AP Biology 2007-2008
Synthesis Of DNA Presentation By G.H Ngubane
07 March 2014@
University of Johannesburg
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DNA synthesis occurs by the process of replication.
During replication, each of the two parental strands of DNA serves as a template for the synthesis of aComplementary strand.
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Each molecule generated by the replication process contains
one intact parental strand and one newly synthesized strand.
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In eukaryotes, DNA replication occurs during the S phase of the cell cycle
The cell divides during the next phase (M), and each daughter cell receives an exact copy of the DNA of the parent cells.
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Watson and Crick1953 article in Nature
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Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick
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Directionality of DNA You need to
number the carbons! it matters!
OH
CH2
O
4
5
3 2
1
PO4
N base
ribose
nucleotide
This will beIMPORTANT!!
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The DNA backbone Putting the DNA
backbone together refer to the 3 and 5
ends of the DNA the last trailing carbon
OH
O
3
PO4
base
CH2
O
base
OPO
C
O–O
CH2
1
2
4
5
1
2
3
3
4
5
5
Sounds trivial, but…this will be
IMPORTANT!!
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Anti-parallel strands Nucleotides in DNA
backbone are bonded from phosphate to sugar between 3 & 5 carbons DNA molecule has
“direction” complementary strand runs
in opposite direction
3
5
5
3
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Bonding in DNA
….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalentphosphodiester
bonds
hydrogenbonds
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Base pairing in DNA Purines
adenine (A) guanine (G)
Pyrimidines thymine (T) cytosine (C)
Pairing A : T
2 bonds C : G
3 bonds
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Copying DNA Replication of DNA
base pairing allows each strand to serve as a template for a new strand
new strand is 1/2 parent template & 1/2 new DNA
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DNA Replication Large team of enzymes coordinates replication
Let’s meetthe team…
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Replication: 1st step Unwind DNA
helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins
single-stranded binding proteins replication fork
helicase
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DNAPolymerase III
Replication: 2nd step
But…We’re missing
something!What?
Where’s theENERGY
for the bonding!
Build daughter DNA strand add new
complementary bases DNA polymerase III
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Energy of Replication The nucleotides arrive as nucleosides
DNA bases with P–P–P P-P-P = energy for bonding
DNA bases arrive with their own energy source for bonding
bonded by enzyme: DNA polymerase III
ATP GTP TTP CTP
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Adding bases can only add
nucleotides to 3 end of a growing DNA strand need a “starter”
nucleotide to bond to
strand only grows 53
DNAPolymerase III
DNAPolymerase III
DNAPolymerase III
DNAPolymerase III
energy
energy
energy
Replication energy
3
3
5B.Y.O. ENERGY!
The energy rulesthe process
5
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Limits of DNA polymerase III can only build onto 3 end of
an existing DNA strand
Leading & Lagging strands
5
5
5
5
3
3
3
53
53 3
Leading strand
Lagging strand
Okazaki fragments
ligase
Okazaki
Leading strand continuous synthesis
Lagging strand Okazaki fragments joined by ligase
“spot welder” enzyme
DNA polymerase III
3
5
growing replication fork
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DNA polymerase III
Replication fork / Replication bubble
5
3 5
3
leading strand
lagging strand
leading strand
lagging strandleading strand
5
3
3
5
5
3
5
3
5
3 5
3
growing replication fork
growing replication fork
5
5
5
5
53
3
5
5lagging strand
5 3
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DNA polymerase III
RNA primer built by primase serves as starter sequence
for DNA polymerase III
Limits of DNA polymerase III can only build onto 3 end of
an existing DNA strand
Starting DNA synthesis: RNA primers
5
5
5
3
3
3
5
3 53 5 3
growing replication fork
primase
RNA
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DNA polymerase I removes sections of RNA
primer and replaces with DNA nucleotides
But DNA polymerase I still can only build onto 3 end of an existing DNA strand
Replacing RNA primers with DNA
5
5
5
5
3
3
3
3
growing replication fork
DNA polymerase I
RNA
ligase
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DNA polymerases DNA polymerase III
1000 bases/second! main DNA builder
DNA polymerase I 20 bases/second editing, repair & primer removal
DNA polymerase III enzyme
Arthur Kornberg1959
Roger Kornberg2006
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Editing & proofreading DNA 1000 bases/second =
lots of typos!
