DNA and DNA and ReplicatiReplicationon

Chargaff’s RuleChargaff’s Rule

•AdenineAdenine must pair with ThymineThymine•GuanineGuanine must pair with CytosineCytosine

•The bases form weak hydrogen bonds

T A G C

DNA DNA StructurStructureeReviewReview

DNA Double HelixDNA Double Helix

Phosphate &Phosphate &Sugar BackboneSugar Backbone

““Legs of ladder”Legs of ladder”

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

““Rungs of ladder”Rungs of ladder”

DNADNA•Stands for Deoxyribonucleic acid•Made up of subunits called nucleotidesnucleotides

•NucleotideNucleotide made of: made of:1. Phosphate Phosphate

groupgroup2. 5-carbon sugar5-carbon sugar3. Nitrogenous Nitrogenous

basebase

DNA NucleotideDNA Nucleotide

CH2

O

C1C4

C3 C2

5

SugarSugar(deoxyribose)(deoxyribose)

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

OO=P-O O

PhosphatePhosphate GroupGroup

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

DNADNA

Nitrogenous BasesNitrogenous Bases

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)

A or G

T or C

Base-PairingsBase-Pairings•Purines only pair with Pyrimidines

•Three hydrogen bonds required to bond Guanine & Cytosine

CG

3 H-bonds

Two hydrogen bonds are required to bond Adenine & Thymine

T A

Question:Question:

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

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

What ensures that What ensures that future copies of DNA future copies of DNA are identical to the are identical to the

original DNA original DNA molecule sequence?molecule sequence?

DNA DNA ReplicationReplication

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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 the DNA is copied during the S S or synthesis phase of or synthesis phase of interphaseinterphase•New cells will need New cells will need identical identical DNA strandsDNA strands

Synthesis Phase (S phase)Synthesis Phase (S 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.

DNA ReplicationDNA Replication•As the 2 DNA strands As the 2 DNA strands open at the origin, open at the origin, Replication BubblesReplication Bubbles form form•Eukaryotic chromosomes have MANY bubbles•Prokaryotes (bacteria) have a single bubble

Bubbles Bubbles

DNA ReplicationDNA Replication•Enzyme Enzyme HelicaseHelicase unwinds and unwinds and separates the separates the 2 DNA strands 2 DNA strands by breaking by breaking the the weak weak hydrogen hydrogen bondsbonds

DNA ReplicationDNA ReplicationDNA polymeraseDNA polymerase can only can only add nucleotides to the add nucleotides to the 3’ 3’ endend of the DNA of the DNA This causes the This causes the NEWNEW strand to be built in a strand to be built in a 5’ to 5’ to 3’ direction3’ direction

RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase

NucleotideNucleotide

5’

5’ 3’

Direction of ReplicationDirection of Replication

Remember the Strands Remember the Strands are Antiparallelare Antiparallel

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

So, what ensures that future So, what ensures that future copies of DNA are identical to copies of DNA are identical to the original DNA molecule the original DNA molecule sequence?sequence?

So, what ensures that future So, what ensures that future copies of DNA are identical to copies of DNA are identical to the original DNA molecule the original DNA molecule sequence?sequence?

Complimentary base pairing-Complimentary base pairing-

A = T and C = GA = T and C = G

Question:Question:

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

DNA DNA 5’-GCGTATG-3’5’-GCGTATG-3’

Answer:Answer:

DNA 5’-GCGTATG-3’DNA 5’-GCGTATG-3’DNA 3’-CGCATAC-5’DNA 3’-CGCATAC-5’