicks in both strands of DNA, allowing
  them to swivel around one another and then
  resealing the cut strands.
• Similar enzymes perform the task in eukaryotes.

10
Replication

• Replication begins in 2 directions from the ori
  s as a region of the DNA is unwound.
• DNA replication proceeds toward the direction of
  the replication fork (leading strand) on one
  strand and away from the fork (lagging strand) on
  the other strand.
• In eukaryotes, when 2 replication forks are too
  near, a replication bubble forms

11
Text page 220, figure 4
12
Building Complimentary Strands

• In prokaryotes, there are 3 enzymes known to
  function in replication repair
• DNA polymerase I, II III
• In eukaryotes,
• there are 5 enzymes
• known to function
• in replication repair

13
Building Complimentary Strands in prokaryotes

• DNA polymerase III builds the complimentary
  strand of DNA
• It only functions under certain conditions
• DNA polymerase III adds complimentary nucleotides
  (deoxyribonucleoside triphosphates) in the 5 to
  3 direction, using RNA primers as starting
  points
• The segments are called Okazaki fragments

14
Text page 221, figure 7 a
15
Building Complimentary Strands in prokaryotes

• RNA primers are synthesized by primase and are
  temporary
• The leading strand (uses 3-5 template) is
  synthesized continuously
• The lagging strand (uses 5-3 template) is
  synthesized discontinuously in short fragments

Text page 220, figure 5
16
Building Complimentary Strands in prokaryotes

• DNA polymerase I removes the RNA primers from the
  leading strand and fragments from the lagging
  strand and replaces them with the appropriate
  deoxyribonucleotides.

Text page 221, figure 7b
17
Building Complimentary Strands in prokaryotes

• DNA ligase joins the Okazaki fragments into one
  strand on the lagging strand of DNA through the
  formation of a phosphodiester bond.

Text page 221, figure 7 c
18
Building Complimentary Strands in prokaryotes

• As the 2 new strands of DNA are synthesized, 2
  d.s. DNA molecules are produced that
  automatically twist into a helix.
• TAKE A MOMENT AND VISUALIZE THIS

19
Text page 222, figure 8
20
Guided Practice Activity
21
Text page 222, figure 8
22
DNA Repair

• DNA polymerase III and DNA polymerase I proofread
  the newly synthesized DNA strands.
• When mistakes occur, either enzyme can function
  as an exonuclease.
• The enzyme backtracks and excises the incorrectly
  paired nucleotide
• Then it continues forward adding nucleotides to
  the complimentary strand

23
DNA Repair

• Repairs must be made immediately to avoid errors
  being copied in subsequent replications.
• Errors missed by proofreading can be corrected by
  one of several repair mechanisms that operate
  after the completion of DNA replication.

24
DNA Replication Repair

• Summary page 222

Homework -page 223, q 1, 2 (due
tomorrow) -something, in the format of your
choice, describing a)structure of DNA
b)replication c)methods of repair following
an error.