kuliah ke tiga (dna replication)
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History of DNA
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History of DNA
Early scientists thought proteinwas the cells hereditary material
because it was more complexthan DNA
Proteins were composed of 20
different amino acids in longpolypeptide chains
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Transformation
Fred Griffith worked with virulent Sand nonvirulent R strainPneumoccocus bacteria
He found that R strain could becomevirulent when it took in DNA fromheat-killed S strain
Study suggested that DNA wasprobably the genetic material
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Griffith Experiment
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History of DNA
Chromosomes are made ofboth DNA and protein
Experiments on
bacteriophage viruses byHershey & Chase provedthat DNA was the cellsgenetic material
Radioactive 32P was injected into bacteria!
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Discovery of DNA Structure
Erwin Chargaff showed theamounts of the four bases on
DNA ( A,T,C,G) In a body or somatic cell:
A = 30.3%
T = 30.3%G = 19.5%
C = 19.9%
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Chargaffs Rule
Adenine must pair with Thymine Guanine must pair with Cytosine
The bases form weak hydrogen
bonds
G CT A
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DNA Structure
Rosalind Franklin tookdiffraction x-ray photographsof DNA crystals
In the 1950s, Watson & Crickbuilt the first model of DNA
using Franklins x-rays
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Rosalind Franklin
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DNA Structure
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DNATwo strands coiled called a
double helix
Sides made of a pentose sugar
Deoxyribose bonded tophosphate (PO4) groups byphosphodiester bonds
Center made of nitrogenbases bonded together byweak hydrogen bonds
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DNA Double Helix
NitrogenousBase (A,T,G or C)
Rungs of ladder
Legs of ladder
Phosphate &Sugar Backbone
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Helix
Most DNA has a right-hand twistwith 10 base pairs in a completeturn
Left twisted DNA is called Z-DNAor southpaw DNA
Hot spots occur where right and
left twisted DNA meet producingmutations
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DNA
Stands for Deoxyribonucleic acid
Made up of subunits callednucleotides
Nucleotide made of:
1. Phosphate group
2. 5-carbon sugar3. Nitrogenous base
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DNA Nucleotide
O=P-OO
PhosphateGroup
NNitrogenous base(A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
O
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Pentose Sugar
Carbons are numbered clockwise 1 to5
CH2
O
C1
C4
C3 C2
5
Sugar(deoxyribose)
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DNA
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
P
O
O
O
1
2 3
4
5
5
3
5
3
G C
T A
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Antiparallel Strands
One strand of DNAgoes from 5 to 3(sugars)
The other strand isopposite indirectiongoing 3
to 5 (sugars)
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Nitrogenous Bases
Double ring PURINES
Adenine (A)
Guanine (G)
Single ring PYRIMIDINES
Thymine (T)Cytosine (C) T or C
A or G
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Base-Pairings
Purines only pairwithPyrimidines
Three hydrogen bonds
required to bond Guanine &Cytosine
CG
3 H-bonds
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T A
Two hydrogen bonds are
required to bond Adenine &Thymine
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The bases always pair up in the same way
Adenine forms a bond with Thymine
and Cytosine bonds with Guanine
Bonding 1
10
Adenine Thymine
Cytosine Guanine
Bonding 2 11
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PO4
PO4
PO4
thymine
PO4
PO4
PO4
PO4
adenine
cytosine
PO4
guanine
Bonding 2 11
Pairing up 12
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PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
Pairing up 12
13
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The paired strands are coiled into a spiral called
A DOUBLE HELIX
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sugar-phosphate
chain
bases
THE DOUBLEHELIX
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replication 16
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Before a cell divides, the DNA strands unwind
and separate
Each strand makes a new partner by adding
the appropriate nucleotides
The result is that there are now two double-stranded DNA
molecules in the nucleus
So that when the cell divides, each nucleus
contains identical DNA
This process is called replication
replication 16
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PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
The strands
separate
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18
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PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
Each strand builds up its partner by adding
the appropriate nucleotides
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Genetic code 1 19
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The sequence of bases in DNA forms the
Genetic Code
A group of three bases (a triplet) controls
the production of a particular amino acid in
the cytoplasm of the cell
The different amino acids and the order in which they
are joined up determines the sort of protein being
produced
Genetic code 1 19
Genetic code 2 20
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Ser-Cyst-Val-Gly-Ser-Cyst Ala
ValVal-Cyst-Ser-Ala-Ser-Cyst-Gly
Val- Cyst-Ala-Ala-Ser-Gly
This is a small, imaginary protein molecule showing
how a sequence of 5 different amino acids coulddetermine the shape and identity of the molecule
Each amino acid (Serine, Cysteine, Valine, Glycine and
Alanine) is coded for by a particular triplet of bases
Genetic code 2 20
Coding 21
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For example
Cytosine
Adenine Codes for Valine
Cytosine (C)
Guanine (G)
Adenine (A)
Codes for Alanine
Thymine
Coding 21
Triplet code 22
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This is known as the triplet code
Each triplet codes for a specific amino acid
CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA -
Ala Val Gly Gly Arg Pro Leu Gly
Ala Val Gly Gly Arg Pro Leu Gly
The amino acids are joined together in the correct
sequence to make part of a protein
Triplet code 22
DNA and enzymes 23
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The proteins build the cell structures
They also make enzymes
The DNA controls which enzymes are made and
the enzymes determine what reactions take place
The structures and reactions in the cell determine
what sort of a cell it is and what its function is
So DNA exerts its control through the enzymes
DNA and enzymes 23
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Question:
If there is 30% Adenine,
how much Cytosine ispresent?
