Kuliah Ke Tiga (DNA Replication)

7/28/2019 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

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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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Kuliah Ke Tiga (DNA Replication)

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