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DNA StructureDNA Structure•Rosalind Franklin took
diffraction x-ray photographs of DNA crystals
• 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
•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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Nitrogenous Nitrogenous BasesBases
• Double ring Double ring PURINESPURINESAdenine (A)Adenine (A)Guanine (G)Guanine (G)
• Single ring Single ring PYRIMIDINESPYRIMIDINES
Thymine (T)Thymine (T)Cytosine (C)Cytosine (C) T or C
A or G
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Base-PairingsBase-Pairings•Purines only pair with
Pyrimidines•Three hydrogen bonds
required to bond Guanine & Cytosine
CG
3 H-bonds
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Chargaff’s RuleChargaff’s Rule• AdenineAdenine must pair with
ThymineThymine• GuanineGuanine must pair with
CytosineCytosine• The bases form weak
hydrogen bonds
G CT A
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Answer:Answer:•There would be 20%
CytosineCytosine• Adenine (30%) = Adenine (30%) =
Thymine (30%)Thymine (30%)• Guanine (20%) = Guanine (20%) =
Cytosine (20%)Cytosine (20%)• Therefore, Therefore, 60% A-T 60% A-T
and 40% C-Gand 40% C-G
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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 the the SS or synthesis phase or synthesis phase of of interphaseinterphase
• New cells will need New cells will need identical identical DNA strandsDNA strands
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Synthesis Phase (S Synthesis Phase (S phase)phase)
• S phase during interphase of the cell cycle
• Nucleus of eukaryotes
Mitosis-prophase-metaphase-anaphase-telophase
G1 G2
Sphase
interphase
DNA replication takesDNA replication takesplace in the S phase.place in the S phase.
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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)
• 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• As the 2 DNA strands open at As the 2 DNA strands open at
the origin, the origin, Replication Replication BubblesBubbles form form
• Eukaryotic chromosomes have MANY bubbles
• Prokaryotes (bacteria) have a single bubble
Bubbles Bubbles
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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• 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
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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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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’