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History

• Before the 1940’s scientists didn’t know what material caused inheritance.

• They suspected it was either DNA or proteins.

History

• A series of experiments proved that DNA was the genetic material responsible for inheritance.

History

• In 1952, Alfred Hershey and Martha Chase did an experiment using a virus that infects E. coli bacteria.

• The experiment proved that DNA and not protein is the factor that influences inheritance.

History

• Erwin Chargaff discovered the base pairing rules and ratios for different species.

• Adenine pairs with Thymine

• Cytosine pairs with Guanine.

History• Rosalind Franklin & Maurice Wilkins had

taken the 1st pictures of DNA using X-ray crystallization

This proved that DNA had a helical shape.

History• The Nobel Prize in Medicine 1962

Francis Harry Compton Crick

James Dewey Watson

Maurice Hugh Frederick Wilkins

Rosalind Franklin(Died of cancer 1958)

Wilkins has become a historical footnote and

Watson & Crick are remembered as the

Fathers of DNA

Watson Crick

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The Double Helix Molecule

• The DNA double helix has two strands twisted together.

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DNA• DNA is often called

the blueprint of life.• In simple terms,

DNA contains the instructions for making proteins within the cell.

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Chromosomes and DNA

• Our genes are on our chromosomes.

• Chromosomes are made up of a chemical called DNA.

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DNA• Stands for

Deoxyribonucleic acid• Made up of subunits called

nucleotides • Nucleotide made of:

1. Phosphate group2. 5-carbon sugar3. Nitrogenous base

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One Strand of DNA• The backbone of

the molecule is alternating phosphates and deoxyribose sugar

• The teeth are nitrogenous bases.

phosphate

deoxyribose

bases

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One Strand of DNA

• One strand of DNA is a polymer of nucleotides.

• One strand of DNA has many millions of nucleotides.

nucleotide

DNA

OO=P-O O

Phosphate Group

N

Nitrogenous base (A, T, G, C)CH2

O

C1C4

C3 C2

5

Sugar(deoxyribose)

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Four nitrogenous bases

• Cytosine C• Thymine T

• Adenine A • Guanine G

DNA has four different bases:

Nitrogen Bases

• 2 types of Nitrogen Bases–Purines

• Double ring–G & A

–Pyrimidines• Single ring

–C & U & T

PGA

CUT PY

A or G

T or C

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Two Stranded DNA• Remember, DNA

has two strands that fit together something like a zipper.

• The teeth are the nitrogenous bases but why do they stick together?

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C

C

C

C

N

N

O

N

C

C

C

C

N N

O

N

N

N

C

Hydrogen Bonds• The bases attract each

other because of hydrogen bonds.

• Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.

• The bonds between cytosine and guanine are shown here with dotted lines

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Hydrogen Bonds, cont.• When making

hydrogen bonds, cytosine always pairs up with guanine

• Adenine always pairs up with thymine

• Adenine is bonded to thymine here

C

C

CC

N

N

N

N

N

C

C

C

C

C

N

N

O

O

C

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Chargaff’s Rule:

• Adenine and Thymine always join together

A T• Cytosine and Guanine

always join togetherC G

DNA• DNA is a double-

stranded molecule.• The strands are

connected by complementary nucleotide pairs (A-T & C-G) like rungs on a ladder.

• The ladder twists to form a double helix.

DNA - double helix

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

T A

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Question:

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

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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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DNA by the Numbers• Each cell has about 2 m

of DNA.• The average human has

75 trillion cells.• The average human has

enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from the sun.