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DNA and DNA and ReplicationReplication

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History History of DNAof DNA

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History of DNA

• Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA

• Proteins were composed of 20 different amino acids in long polypeptide chains

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History of DNA• Chromosomes are

made of both DNA and protein

• Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material

Radioactive 32P was injected into bacteria!

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Discovery of DNA Discovery of DNA StructureStructure

• Erwin Chargraff showed the amounts 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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Chargaff’s RuleChargaff’s Rule• AdenineAdenine must pair with

Thymine Thymine AA - T - T• GuanineGuanine must pair with

Cytosine Cytosine GG - C - C• The bases form weak

hydrogen bonds

G CT A

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Rosalind FranklinRosalind Franklin

Rosalind Franklin took diffraction x-ray photographs of DNA crystals

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Watson and Crick•In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays

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DNA DNA StructurStructur

ee

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DNADNA•Two strands coiled called

a double helix•Sides made of a pentose

sugar Deoxyribose bonded to phosphate (PO4) groups

•Center made of nitrogen bases bonded together by weak hydrogen bonds

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DNA Double HelixDNA Double Helix

NitrogenousNitrogenousBase (A,T,G or C)Base (A,T,G or C)

““Rungs of ladder”Rungs of ladder”

““Legs of ladder”Legs of ladder”

Phosphate &Phosphate &Sugar BackboneSugar Backbone

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DNADNA•Stands for

Deoxyribonucleic acid•Made up of subunits

called nucleotidesnucleotides • NucleotideNucleotide made of: made of:

1. Phosphate groupPhosphate group2. 5-carbon sugar5-carbon sugar3. Nitrogenous baseNitrogenous base

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DNA NucleotideDNA Nucleotide

OO=P-O O

PhosphatePhosphate GroupGroup

NNitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T)

CH2

O

C1C4

C3 C2

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SugarSugar(deoxyribose)(deoxyribose)

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DNADNA

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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Antiparallel Antiparallel StrandsStrands

• One strand of DNA goes from 5’ to 3’ (sugars)

• The other strand is opposite in direction going 3’ to 5’ (sugars)

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DNA DNA ReplicatiReplicati

onon

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Replication FactsReplication Facts

• DNA has to be copied DNA has to be copied before a cell dividesbefore a cell divides

• DNA is copied during DNA is copied during interphaseinterphase of mitosis of mitosis

• New cells will need New cells will need identical identical DNA strandsDNA strands

Three Phases of Replication

• Initiation: Unzipping DNA

• Elongation: Making new DNA strand

• Termination: Ends at telomeres (specific repeated bases) Zipping DNA back together.

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DNA ReplicationDNA Replication

• Begins atBegins at Origins of ReplicationOrigins of Replication• Two strands open forming Two strands open forming

Replication Forks (Y-shaped Replication Forks (Y-shaped region) region) The “unzipping of the DNA The “unzipping of the DNA strand is carried out by an enzymestrand is carried out by an enzyme DNA helicaseDNA helicase..

• New strands grow at the forksNew strands grow at the forks

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

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DNA ReplicationDNA Replication

• Enzyme Enzyme HelicaseHelicase unwinds and separates unwinds and separates the 2 DNA strands by the 2 DNA strands by breaking the breaking the weak weak hydrogen bondshydrogen bonds

• Single-Strand Binding Single-Strand Binding ProteinsProteins attach and keep the 2 DNA strands separated and untwisted

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DNA ReplicationDNA Replication• BeforeBefore new DNA strands can

form, there must be RNA RNA primersprimers present to start the addition of new nucleotides

• DNA polymerase can then add the new nucleotides

• The joining of RNA primers and DNA polymerase creates an Okazaki fragment. Only occurs on lagging strand

Leading and Lagging Strands

• Leading strand: starts at the 3’ end of the parent DNA forming the complementary strand of 5’ to 3’

• Lagging strand: is discontinuous (patchwork for DNA); requires many RNA primers for Okazaki fragments

Okazaki Fragment

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DNA ReplicationDNA Replication• DNA polymeraseDNA polymerase can only add can only add

nucleotides to the nucleotides to the 3’ end3’ end of of the DNA the DNA

• This causes the This causes the NEWNEW strand to strand to be built in a be built in a 5’ to 3’ direction5’ to 3’ direction

RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase

NucleotideNucleotide

5’

5’ 3’

Direction of ReplicationDirection of Replication

Finishing the New DNA Strands

• DNA Ligase “zips up” the lagging DNA strand– This is done by

joining the old and new strands of DNA with hydrogen bonds.

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Remember the Strands are Remember the Strands are AntiparallelAntiparallel

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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Replication of Replication of StrandsStrands

Replication Fork

Point of Origin

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Proofreading New Proofreading New DNADNA

• DNA polymerase initially DNA polymerase initially makes about makes about 1 in 10,0001 in 10,000 base base pairing errorspairing errors

• EnzymesEnzymes proofread and proofread and correct these mistakescorrect these mistakes

• The new error rate for DNA The new error rate for DNA that has been proofread is that has been proofread is 1 1 in 1 billionin 1 billion base pairing errors base pairing errors

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Semiconservative Model Semiconservative Model of Replicationof Replication

• Idea presented by Idea presented by Watson & CrickWatson & Crick• TheThe two strands of the parental

molecule separate, and each acts as a template for a new complementary strand

• New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA

Parental DNA

DNA Template

New DNA

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DNA Damage & DNA Damage & RepairRepair

• Chemicals & ultraviolet radiation damage the DNA in our body cells

• Cells must continuously repair DAMAGED DNA

• Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA

• DNA polymerase and DNA ligase replace and bond the new nucleotides together

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Question:Question:

•What would be the complementary DNA strand for the following DNA sequence?

DNA 5’-CGTATG-3’DNA 5’-CGTATG-3’

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Answer:Answer:

DNA 5’-GCGTATG-3’DNA 5’-GCGTATG-3’

DNA 3’-CGCATAC-5’DNA 3’-CGCATAC-5’

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DNA by the NumbersDNA by the Numbers• Each cell has about 2

m of DNA.• The average human

has 75 trillion cells.• The average human

has enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from the sun.