Discovering the Structure

of DNA

What is DNA?

• DNA = deoxyribonucleic acid• Stores, transmits and copy all information• Located in the cell’s nucleus

What do you know about DNA?

• Codes for proteins• nucleic acid• Monomer is a nucleotide• The three parts of a nucleotide:

– 1. Phosphate group– 2. Sugar (deoxyribose)– 3. Nitrogen base

Nitrogen bases• nitrogen two types:

purine- 2 carbon rings pyrimidine. -1 carbon

ring

More about nitrogen bases• DNA has 4 nitrogen

bases:– Thymine (T)– Adenine (A)– Cytosine (C)– Guanine (G)

• A G - purines• C T - pyrimidines.

YouTube:DNA

Structure of DNA

A collaborative effort!

• Early 1900s– known: information is passed from cell to cell. – Unknown: what carried the information?

• scientists thought a protein • others that it was a nucleic acid.• Three major experiments helped shows

nucleic acid carried cell information:– Griffith– Avery– Hershey-Chase

Frederick Griffith

• studied pneumonia bacteria

• 1928• isolated two strains of

pneumonia• injected them into

mice

Griffith got lucky?– Something transferred

from heat-killed bacteria

– What happened?

• Griffith’s conclusions:• Transformation = process

by which one strain of bacteria changes the gene(s) of another bacteria

Avery• DNA was transforming bacteria • 1943• Uses pneumonia ( just like

Griffith)• Showed gene is made of DNA• Scientists were slow to accept

the results

Hershey and Chase

• bacteriophages to see if information is carried on proteins or DNA

• 1950

• Further supported Avery’s experiment that genetic material is DNA

http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif

Discovery of the structure of DNA

• Many scientists contributed to determining the structure of DNA– Erwin Chargaff– Rosalind Franklin– James Watson &

Francis Crick

Erwin Chargaff • Base pairing rule• 1950

• percentages of bases in DNA

• 10% A = 10% T• 40% C = 40% G

• This is Chargaff’s Rule!

Rosalind Franklin• x-ray photography to try

to find DNA structure• 1952• Her “Photo 51” revealed

DNA’s structure• Died of cancer in 1958

Watson and Crick• Credited with finding

the structure of DNA)

• 1953

• Watson got a sneak peak at Franklin’s x-ray photos and used them with other evidence

• Did not give credit to Franklin

http://teachers.sduhsd.k12.ca.us/lolson/images/watson_crickjpg

DNA structure

• Twisted ladder • made of nucleotides• Sides- phosphate/sugar • Rung- nitrogen bases

What bonds with what?

– A bonds with T– G bonds with C – Bonds

sides- covalent- strong rungs - hydrogen - weak

Your turn...the structure of DNA• On the

diagram:– Circle and

label a nucleotide.

– Label the sugar and phosphate molecules.

– Label the bases

– Label a base pair.

– Label the sugar-phosphate backbones.

– Label the hydrogen bonds.

nucleotide

P

S

A

C

A

A

T

T

CG

G

Base pairSugar /P

backbone

G

Sugar /P

backbone

Hydrogen bonds