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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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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
5
SugarSugar(deoxyribose)(deoxyribose)
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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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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
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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
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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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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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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.