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12–1 DNA

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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 made two observations:

(1) The disease-causing strain of bacteria grew into smooth colonies on culture plates.

(2) The harmless strain grew into colonies with rough edges.

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Griffith's Experiments

Griffith set up four

individual experiments.

Experiment 1: Mice

were injected with the

disease-causing strain

of bacteria. The mice

developed pneumonia

and died.

Movies/vagriffi.mov

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Experiment 2: Mice were

injected with the harmless

strain of bacteria. These

mice didn’t get sick.

Harmless bacteria

(rough colonies)

Lives

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Experiment 3: Griffith

heated the disease-

causing bacteria. He then

injected the heat-killed

bacteria into the mice.

The mice survived.

Heat-killed disease-

causing bacteria (smooth

colonies)

Lives

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Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died.

Live disease-

causing bacteria

(smooth colonies)

Dies of pneumonia


causing bacteria

(smooth colonies)

Harmless bacteria

(rough colonies)

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Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.

Live disease-

causing bacteria

(smooth colonies)


causing bacteria

(smooth colonies)

Harmless bacteria

(rough colonies)

Dies of pneumonia

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Transformation

Griffith called this process transformation

because one strain of bacteria (the harmless strain)

had changed permanently into another (the

disease-causing strain).

Griffith hypothesized that a factor must contain

information that could change harmless bacteria

into disease-causing ones.

Transformers!

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../../../../../../Scott.Taubitz/My Documents/My Documents Backup/BIOLOGY/Chapter Presentations/Ch. 12/Transformers S1 Opening.mp3

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Avery and DNA

Avery and DNA

Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation.

Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes.

The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA.

Transformation still occurred.

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Avery and DNA

Avery and other scientists repeated the experiment using enzymes that would break down DNA.

When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor.

Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

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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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When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.

The viral genes produce many new bacteriophages, which eventually destroy the bacterium.

When the cell splits open, hundreds of viruses burst out.

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).

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If 35S was found in the bacteria, it would mean that

the viruses’ protein had been injected into the

bacteria.

Bacteriophage with

sulfur-35 in protein coat

Phage infects

bacterium No radioactivity

inside bacterium

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If 32P was found in the bacteria, then it was the DNA

that had been injected.

Bacteriophage with

phosphorus-32 in DNA Phage infects

bacterium Radioactivity

inside bacterium

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Nearly all the radioactivity in the bacteria was from phosphorus (32P).

Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

The Components and Structure of DNA

• DNA is made up of nucleotides.

• A nucleotide is a monomer of nucleic acids made up of a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.

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There are four kinds of bases in in DNA:

• adenine

• guanine

• cytosine

• thymine

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The backbone of a DNA chain is formed by sugar

and phosphate groups of each nucleotide joined

together in any order.

Chargaff's Rules

• The percentages of guanine [G] and cytosine

[C] bases are almost equal in any sample of

DNA. So G and C pair up!

• The percentages of adenine [A] and thymine

[T] bases are almost equal in any sample of

DNA. So A and T pair up!

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X-Ray Evidence

Rosalind Franklin used X-ray

diffraction to get information

about the structure of DNA.

She aimed an X-ray beam at

concentrated DNA samples

and recorded the scattering

pattern of the X-rays on film.

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The Double Helix

Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied.

Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.

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.

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The Components and Structure of

DNA

DNA Double Helix

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Avery and other scientists discovered that

a. DNA is found in a protein coat.

b. DNA stores and transmits genetic

information from one generation to the next.

c. transformation does not affect bacteria.

d. proteins transmit genetic information from

one generation to the next.

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The Hershey-Chase experiment was based on

the fact that

a. DNA has both sulfur and phosphorus in its

structure.

b. protein has both sulfur and phosphorus in

its structure.

c. both DNA and protein have no phosphorus

or sulfur in their structure.

d. DNA has only phosphorus, while protein

has only sulfur in its structure.

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DNA is a long molecule made of monomers

called

a. nucleotides.

b. purines.

c. pyrimidines.

d. sugars.

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Chargaff's rules state that the number of

guanine nucleotides must equal the number of

a. cytosine nucleotides.

b. adenine nucleotides.

c. thymine nucleotides.

d. thymine plus adenine nucleotides.

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In DNA, the following base pairs occur:

a. A with C, and G with T.

b. A with T, and C with G.

c. A with G, and C with T.

d. A with T, and C with T.