AP Molecular Heredity MC Monday #1

Meselson and Stahl cultured E. coli for several generations in a medium with a heavy isotope of nitrogen, ¹⁵N. They transferred the bacteria to a medium with a light isotope of nitrogen ¹⁴ N. After two rounds of DNA replication, half the DNA molecules were light (both strands had ¹⁴N) and half were hybrids (¹⁵N-¹⁴N). What did the researchers conclude from these results?

• The nitrogenous bases in DNA molecule incorporate both ¹⁵N and ¹⁴N.

• DNA replication is semiconservative.

• Opposite DNA strands are complementary to each other.

• DNA replication is conservative.

For a science fair project, two students decided to repeat the Hershey Chase experiment, with modifications. They decided to label the nitrogen of the DNA, rather than the phosphate. They reasoned that each nucleotide has only one phosphate and two to five nitrogens. Thus, labeling the nitrogens would provide a stronger signal than labeling the phosphates. Why won't the experiment work?

• Although there are more nitrogens in a nucleotide, labeled phosphates actually have 15 extra neutrons: therefore, they are more radioactive.

• Amino acids (and thus proteins) also have nitrogen atoms, thus, the radioactivity would not distinguish between DNA and proteins.

• There is no radioactive isotope of nitrogen

• Radioactive nitrogen has a half-life of 100,000 years, and the material would be too dangerous for too long.

You briefly expose bacteria undergoing DNA replication to radioactively labeled nucleotides. When you centrifuge the DNA isolated from the bacteria, the DNA separates into two classes. One class of labeled DNA includes very large molecules (thousands or even millions of nucleotides long) and the other includes short stretches of DNA (several hundred to a few thousand nucleotides in length). These two classes of DNA probably represent

• RNA primers and mitochondrial DNA

• Okazaki fragments and RNA primers

• leading strands and Okazaki fragments

• leading strands and RNA primers

In E. coli replication the enzyme primase is used to attach a 5 to 10 base ribonucleotide strand complementary to the parental DNA strand. THe RNA strand serves as a starting point for the DNA polymerase that replicates the DNA. If a mutation occurred in the primase gene, which of the following would you expect?

• Replication would not be affected as the enzyme primase involved with RNA synthesis.

• Replication would not occur on either the leading or lagging strand

• Replication would only occur on the leading strand

• Replication would only occur on the lagging strand

Hershey and Chase used a DNA-based virus for their work. What would the results have been if they had used an RNA virus?

• with an RNA virus radioactive RNA would have been in the final pellet.

• with an RNA virus neither sample would have had a radioactive pellet

• With an RNA virus the protein shell would have been radioactive in both samples

• with an RNA virus radioactive protein would have been in the final pellet.

The lagging strand is characterized by a series of short segments of DNA (Okazaki fragments) that will be joined together to form a finished lagging strand. The experiments that led to the discovery of Okazaki fragments gave evidence for which of the following ideas?

• DNA polymerase is a directional enzyme that synthesizes leading and lagging strands during replication.

• DNA is a polymer consisting of four monomers: adenine, thymine, guanine, and cytosine.

• DNA is the genetic material

• Bacterial replication is fundamentally different from eukaryotic replication

After mixing a heat-killed phosphorescent (light emitting) strain of bacteria with a living, nonphosphorescent strain, you discover that some of the living cells are now phosphorescent. Which observation(s) would provide the best evidence that the ability to phosphoresce is a heritable trait?

• DNA passed from the heat-killed strain to the living strain

• Protein passed from the heat-killed strain to the living strain

• Both DNA and protein passed from the heat-killed strain to the living strain.

• Descendants of the living cells are also phosphorescent.

In trying to determine whether DNA or protein is the genetic material, Hershey and Chase made use of which of the following facts?

• DNA contains nitrogen, whereas protein does not.

• RNA includes ribose, whereas DNA includes deoxyribose sugars.

• DNA contains phosphorus, whereas protein does not.

• DNA contains purines, whereas protein includes pyrimidines.

Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

• 42%

• 8%

• 16%

• 31%

In an analysis of the nucleotide composition of DNA, which of the following will be found?

• A + C = G + T

• G + C = T + A

• A = C

• A = G and C = T