DNA: The Stuff of Life

Griffith and Transformation• Griffith and Transformation

• In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia.

• He isolated two different strains of pneumonia bacteria from mice and grew them in his lab.

Griffith and Transformation Performed the first major experiment that led to the

discovery of DNA as the genetic material

Griffith and Transformation– Transformation 

• Griffith determined that bacteria could pass genetic information from one to another.

• Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain).

Oswald Avery Oswald Avery repeated Griffith’s work to

determine which molecule was most important for transformation.

Identified the molecule that transformed the R strain of bacteria into the S strain

Concluded that when the S cells were killed, DNA was released

R bacteria incorporated this DNA into their cells and changed into S cells.

The Hershey-Chase Experiment

• The Hershey-Chase Experiment– Alfred Hershey and Martha Chase studied

viruses—nonliving particles smaller than a cell that can infect living organisms.

• Bacteriophages • A virus that infects bacteria is

known as a bacteriophage. • Bacteriophages are composed

of a DNA or RNA core and a protein coat.

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The Hershey-Chase Experiment

• If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA.

• They grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

Hershey and Chase Used radioactive labeling to

trace the DNA and protein

Concluded that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses

DNA Structure Nucleotides

Consist of a five-carbon sugar, a phosphate group, and a nitrogenous base

Two Kinds of Bases in DNA

• Pyrimidines are single ring bases.– Thymine & Cytosine

• Purines are double ring bases.– Adenine & Guanine

CC

CC

N

NO

N

CC

CC

N

N

N

N

NC

Chargaff Chargaff’s rule: C = G and T = A

• The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA.

• The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.

X-ray Diffraction

Rosalind Franklin’s X-ray diffraction data helped solve the structure of DNA

Indicated that DNA was a double helix

Watson and Crick

Built a model of the double helix that conformed to the others’ research

1. two outside strands consist of alternating deoxyribose and phosphate 2. cytosine and guanine bases pair to each other by three hydrogen bonds 3. thymine and adenine bases pair to each other by two hydrogen bonds

DNA Structure

DNA often is compared to a twisted ladder.

Rails of the ladder are represented by the alternating deoxyribose and phosphate.

The pairs of bases (cytosine–guanine or thymine–adenine) form the steps.

• DNA Double Helix

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The Components and Structure of DNA

• Watson and Crick discovered that hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine.

• This principle is called base pairing.

DNA and Chromosomes– Chromosome Structure

• Eukaryotic chromosomes contain DNA and protein.• The DNA is tightly coiled around proteins called histones.• DNA and histone molecules form nucleosomes. • Nucleosomes pack together, forming a thick fiber called

chromatin.• When chromatin is super-coiled it forms the chromosome.

DNA (4:00)

DNA Replication• DNA must be copied

• The DNA molecule produces 2 IDENTICAL new complementary strands following the rules of base pairing:

A-T, G-C

• Each strand of the original DNA serves as a

template for the new strand

DNA Replication• Semiconservative

Model:

1. Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand.

. Parental DNA

DNA Template

New DNA

1. Why is replication necessary?

2. When does replication occur?

3. Describe how replication works.

4. Use the complementary rule to create the complementary strand:

A---?G---?C---?T---?A---?G---?A---?G---?C---?A---?G---?T---?

Replication Quiz

1. Why is replication necessary?So both new cells will have the correct DNA2. When does replication occur?During interphase (S phase).3. Describe how replication works.Enzymes unzip DNA and complementary

nucleotides join each original strand.4. Use the complementary rule to

create the complementary strand:

A---TG---CC---GT---AA---TG---CA---TG---CC---GA---TG---CT---A

Replication Quiz

– Avery and other scientists discovered that • DNA is found in a protein coat.• DNA stores and transmits genetic information from

one generation to the next.• transformation does not affect bacteria.• proteins transmit genetic information from one

generation to the next.

Questions

– The Hershey-Chase experiment was based on the fact that• DNA has both sulfur and phosphorus in its structure• Protein has both sulfur and phosphorus in its

structure.• both DNA and protein have no phosphorus or sulfur

in their structure.• DNA has only phosphorus, while protein has only

sulfur in its structure.

Questions

– DNA is a long molecule made of monomers called • nucleotides.• purines.• pyrimidines.• sugars.

Questions

– Chargaff's rules state that the number of guanine nucleotides must equal the number of • cytosine nucleotides.• adenine nucleotides.• thymine nucleotides.• thymine plus adenine nucleotides.

Questions

– In DNA, the following base pairs occur:• A with C, and G with T.• A with T, and C with G.• A with G, and C with T.• A with T, and C with T.

Questions