DNA

The Genetic Material

The genetic material must meet 4 criteria 1. Information: 2. Transmission: 3. Replication: 4. Variation:

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IDENTIFICATION OF DNA AS THE GENETIC MATERIAL

Two strains of Streptococcus pneumoniae S Smooth

Secrete a polysaccharide capsule Produce smooth colonies on solid media virulent

R Rough Unable to secrete a capsule Produce colonies with a rough appearance avirulent

Two coat types II III

Four possible phenotypes IIS or IIIS or IIR or IIIR

1928 - Frederick Griffith’s Transformation Experiments

Griffith’s Experiments

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Figure 9.1

Rare mutations convert strains S into R strain & vice versa

But mutations do not change coat types II into III or vice versa

9-8Figure 9.2

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Something from the dead type IIIS transformed type IIR into type IIIS

Called this process transformation

The unknown substance was termed the transforming principle

Griffith’s Conclusions

Griffith’s transforming principle was the genetic material

Transformation assay to identify actual biomolecule

Major constituents - DNA, RNA, proteins, carbohydrates, & lipids

Made cell extracts from type IIIS cells containing each of these macromolecules

1944 - Avery, MacLeod & McCarty Identify the Genetic Material

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9-11Figure 9.3

Mixed each extract with type IIR cells to test for transformation Only extract containing purified DNA transformed type IIR to type IIIS Verify that DNA, not RNA or protein, is the genetic material

Avery’s Experiments

1952- Hershey & Chase Confirm DNA is Genetic Material Using Phage T2

Figure 9.5

Life cycle of phage T2

Phage Attached to E. coli Cell

The Hershey and Chase experiment:

Used radioisotopes to distinguish DNA from proteins 32P labels DNA specifically 35S labels protein specifically

Infect non-radioactive E. coli with radioactively-labeled phages

Remove phage coats from cells Is 32P or 35S inside bacteria?

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Hershey & Chase Experiment

S

Hershey & Chase Experiment

The Data

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9-20Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

In 1956, A. Gierer and G. Schramm isolated RNA from the tobacco mosaic virus (TMV)

Purified TMV RNA caused the same lesions as intact TMV viruses Therefore, the viral genome is composed of RNA

Since that time, many RNA viruses have been found

RNA is Genetic Material in Some Viruses

9-21Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display