DeoxyriboNucleicAcid

The 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 vs. DNA which is composed of a nitrogenous bases, a sugar, and a phosphate in a monomer called a nucleotide.

History of DNA

Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material

Erwin Chargaff 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%

T A

G C

The All-important Photo 51…

Rosalind Franklin

James Watson & Francis Crick

Rosalind Franklin (x-ray crystallography) –gave them an idea of the helical shape of the molecule. helped to uncover the 3D shape of the molecule and that there were 2 strands (Linus Pauling’s model hypothesized that there were 3)

Watson & Crick – used the information from others to synthesize the molecular shapes and bonding present to determine the correct structure for DNA!!

Structure of DNA

The structure of DNA (deoxyribonucleic acid)

• Carbons are numbered clockwise 1’ to 5’ starting at the oxygen.

CH2

O

C1C4

C3 C2

5

Pentose sugar(deoxyribose)

Structure of a nucleotideA nucleotide is the

monomer (single unit) of DNA and is made of 3 components:

A Pentose sugar This is a 5 carbon sugar

(numbered) The sugar in DNA is

deoxyribose. The sugar in RNA is

ribose.

CH2

O

C1C4

C3 C2

5

Pentose sugar(deoxyribose)

Structure of a nucleotide A Phosphate group Phosphate groups are

important because they link the sugar on one nucleotide onto the phosphate of the next nucleotide to make a polynucleotide.

O=P-OO

PhosphateGroup

O

O=P-OO

N

CH2

O

C1C4

C3 C2

5O

Do you know the three parts?

Nitrogenous Bases (rungs of the DNA ladder)

Double ring PURINESAdenine (A)Guanine (G)

Single ring PYRIMIDINESThymine (T)Cytosine (C) T or C

A or G

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Base-Pairings• Purines only pair with Pyrimidines

(remember Chargaff?) • The fit works so well that they are called

“complementary”.• Three hydrogen bonds required to bond

Guanine & Cytosine (remember Watson & Crick)

CG

3 H-bonds

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T A

•Two hydrogen bonds are required to bond Adenine & Thymine

DNA is sooo anti… parallel (Meselson & Stahl found discovered this)

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

Question:

• If there is 30% Adenine, how much Cytosine is present?

Answer:• There would be 20% Cytosine• Adenine (30%) = Thymine (30%)• Guanine (20%) = Cytosine (20%)• Therefore, 60% A-T and 40% C-G

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Antiparallel Strands• One strand of DNA

goes from 5’ to 3’ (sugars)

• The other strand is opposite in directiongoing 3’ to 5’ (sugars)