CH 12.1 DNAThe Genetic Material

Learning Objectives: 1. Summarize the experiments leading

to the discovery of DNA as the genetic material.

2. Diagram and Label the basic structure of DNA

Scientific History • The march to understanding that DNA is

the genetic material– T.H. Morgan (1908)– Frederick Griffith (1928)– Avery, McCarty & MacLeod (1944)– Erwin Chargaff (1947)– Hershey & Chase (1952)– Watson & Crick (1953)– Meselson & Stahl (1958)

Chromosomes related to phenotype • T.H. Morgan

– working with Drosophila • fruit flies

– associated phenotype with specific chromosome

• white-eyed male had specific X chromosome

1908 | 1933

Genes are on chromosomes • Morgan’s conclusions

– genes are on chromosomes– but is it the protein or the DNA

of the chromosomes that are the genes?

• initially proteins were thought to be genetic material… Why?

1908 | 1933

What’s so impressiveabout proteins?!

The “Transforming Principle” 1928• Frederick Griffith

– Streptococcus pneumonia bacteria• was working to find cure for pneumonia

– harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice

– a substance passed from dead bacteria to live bacteria to change their phenotype

• “Transforming Principle”

The “Transforming Principle”

Transformation = change in phenotypesomething in heat-killed bacteria could still transmit disease-causing properties

live pathogenicstrain of bacteria

live non-pathogenicstrain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenicbacteria

mice live mice die

A. B. C. D.

DNA is the “Transforming Principle”

• Avery, McCarty & MacLeod– purified both DNA & proteins separately from Streptococcus pneumonia bacteria• which will transform non-pathogenic bacteria?

– injected protein into bacteria• no effect

– injected DNA into bacteria• transformed harmless bacteria into

virulent bacteria

1944

What’s theconclusion?

mice die

Oswald Avery Maclyn McCarty Colin MacLeod

Avery, McCarty & MacLeod• Conclusion

– first experimental evidence that DNA was the genetic material

1944 | ??!!

Confirmation of DNA

• Hershey & Chase– classic “blender” experiment– worked with bacteriophage

• viruses that infect bacteria

– grew phage viruses in 2 media, radioactively labeled with either • 35S in their proteins• 32P in their DNA

– infected bacteria with labeled phages

1952 | 1969Hershey

Why useSulfur

vs.Phosphorus?

Protein coat labeledwith 35S

DNA labeled with 32P

bacteriophages infectbacterial cells

T2 bacteriophagesare labeled with

radioactive isotopesS vs. P

bacterial cells are agitatedto remove viral protein coats

35S radioactivityfound in the medium

32P radioactivity foundin the bacterial cells

Which radioactive marker is found inside the cell?

Which molecule carries viral genetic info?

Hershey &

Chase

Blender experiment• Radioactive phage & bacteria in blender

– 35S phage• radioactive proteins stayed in supernatant• therefore viral protein did NOT enter bacteria

– 32P phage• radioactive DNA stayed in pellet• therefore viral DNA did enter bacteria

– Confirmed DNA is “transforming factor”

Taaa-Daaa!

Hershey & Chase

Alfred HersheyMartha Chase

1952 | 1969Hershey

Chargaff• DNA composition: “Chargaff’s rules”

– varies from species to species– all 4 bases not in equal quantity– bases present in characteristic ratio

• humans:A = 30.9%

T = 29.4% G = 19.9% C = 19.8%

1947

That’s interesting!What do you notice?

RulesA = TC = G

Structure of DNA

Now scientists agreed that DNA was in fact the genetic material….

But what did it look like?

Let’s take a closer look at DNA.

Structure of DNA• Watson & Crick

– developed double helix model of DNA• other leading scientists working on question:

– Rosalind Franklin– Maurice Wilkins– Linus Pauling

1953 | 1962

Franklin Wilkins Pauling

Watson and Crick1953 article in Nature

CrickWatson

Rosalind Franklin (1920-1958)

DNA

Stands for

Deoxyribonucleic Acid

proteinsproteins

DNADNA

Nucleic Acids• Function:

– genetic material• stores information

–genes–blueprint for building proteins

»DNA RNA proteins• transfers information

– blueprint for new cells– blueprint for next generation

AA

A

A

TC

G

CG

TG

C

T

Nucleic Acids• Examples:

– RNA (ribonucleic acid)• single helix

– DNA (deoxyribonucleic acid)• double helix

• Structure:– monomers = nucleotides

RNADNA

Nucleotides• 3 parts

– nitrogen base (C-N ring)

– pentose sugar (5C)• ribose in RNA• deoxyribose in DNA

– phosphate (PO4) group

Are nucleic acidscharged molecules?

Nitrogen baseI ’m the

A,T,C,G or Upart!

Types of nucleotides

• 2 types of nucleotides– different nitrogen bases – purines

• double ring N base • adenine (A)• guanine (G)

– pyrimidines• single ring N base • cytosine (C)• thymine (T)• uracil (U)

Purine = AGPure silver!

Nucleic polymer• Backbone

– sugar to PO4 bond

– phosphodiester bond• new base added to sugar of

previous base• polymer grows in one direction

– N bases hang off the sugar-phosphate backbone

Dangling bases?Why is this important?

Pairing of nucleotides• Nucleotides bond between

DNA strands– H bonds– purine :: pyrimidine

– A :: T• 2 H bonds

– G ::: C• 3 H bonds

Matching bases?Why is this important?

DNA molecule

Shape = Double helix– H bonds between bases

join the 2 strands• A :: T• C ::: G

H bonds?Why is this important?

Time for Questions!!!Learning Objectives: 1. Summarize the experiments leading to the discovery of DNA

as the genetic material.

Name that Scientist(s)….1. The double helix structure of DNA was first described by

________.2. The first major experiment that led to the discovery of DNA as

the genetic material was conducted by ______. He used heat-killed bacteria in mice.

3. The scientist who identified the transforming agent in Griffith’s famous experiment as DNA was _______.

4. These scientists preformed the famous “blender experiment” to demonstrate that DNA is the genetic material in viruses.

5. This scientist’s X-ray diffraction data helped Watson and Crick solve the structure of DNA.

Answers: 1. Watson and Crick 2. Griffith 3. Avery 4. Hershey and Chase 5. Rosalind Franklin

Learning Objective 2:

Diagram and Label the basic structure of DNA.

1. Use the following words to label this piece of DNA:

– Deoxyribose – Phosphate – Adenine – Thymine – Cytosine – Guanine

2. Circle a nucleotide

3. Put a star by the purines

4. Underline the pyrimidines