recap

13.1 recap

Experiments on bacteria and on viruses demonstrated that DNA is the genetic material. DNA from one genetic strain of bacteria was able to genetically transform another strain into the donor strain. Viral DNA was shown to be injected into a host cell and to genetically change that cell into a virus factory.

learning outcomes

You should be able to:

  • Describe evidence from cell staining techniques that DNA was the genetic material.

  • Explain how experiments conducted by Griffith and Avery provided evidence that DNA was the genetic material.

  • Justify the experimental design used by Hershey and Chase.

  • Design an experiment to test the hypothesis that DNA and not protein is the cellular material that carries genetic information.

Question 1

At the time of Griffith’s experiments in the 1920s, what circumstantial evidence suggested to scientists that DNA might be the genetic material?

DNA was located in the eukaryotic cell nucleus, where chromosomes carrying genes were located. The amount of DNA was the same in somatic cells of an organism, and halved in the products of meiosis, as expected by the genetic material. Different species had different amounts of DNA, just as they seemingly had different numbers of genes.

Question 2

How did the experiments of Avery and his colleagues rule out protein as the genetic material?

Avery and his colleagues performed genetic experiments using cell extracts of one strain of bacteria (A) that could change a recipient strain (B) that was genetically different into the extracted strain (A). When they pretreated the extract of the donor strain with enzymes that hydrolyzed proteins, transformation still occurred. So proteins in the donor extract were not responsible for genetic transformation.

Question 3

What attributes of bacteriophage T2 were key to the Hershey–Chase experiments demonstrating that DNA, rather than protein, is the genetic material?

Bacteriophage T2 has only two types of molecules, DNA and protein. So labeling one or the other could indicate which got into a host cell to cause genetic changes.

Question 4

Outline a series of experiments using radioactive isotopes (such as 32P and 35S) to show that it is DNA and not protein that moves from the donor cell to the recipient cell and is responsible for bacterial transformation.

The proposed experiments might use S strain pneumococcus and transform R strain, as in Figure 13.1. Incubate separate batches of S strain bacteria in 32P or 35S. Make cell-free extracts of the S strains. Incubate with R cells and look for their transformation to the S phenotype. Then check to see if there is 32P or 35S label in the newly transformed cells. It would be expected that only 32P label (DNA) would enter the cells.

The transformation and viral infection experiments convinced biologists that the genetic material is DNA. It had been known for several decades that chemically, DNA is a polymer of nucleotides. Next, scientists turned to DNA’s precise three-dimensional structure.