DNA polymerase I proofreads & corrects
typos repairs mismatched bases removes abnormal bases
repairs damage throughout life
reduces error rate from 1 in 10,000 to 1 in 100 million bases
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Fast & accurate! It takes E. coli <1 hour to copy
5 million base pairs in its single chromosome divide to form 2 identical daughter cells
Human cell copies its 6 billion bases & divide into daughter cells in only few hours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle
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The importance of nucleotide sequence
The sequence of nucleotides form the unique genetic information of an organism.
The more closely related two organisms are, the more alike the sequence of nucleotides in the DNA will be.
Ex: Gel Electrophoresis
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Gel Electrophoresis
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REACTION:oThe DNA occurs simultaneously
forming sister chromatids.oNucleotides should always be in pair.oNucleotides are held together with
loose hydrogen bonds.oEvery cell in our body has the same
copy of DNA, and the DNA will copy itself trillion of times in our lifetime.
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Structure of a nucleotideA nucleotide is made
of 3 components: A Pentose sugar This is a 5 carbon
sugar The sugar in RNA
is ribose. The sugar in DNA
is deoxyribose.
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Structure of a nucleotide A Phosphate group Phosphate groups
are important because they link the sugar on one nucleotide onto the phosphate of the next nucleotide to make a polynucleotide.
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Structure of a nucleotide A Nitogenous base
In DNA the four bases are: Thymine Adenine Cytosine Guanine
In RNA the four bases are: Uracil Adenine Cytosine Guanine
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Which came first … The language ?
The storage medium ? The translation machinery ?
The transcription machinery ?
The transfer medium ?
The letters ?
The ATP power plant ?
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Every component used in RNA transcription is made by RNA transcription.
ATP Synthase
tRNA SynthaseRNA PolymeraseRibosomes
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Even if every component is present and in it’s place. Once a cell dies it will not spontaneously revive. It’s order begins to decay.
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The chemicals, components and arrangements necessary for life spontaneously disassemble if the processes of a living cell which maintain order cease to function.
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Acknowledgements 1. Ramesh Mahindrakar
2. RAD DAD GONE MAD Doug Hove
3. MBBS IMS MSU
4. Claudeaa
5. Valerie Evans
To view their work click on their names or see links in the
reference list.
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Reference List1.DNA code of life by Ramesh Mahindrakar.
Working at G.E. SOCIETY'S J.S.S.COLLEGE GOKAK
On Nov 08, 2013
http://www.slideshare.net/RameshMahindrakar/ii-puc-dna?v=qf1&b=&from_search=2
2.The DNA Code by RAD DAD GONE MAD Doug Hove. Mechanic Electrian / Fundamental Independent Baptist at Pleasant Valley Baptist Church
On Jan 24, 2008
http://www.slideshare.net/CreationCD/the-dna-code?v=qf1&b=&from_search=7
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Reference List3.Bio-Chem synthesis of DNA by MBBS IMS MSU
On Jun 21, 2010
http://www.slideshare.net/ananthatiger/biochem-synthesis-of-dna?v=qf1&b=&from_search=9
4. DNA replication by Claudeaa
On Mar 12, 2012
http://www.slideshare.net/claudeaa/dna-replication-11970385?v=default&b=&from_search=2
AP Biology
Reference List
5. Chapter 12.2 DNA replication by Valerie Evans
On Dec 01, 2011
http://www.slideshare.net/evansv1/chapter-122-dna-replication
To view my Slide Share PowerPoint presentation click here slide share PowerPoint
Or click on this link http://www.slideshare.net/Gordon-3