Historical perspectives- Early evidence , DNA is the

genetic material-II

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Lecture no. 3

OutlineOutline

• How we know DNA is the genetic material?• Search for genetic material---is it composed of

protein or nucleic acid /DNA or RNA?

Griffith’s Transformation Experiment Avery’s Transformation Experiment Hershey-Chase Bacteriophage Experiment Tobacco Mosaic Virus (TMV) Experiment Chargaff’s Rules

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Lecture no. 3

3 Characteristics of3 Characteristics of“The Genetic Material”:“The Genetic Material”:

Must be a stable form containing information about cell form and function.

Must replicate accurately. Capable of change to allow evolution..

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• Until the 1940s, the case for protein seemed more likely. Why?

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Lecture no. 3

Search for genetic materialSearch for genetic material

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Timeline of events:• 1890: Weismann - substance in the cell nuclei controls

development.• 1900: Chromosomes shown to contain hereditary

information, later shown to be composed of protein & nucleic acids.

• 1928: Griffith’s Transformation Experiment.• 1944: Avery’s Transformation Experiment.• 1953: Hershey-Chase Bacteriophage Experiment.• 1956: First demonstration that RNA is viral genetic

material.

Lecture no. 3

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Toward The Real Toward The Real ConclusionConclusion

Griffith’s Transformation

Experiment

Tobacco Mosaic Virus (TMV) Experiment

Avery’s Transformation

Experiment

Hershey-Chase Bacteriophage

Experiment

19281944

1953 1956

Lecture no. 3

First Real Break- Frederick Griffith’s Transformation Experiment.

• Griffith worked with two strains of bacteria Streptococcus pneumoniae, IIR(non virulent) and IIIS (virulent).

• His experiment was one of the first suggesting that bacteria are capable of transferring genetic information through a process known as transformation.

Lecture no. 3

Continue…Continue…

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Griffith concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming factor“.

Griffith hypothesized that the transforming factor was a “IIIS” protein.

Griffith concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming factor“.

Griffith hypothesized that the transforming factor was a “IIIS” protein.

1928

Lecture no. 3

Griffith’s “transforming factor” was different from proteins because:

i. The heat treatment did not denature the unidentified factor;

ii. The factor was able to transfer information from one type of bacterial cell to another;

iii. The factor transformed the host cells by giving them new properties.

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Lecture no. 3

Continue…Continue… Avery’s Transformation Experiment

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When DNA was destroyed, the transforming activity was lost ,but when DNA was left intact, the transforming activity survived

When DNA was destroyed, the transforming activity was lost ,but when DNA was left intact, the transforming activity survived

1944

Lecture no. 3

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When DNA was destroyed, the transforming activity was lost, but when DNA was left intact, the transforming activity survived.

Continue…Continue… Avery’s Transformation Experiment

1944

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Continue…Continue… Hershey-Chase Bacteriophage Experiment

Because DNA and not protein entered the cell, then DNA must be the heritable material (i.e., responsible for the function and reproduction of phage).

A.H. received Nobel Prize in 1969 for this & other work

Because DNA and not protein entered the cell, then DNA must be the heritable material (i.e., responsible for the function and reproduction of phage).

A.H. received Nobel Prize in 1969 for this & other work

1953

Lecture no. 3

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Where was radioactive sulfur found at the end of experiment 1?

Where was it found at the end of experiment 2?

Based on the Hershey–Chase experiments, is it reasonable to assume that Griffith’s “transforming factor” was DNA, not protein? Why or why not? What is the connection between the two experiments?

Lecture no. 3

- Gierer & Schramm (TMV) Experiment

Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957 Used 2 viral strains to demonstrate RNA is the genetic material of TMV.Used 2 viral strains to demonstrate RNA is the genetic material of TMV.Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957 Used 2 viral strains to demonstrate RNA is the genetic material of TMV.Used 2 viral strains to demonstrate RNA is the genetic material of TMV.

RNA as Genetic MaterialRNA as Genetic Material

1956

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In conclusionIn conclusion

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DNA (not RNA) is transforming agentDNA (not RNA) is transforming agent

DNA (not protein) is the genetic materialDNA (not protein) is the genetic material

RNA (not protein) is genetic material of some RNA (not protein) is genetic material of some virusesviruses

Lecture no. 3

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Chargaff’s Rules 1949-1953, evidence for DNA as Chargaff’s Rules 1949-1953, evidence for DNA as genetic material. genetic material.

Digested many DNAs and subjected products to chromatographic separationResults: A = T, C = G

A + G = C + T (purine = pyrimidine) A + T does not equal C + GMembers of a species similar but different species vary in AT/CG ratio

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What could these observations suggest?

What are possible relationships between (A) and (T), and between (C) and (G)?

Lecture no. 3