DNA

12–1 DNA

Griffith and Transformation

I Griffith and TransformationI Griffith and Transformation

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

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


Griffith made two observations:Griffith made two observations:

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

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


Griffith's ExperimentsGriffith's Experiments

Griffith set up four Griffith set up four individual individual experiments. experiments.

Experiment 1:Experiment 1: Mice Mice were injected with were injected with the disease-causing the disease-causing strain of bacteria. strain of bacteria. The mice developed The mice developed pneumonia and died.pneumonia and died.


Experiment 2: Mice Experiment 2: Mice were injected with the were injected with the harmless strain of harmless strain of bacteria. These mice bacteria. These mice didn’t get sick.didn’t get sick.

Harmless bacteria (rough colonies)

Lives


Experiment 3: Griffith Experiment 3: Griffith heated the disease-heated the disease-causing bacteria. He causing bacteria. He then injected the then injected the heat-killed bacteria heat-killed bacteria into the mice. The into the mice. The mice survived.mice survived.

Heat-killed disease-causing bacteria (smooth colonies)

Lives

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

Live disease-causing bacteria(smooth colonies)

Dies of pneumonia




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

causing bacteria(smooth colonies)



Dies of pneumonia


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.

Avery and DNA

II Avery and DNAII Avery and DNAOswald Avery repeated Griffith’s work to Oswald Avery repeated Griffith’s work to

determine which molecule was most important determine which molecule was most important for transformation.for transformation.

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

Avery and DNAAvery and DNA

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

Transformation still occurred.

Avery and DNA

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

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

Avery and DNA

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

The Hershey-Chase Experiment

III The Hershey-Chase ExperimentIII The Hershey-Chase Experiment

Alfred Hershey and Martha Chase studied Alfred Hershey and Martha Chase studied viruses—nonliving particles smaller than viruses—nonliving particles smaller than a cell that can infect living organisms.a cell that can infect living organisms.


Bacteriophages Bacteriophages

A virus that infects bacteria is known as a A virus that infects bacteria is known as a bacteriophage. bacteriophage.

Bacteriophages are composed of a DNA or Bacteriophages are composed of a DNA or RNA core and a protein coat.RNA core and a protein coat.

They grew viruses in cultures containing They grew viruses in cultures containing radioactive isotopes of phosphorus-32 radioactive isotopes of phosphorus-32 ((3232P) and sulfur-35 (P) and sulfur-35 (3535S).S).


If If 3535S was found in the bacteria, it would S was found in the bacteria, it would mean that the viruses’ protein had been mean that the viruses’ protein had been injected into the bacteria.injected into the bacteria.

Bacteriophage withsuffur-35 in protein coat

Phage infects bacterium

No radioactivity inside bacterium


If 32P was found in the bacteria, then it was the DNA that had been injected.

Bacteriophage withphosphorus-32 in DNA

Phage infects bacterium

Radioactivity inside bacterium


Nearly all the radioactivity in the Nearly all the radioactivity in the bacteria was from phosphorus (bacteria was from phosphorus (3232P).P).

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

The Components and Structure of DNA

I The Components and Structure of DNAI The Components and Structure of DNA

DNA is made up of nucleotides. DNA is made up of nucleotides.

A nucleotide is a monomer of A nucleotide is a monomer of nucleic acids made up of:nucleic acids made up of:

• Deoxyribose – 5-carbon SugarDeoxyribose – 5-carbon Sugar

• Phosphate GroupPhosphate Group

• Nitrogenous BaseNitrogenous Base


There are four There are four kinds of bases kinds of bases in in DNA:in in DNA:

• adenineadenine• guanine guanine • cytosinecytosine• thyminethymine


Chargaff's RulesChargaff's Rules

Erwin Chargaff discovered that:Erwin Chargaff discovered that:–The percentages of guanine [G] and The percentages of guanine [G] and

cytosine [C] bases are almost equal in cytosine [C] bases are almost equal in any sample of DNA. any sample of DNA.

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


X-Ray Evidence X-Ray Evidence

Rosalind Franklin used Rosalind Franklin used X-ray diffraction to get X-ray diffraction to get information about the information about the structure of DNA.structure of DNA.

She aimed an X-ray She aimed an X-ray beam at concentrated beam at concentrated DNA samples and DNA samples and recorded the recorded the scattering pattern of scattering pattern of the X-rays on film.the X-rays on film.


The Double Helix The Double Helix

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

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


DNA Double Helix


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

This principle is called “the base pairing This principle is called “the base pairing rule”.rule”.

END OF SECTION

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