Step-by-Step Guidance for Bio 2.1 – Practice Exam for Unit #3

Q1. What did Griffith’s studies of Streptococcus pneumoniae demonstrate?

Background

Topic: History of DNA as Genetic Material

This question tests your understanding of classic experiments that led to the discovery of DNA as the genetic material, specifically the phenomenon of bacterial transformation.

Key Terms:

• Bacterial transformation: The process by which bacteria take up foreign genetic material from their environment.

• Streptococcus pneumoniae: The bacterium used in Griffith's experiments.

Step-by-Step Guidance

1. Recall the setup of Griffith’s experiment: He worked with two strains of S. pneumoniae—one virulent (smooth, S) and one non-virulent (rough, R).

2. Consider what happened when he injected mice with live R strain, live S strain, heat-killed S strain, and a mixture of live R and heat-killed S strains.

3. Think about the outcome when the non-virulent R strain was mixed with heat-killed S strain and injected into mice. What was observed?

4. Ask yourself: What did this result suggest about the ability of bacteria to acquire new traits?

Try solving on your own before revealing the answer!

Q2. Which enzyme did Avery, MacLeod, and McCarty use to abolish the transforming ability of bacterial extract?

Background

Topic: Identification of DNA as the Transforming Principle

This question focuses on the experiment that identified DNA as the molecule responsible for transformation in bacteria.

Key Terms:

• Enzymes: Protease (digests proteins), RNase (digests RNA), DNase (digests DNA), Lipase (digests lipids).

• Transforming ability: The capacity of a substance to induce genetic change in bacteria.

Step-by-Step Guidance

1. Recall that Avery, MacLeod, and McCarty treated bacterial extracts with different enzymes to determine which macromolecule was responsible for transformation.

2. Think about which enzyme would specifically degrade DNA, and what effect this would have on the transforming ability of the extract.

3. Consider the results: When the extract was treated with this enzyme, transformation did not occur.

Try solving on your own before revealing the answer!

Q3. Who performed the classic experiments supporting the semiconservative model of DNA replication?

Background

Topic: DNA Replication Models

This question tests your knowledge of the experiments that demonstrated how DNA is replicated in cells.

Key Terms:

• Semiconservative replication: Each new DNA molecule consists of one parental and one newly synthesized strand.

• Key scientists: Watson, Crick, Meselson, Stahl, Hershey, Chase, Franklin, Wilkins.

Step-by-Step Guidance

1. Recall the experiment involving isotopic labeling of DNA with heavy and light nitrogen.

2. Think about which scientists used density gradient centrifugation to distinguish between old and new DNA strands.

3. Identify the pair of scientists whose work provided strong evidence for the semiconservative model.

Try solving on your own before revealing the answer!

Q4. Why wouldn’t labeling the nitrogen of DNA (instead of phosphate) in the Hershey and Chase experiment work?

Background

Topic: Experimental Design in Molecular Biology

This question examines your understanding of how isotopic labeling distinguishes DNA from protein in classic experiments.

Key Terms:

• Isotopic labeling: Using radioactive isotopes to track molecules.

• DNA vs. protein composition: Both contain nitrogen, but only DNA contains significant phosphate.

Step-by-Step Guidance

1. Recall that Hershey and Chase used radioactive phosphorus to label DNA and sulfur to label protein.

2. Consider what would happen if nitrogen was used instead of phosphorus to label DNA.

3. Think about whether proteins also contain nitrogen, and how this would affect the specificity of the labeling.

Try solving on your own before revealing the answer!

Q5. Which three scientists are most credited with the discovery of the structure of DNA?

Background

Topic: Discovery of DNA Structure

This question tests your knowledge of the key contributors to the discovery of the double helix structure of DNA.

Key Terms:

• Double helix: The structure of DNA as determined by X-ray crystallography and model building.

• Key contributors: Watson, Crick, Franklin, Wilkins, Pauling.

Step-by-Step Guidance

1. Recall which scientists built the first accurate model of DNA’s structure.

2. Consider who provided critical X-ray diffraction data.

3. Identify the three individuals most often credited in textbooks and Nobel Prizes for this discovery.

Try solving on your own before revealing the answer!