DNA
The Molecule of Life:
Replication
Replication:

Why?


When cells replicate, each new cell needs it’s
own copy of DNA.
Where?
Nucleus in Eukaryotes.
 Cytosol in Prokaryotes

Replication:

When?


What?


S phase of cell cycle (S for synthesis, during
interphase)
Many proteins involved as enzymes: major one
is DNA Polymerase
How?
Replication (def’ns)

Using one side of the
DNA strand as a
TEMPLATE to make a
complementary side.
Replication - (def’ns-cont’d)
Template – parental strand
 Complementary copy – daughter strand
 Both sides of the DNA double strand do this.
 End result – two identical double helixes
(helices).

DNA Replication is
Semiconservative
Each double helix has one
parent strand that is
conserved and one daughter
strand that is new
 The new strand forms by
base-pairing that is
complementary to the parent
strand. (A-T and C-G)


Meselson and Stahl Experiment
http://preuniversity.grkraj.org/html/10_MOLECULAR_
BIOLOGY.htm
Semiconservative:
http://www.cpsd.us/CRLS/LC_R/classrooms/AUGUSTINE/DNA_structure_function/index_files/frame.html
Replication: Here’s How

How?
Enzymes!
 5’3’ directionality
 Starts with RNA primer
 Leading Strand
 Lagging Strand

 Okasaki
Fragments
Sequence determined by base pairing
 DNA Replication Animation

Replication: Prerequisites

Parent DNA needs to unwind and “unzip”
The hydrogen bonds between the paired bases
break.
 RECALL hydrogen bonds are not really bonds,
they are forces between atoms of H and either N
or O (for DNA and RNA).
 Weaker than covalent bonds in rungs

 Phosphate/sugar

bond
Nucleotide bases are always present in the
nucleus
Replication Enzymes
1.
2.
Helicase: the enzyme required to unwind
and split the H-bonds in the DNA double
helix – exposing the bases.
Primase is the enzyme that lays down the
RNA primer (establishes starting point)

Needs a “primer” or a place to start from—they cannot
pick up the first two required nucleotides unless they
have the RNA primer (primase).
DNA Polymerase:
3.
4.
DNA Polymerase III recognizes the
exposed bases and matches them up with
free, complementary nucleotides (H bonds).
The enzyme then bonds the sugars and
phosphates together to form the backbone
of the new strand
Proofreading
5. Makes only one mistake every 10^8 or
10^11 bases it adds. Has proof-reading
abilities!



A single strand of DNA has about 10^7 –
10^11 base pairs
DNA replication occurs fairly quickly; as many
as 4000 nucleotides per second are replicated.
This helps explain why bacterial cells, under
ideal conditions, can reproduce in 20 minutes.
Proofreading (cont’d)
6. DNA polymerase I removes the RNA
primers when done and fills in the RNA gaps
with DNA.
 DNA ligase seals any nicks in the DNA by
linking up the deoxyribose to the
phosphate to seal the new DNA strand
together.
 There are a few others, but this is enough
for now.
Enzyme Summary
http://www.nature.com/nature/journal/v421/n6921/box/nature01407_bx1.html
Deoxy vs ribose sugar
http://www.mun.ca/biology/scarr/iGen3_02-07.html
5’ to 3’ Direction
http://www.sciencegeek.net/Biology/review/U5Storyboard.htm
5’ to 3’
http://click4biology.info/c4b/7/pro7.2.htm#one
Animation
Replication is 5’-3’

Replication is always 5’ to 3’. But this
means one strand is made in one long
segment but the other ends up in
fragments…
http://www.britannica.com/EBchecked/media/110068/In-semiconservativeDNA-replication-an-existing-DNA-molecule-is-separated
One strand: Continuous

One strand of DNA is synthesized
continuously from 5’ to 3’ end.
This strand is called the leading strand
 It is called this because it requires only one
primer
 Replication proceeds in the same
direction/towards as the replication fork in the
DNA strand
 At the end of the strand, a TELOMERE is added
by another enzyme.
 Telomere: none-coding redundant sequences.

Other: Discontinuous
The other strand of DNA is synthesized
discontinuously – in pieces – because it
also needs to be made 5’ to 3.
 This strand is called the lagging strand.
 The many small fragments are called
Okazaki fragments

Other: Discontinuous
It requires multiple primers because it
replications AWAY from the replication fork
 As a fork opens up, a RNA primer attaches,
DNA polymerase adds bases 5’ to 3’ until it
reaches the “other” RNA primer.
 This process continues as the fork opens up
until the end of the DNA.
 At the end, another enzyme adds a
telomere.

http://www.esb.utexas.edu/mabrybio211/chapter05/ch5.htm)
DNA polymerase assembles
new complementary strands
http://dnainfo.wikispaces.com/DNA+Replication
SUMMARY and demo

How?





Enzymes make it happen correctly. There are many
5’3’ directionality for both strands
Starts with RNA primer for DNA polymerase to attach
Leading Strand is continuous
Lagging Strand is discontinuous





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Okasaki Fragments are connected by polymerase I
Sequence determined by complementary basepairing
enzyme roles
Replication animation-slow
Animation-johnkyrk
Animation-fast
Sources of info, Assignments

Good sources of info:
Your textbook! Chapter 11 pg. 280-287
 Especially pg 286-287 Copying DNA Fig 11.5


Assignment
Worksheets
 Text pg. 287 Q 1-5 ?


The internet has a LOT of good animations
you could view. Google DNA replication
animations.