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DNA DNA StructureStructure

and and ReplicatioReplicatio

n n

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DNADNA•Two strands coiled called

a double helix•Sides made of a pentose

sugar Deoxyribose bonded to phosphate (PO4) groups by phosphodiester bonds

•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 ladderRungs of ladder””

““Legs of ladderLegs of ladder””

Phosphate &Phosphate &Sugar BackboneSugar Backbone

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HelixHelix

• Most DNA has a Most DNA has a right-right-handhand twist with twist with 10 base 10 base pairspairs in a complete turn in a complete turn

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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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DNADNA

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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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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T A

•Two hydrogen bonds are required to bond Adenine & Thymine

Discovering the structure of DNA

Rosalind Franklin’s DNA image

“Chargoff’s rule”

A = T & C = G

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Rosalind Franklin (early 1950’s)

• used x-ray diffraction to get information about the structure of the DNA molecule

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• Photo shows DNA twisted around each other like coils of string helical

• Angle of X suggests two strands in the structure

Chargaff’s Rules

• [G]=[C]– The percentages of G equal

the percentages of C in DNA

• [A] = [T]– The percentages of A equal

the percentages of T in DNA

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Question:Question:

•If there is 30% AdenineAdenine, how much CytosineCytosine is present?

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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

Discovering the structure of DNA

• Structure was discovered in 1953 by James

Watson and Francis Crick

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Functions of DNA

• DNA holds the genetic information in every cell

• DNA holds the instructions for making proteins

• The sequence of nitrogen bases in DNA determines the traits of an organism

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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 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

• Prokaryotes (bacteria) have a single bubble

• Eukaryotic chromosomes have MANY bubbles

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• BeforeBefore new DNA strands can

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

• PrimasePrimase is the enzyme that synthesizes the RNA Primer

• DNA polymerase can then add the new nucleotides

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

nucleotides to the nucleotides to the 33 ’’ end 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 55 ’’ to 3 to 3’’ direction direction

RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase

NucleotideNucleotide

5’

5’ 3’

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Synthesis of the New Synthesis of the New DNA StrandsDNA Strands

• The The Leading StrandLeading Strand is synthesized as a single single strand strand from the point of origin toward the opening replication fork

RNARNAPrimerPrimerDNA PolymeraseDNA PolymeraseNucleotidesNucleotides

3’5’

5’

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Synthesis of the New DNA Synthesis of the New DNA StrandsStrands

• The The Lagging StrandLagging Strand is is synthesized discontinuouslydiscontinuously against overall direction of replication

• This strand is made in MANY short segments It is replicated from the replication fork toward the origin

RNA PrimerRNA Primer

Leading StrandLeading Strand

DNA PolymeraseDNA Polymerase

5’

5’

3’

3’

Lagging StrandLagging Strand

5’

5’

3’

3’

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Lagging Strand Lagging Strand SegmentsSegments

• Okazaki FragmentsOkazaki Fragments - - series of short segments on the lagging strandlagging strand

• Must be joined together by Must be joined together by an an enzymeenzyme

Lagging Strand

RNARNAPrimerPrimer

DNADNAPolymerasePolymerase

3’

3’

5’

5’

Okazaki FragmentOkazaki Fragment

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Joining of Okazaki Joining of Okazaki FragmentsFragments

• The enzyme The enzyme LigaseLigase joins the joins the Okazaki fragments together Okazaki fragments together to make one strandto make one strand

Lagging Strand

Okazaki Fragment 2Okazaki Fragment 2

DNA ligaseDNA ligase

Okazaki Fragment 1Okazaki Fragment 1

5’

5’

3’

3’

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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

The Results of DNA Replication

• DNA replication produces two DNA molecules that are identical to the original DNA molecule

• Each new DNA molecule has one “old” strand and one “new” strand

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Functions of DNA Polymerase

• Adds nucleotides to the new strands of DNA according to base-pairing rules

• Proof reads the new strands of DNA and corrects any errors

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What is a Complementary Strand of DNA?

A DNA strand that has the base sequence that matches up exactly with the original strand of DNA according to base-pairing rules

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Question:Question:

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

DNA 5DNA 5’’-CGTATG-3-CGTATG-3’’

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Answer:Answer:

DNA 5DNA 5’’-CGTATG-3-CGTATG-3’’DNA 3DNA 3’’-GCATAC-5-GCATAC-5’’

Importance of DNA Replication

• Helps cells prepare for cell division

• Ensures that every cell in your body has a complete, accurate copy of your DNA

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DNA replication

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DNA replication

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DNA replication

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DNA replication

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DNA replication

Original DNA strands44

DNA replication

Newly assembled DNA strands45

DNA replication

Semi-conservative replication46