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Answer:
There would be 20% CytosineAdenine (30%) = Thymine (30%)
Guanine (20%) = Cytosine (20%)Therefore, 60% A-T and 40% C-G
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DNA Replication
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Replication Facts
DNA has to be copied before acell divides
DNA is copied during the S orsynthesis phase of interphase
New cells will need identical
DNA strands
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Synthesis Phase (S phase)
S phase during interphase of the cellcycle
Nucleus of eukaryotes
Mitosis-prophase-metaphase-anaphase-telophase
G1 G2
Sphase
interphase
DNA replication takesplace in the S phase.
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DNA Replication
Begins atOrigins of Replication
Two strands open forming Replication
Forks (Y-shaped region) New strands grow at the forks
ReplicationFork
Parental DNA Molecule
3
5
3
5
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DNA Replication
As the 2 DNA strands open at theorigin, Replication Bubbles form
Prokaryotes (bacteria) have a singlebubble
Eukaryotic chromosomes have MANYbubbles
Bubbles Bubbles
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DNA Replication
Enzyme Helicase unwinds andseparates the 2 DNA strandsby breaking the weak
hydrogen bondsSingle-Strand Binding
Proteins attach and keep the 2
DNA strands separated anduntwisted
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DNA Replication
Enzyme Topoisomerase attaches tothe 2 forks of the bubble to relievestress on the DNA molecule as it
separatesEnzyme
DNA
Enzyme
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DNA Replication
Before new DNA strands can form,there must be RNA primers presentto start the addition of new
nucleotides Primase is the enzyme that
synthesizes the RNA Primer
DNA polymerase can then add thenew nucleotides
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DNA Replication
DNA polymerase can only addnucleotides to the 3 end of the DNA
This causes the NEW strand to be built
in a 5 to 3 direction
RNAPrimerDNA PolymeraseNucleotide
5
5 3
Direction of Replication
R b HOW h C b
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Remember HOW the CarbonsAre Numbered!
O
O=P-OO
PhosphateGroup
N
Nitrogenous base(A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
Remember the Strands are
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Remember the Strands areAntiparallel
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
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Synthesis of the New DNAStrands
The Leading Strand is synthesized asa single strand from the point oforigin toward the opening
replication fork
RNAPrimerDNA PolymeraseNucleotides
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5
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Synthesis of the New DNA Strands
The Lagging Strand is synthesized discontinuouslyagainst overall direction of replication
This strand is made in MANY short segments It isreplicated from the replication fork toward the
origin
RNA Primer
Leading Strand
DNA Polymerase
5
5
3
3
Lagging Strand
5
5
3
3
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Lagging Strand Segments
Okazaki Fragments - series of shortsegments on the lagging strand
Must be joined together by an
enzyme
Lagging Strand
RNA
Primer
DNAPolymerase
3
3
5
5
Okazaki Fragment
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Joining of Okazaki Fragments
The enzyme Ligase joins the Okazakifragments together to make onestrand
Lagging Strand
Okazaki Fragment 2
DNA ligase
Okazaki Fragment 1
5
5
3
3
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Replication of Strands
ReplicationFork
Point of Origin
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Proofreading New DNA
DNA polymerase initially makes about 1in 10,000 base pairing errors
Enzymes proofread and correct thesemistakes
The new error rate for DNA that hasbeen proofread is 1 in 1 billion basepairing errors
Se i o e ti e Model of
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Semiconservative Model ofReplication
Idea presented by Watson & Crick
The two strands of the parental moleculeseparate, and each acts as a template for a
new complementary strandNew DNA consists of 1 PARENTAL
(original) and 1 NEW strand of DNA
Parental DNA
DNA Template
New DNA
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DNA Damage & Repair
Chemicals & ultraviolet radiation damagethe DNA in our body cells
Cells must continuously repair DAMAGEDDNA
Excision repair occurs when any of over 50repair enzymes remove damaged parts ofDNA
DNA polymerase and DNA ligase replaceand bond the new nucleotides together
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Question:
What would be thecomplementary DNA strandfor the following DNAsequence?
DNA 5-CGTATG-3
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Answer:
DNA 5-CGTATG-3
DNA 3-GCATAC-5
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