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F : Genetic control
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Objectives
Todays session, well learn:
.. What are nucleotides
.. Structure & function of DNA & RNA .. DNA replication
.. transcription (DNA RNA)
.. translation (RNA Proteins)
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Recap
Remember Mitosis
What happens before the M phase
Why chromosome must divide before mitosisoccur
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DNA Replication
What is DNA replication/DNA synthesis
is the process of copying a double-stranded DNA strand
doubling DNA
during interphase (S phase of cell cycle)
There were three theories
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Possible mechanisms of DNA replication
i) The conservative model
The parental DNA remains intact
Separate daughter DNA copies are built from newmolecules of deoxyribose, phosphate & organic bases
Outcome: 1 will be an entirely new molecule
1 will be entirely the original material
Conservative Replication
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The semi-conservative model
DNA molecule splits into 2 separate strands & each of the
strands are replicated
Each of the 2 strands will have 1 strand of new material & 1strand of the original material
Semi-conservative Replication
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iii) The dispersive model
parental DNA would be broken down
nucleotides replicated before being dispersed of randomly
throughout the new molecule The new molecules will contain both the new & original
material but are randomly distributed & not necessarily in
the same amount in each molecules
Dispersive Replication
The experiments of Meselsohn & Stahl
http://highered.mcgraw-
hill.com/olc/dl/120076/bio22.swf
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Proven: DNA replicates semi-
conservatively
Each new double helix has 1 originalstrand and 1 newly synthesised strand
DNA replication semi conservative
Original Gen #1 Gen #2
Conservative15N15N 15N15N
14N14N15N15N14N14N
Semiconservative
15N15N15N14N
15N14N14N14N
Dispersive15N15N
VARIED VARIED
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DNA Replication
DNA strands are antiparallel
(like divided highway) & complementary
One strand is 5 to 3 & the complimentary strand is
3 to 5 So each strand can serve as a template for the
reproduction of the opposite strand
The template strand is preserved as a whole piece
The new strand is assembled from nucleotides This process is called semi-conservative replication
Ideally, the two resulting strands are identical
http://www.uam.es/personal_pdi/ciencias/jhermoso/documents/meselsonystahl_000.swf
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3 enzymes
Gyrase (unwinds DNA)
Helicase (breaks H-bonds) DNA Polymerase (binds free nucleotide)
DNA Ligase (joins okazaki fragments of
nucleotide)
DNA endonuclease (proof-reads new DNA double
helix)
DNA Replication
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DNA Replication (i) Unwinding of DNA double strand
Gyrase
Breaking hydrogen bonds between
complementary base pairs
Helicase
Replication forkcreated
Each DNA acts as template
DNAReplication Fork
http://highered.mcgraw-
hill.com/olc/dl/120076/micro04.swf
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DNA polymerase then binds to the single
stranded DNA & moves along the strand
free nucleotides with correct complement base
will bind to template strand
How Nucleotides are Added in DNA Replicationhttp://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf
http://www.susanahalpine.com/anim/Grolier_Anims/DNA.htm
The new strand of DNA is synthesised in
the 5 3 direction
Therefore only 1 strandcan be copied continuously
(continuous/forward
replication)
leading strand
DNA Replication (ii)
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The copying of the other strand has to keep
being started again because the DNA
polymerase moves away from the
unwinding enzyme in the 5 3 direction
This results in small gaps
(okazaki fragments)because the
DNA polymerase cannot bind the 3 endof the newly synthesized strand to the 5 end
This is called (discontinuous/backward replication)
Lagging strand
DNA Replication (ii)
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DNA Replication (iii) DNA ligase joins the okazaki fragments
together to form continuous strand
DNA endonucleaseproof-reads new DNAdouble helix
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DNA Replication (summary)
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DNA Replication
Unwinding of DNA double helixBinds to the single stranded
DNA & moves along the strand
Joins the okazaki fragments
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DNA Replication
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch09/
replicating_dna.html
Interactive DNA Rep with quiz
http://www.bioteach.ubc.ca/TeachingResources/M
olecularBiology/DNAReplication.swf
Animations
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1. Which statement about the strands of a newly replicated DNAmolecule is correct?
A Both strands are made up of newly assembled nucleotides.
B Both strands contain some nucleotides from the original molecule.
C One strand is new and the other is part of the original molecule.D The sugar-phosphate chains are conserved and new bases are
inserted between them.
2. Which statement correctly describes the transcription ofDNA?
AIt is a semi-conservative process.B It occurs at the surface of the ribosome.
C It produces messenger RNA.
DIt produces polypeptides.
Practice Questions
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3. Which structural feature of the DNA molecule varies?
A the arrangement of the sugar-phosphate groups
B the double helical arrangement
C the order of bases on a single nucleotide chain
Dthe pairing of purines with pyrimidines
4. Explain how the structure ofDNA enables it to replicate semi
conservatively.
Practice Questions
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Objectives
Next session, well learn:
.. What are nucleotides
.. Structure & function of DNA & RNA .. DNA replication
.. transcription (DNA RNA)
.. translation (RNA Proteins)
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Homework
Reading
what is gene?
what is transcription?
process of transcription
References
Jones, Fosbery, Taylor & Gregory pg 66,68-70
Taylor Green & Stout pg
Clegg & MacKean pg 205-206