DNADNA

HistoryStructureReplication

History of DNAHistory of DNA

Scientists thought protein was the heredity material

Several Scientists disproved this and proved it was DNA:

GriffithGriffith Worked with virulent S and nonvirulent R strain of

pneumococcus bacteria Made the harmless R strain transform into S strain

when it took in DNA from the killed S strain.

Hershey & ChaseHershey & Chase

Worked with radioactively traced bacteriophages (viruses that attack bacteria).

Further proof that DNA was the cell’s genetic material.

Radioactive 32P was injected into bacteria!

ChargaffChargaff

Developed the base pairing rule by comparing the amounts of A, T, C, G in the cell.

Nucleotide pairing:– Adenine Thymine– Guanine Cytosine

FranklinFranklin

Took x-ray photographs of DNA

Determined double-helix structure

Watson & CrickWatson & Crick

Watson and Crick used Franklin’s x-rays, Chargraff’s rule and developed their model of the DNA molecule (for which they won a Nobel Prize).

Structure of DNAStructure of DNA

Shape of DNAShape of DNA

Double stranded, twisted helix, called a double helix

Shape similar to a twisted ladder

Nucleotide MonomersNucleotide Monomers

DNA Nucleotide is the basic unit (monomer)

Made of:Sugar – DeoxyribosePhosphate group (PO4)Base – A T C G (Adenine, Thymine,

Cytosine, Guanine)

DNA NucleotideDNA Nucleotide((glue picture provided in NB)glue picture provided in NB)

N

OO=P-O O

PhosphatePhosphate GroupGroup

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

CH2

O

C1C4

C3 C2

5

SugarSugar(deoxyribose)(deoxyribose)

Polymer StructurePolymer Structure

Sides made of phosphate group and deoxyribose sugar

Center (rungs) made of nitrogen bases bonded by hydrogen bonds (A = T and C = G)

Direction of NucleotidesDirection of Nucleotides

Each carbon in the sugar is given a number 1’ – 5’

Sides are antiparallel – one side goes 5’ to 3’ and the other 3’ to 5’

This determines the direction that it is “read” by enzymes

DNA StructureDNA Structure(glue picture provided in NB)(glue picture provided in NB)

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

Location of DNALocation of DNA

DNA is a large (macro) molecule, and stays in the nucleus

DNA ReplicationDNA Replication

S Phase of Cell CycleS Phase of Cell Cycle

DNA must be copied before the cell can divide

Occurs during the synthesis (S) part of cell cycle (before mitosis).

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

Helicase enzymeHelicase enzyme

DNA unwinds and unzips with help of DNA helicases

These enzymes break the hydrogen bonds between base pairs.

This point is called the replication fork.

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

Polymerase EnzymePolymerase Enzyme

DNA polymerase moves in new nucleotides Can only add to 3’ end of nucleotide Follows the base-pairing rule Two identical DNA strands are formed

RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase

NucleotideNucleotide

5’

5’ 3’

Direction of ReplicationDirection of Replication

Proofreading enzymesProofreading enzymes

Other enzymes “proofread” the replicated strand looking for errors (mutations).

Incorrect nucleotides are removed and replaced