Question 1

For a science fair project, two students decided to repeat the Hershey and 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 this experiment work?

Accepted Answer

Amino acids contain nitrogen atoms, so if nitrogen was labeled in the DNA, the resulting radioactivity would not distinguish between DNA and proteins. Therefore, this modification to the experiment would not work.

Question 2

Refer to the treatments listed below to answer the following question.
You isolate an infectious substance capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal to analyze the substance and determine the nature of the infectious agent.
I. Treat the substance with enzymes that destroy all nucleic acids and then determine whether the substance is still infectious.
II. Filter the substance to remove all elements smaller than what can be easily seen under a light microscope.
III. Culture the substance on nutritive medium, away from any plant cells.
IV. Treat the sample with proteases that digest all proteins and then determining whether the substance is still infectious.
If you already know that the infectious agent was either bacterial or viral, which method(s) listed above would allow you to distinguish between these two possibilities?

Accepted Answer

Methods II and III can help distinguish between bacteria and viruses. Filtering the substance (II) can remove bacteria because they are larger than viruses, which can pass through the filter. Culturing the substance on nutritive medium (III) can allow bacteria to grow, as they can reproduce on their own in such environments, whereas viruses cannot reproduce without a host cell.

Question 3

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

Accepted Answer

E.coli cells grown on 15N medium have all their DNA labeled with heavy nitrogen (15N). When transferred to 14N medium, the newly synthesized DNA will contain only light nitrogen (14N). After two rounds of DNA replication, there will be two types of DNA in the cells: DNA that is made entirely of heavy nitrogen and DNA that is made of a mix of heavy and light nitrogen. This will result in two bands during the centrifugation: one band with only heavy DNA that will be at the bottom of the tube (low-density) and one band with a mix of heavy and light DNA that will be in the middle of the tube (intermediate-density).

Question 4

In the polymerization of DNA, a phosphodiester bond is formed between a phosphate group of the nucleotide being added and ________ of the last nucleotide in the polymer.

Accepted Answer

A phosphodiester bond is formed between the phosphate group of the nucleotide being added and the 3' hydroxyl (OH) group of the last nucleotide in the polymer. This forms the backbone of the DNA strand. The 5' phosphate group of the nucleotide being added is also involved in the bonding process, but it does not directly bond with the last nucleotide in the polymer. Nitrogen-containing bases also play a role in DNA polymerization, but they do not participate in the formation of phosphodiester bonds.

Question 5

Of the following, which is the most current description of a gene?

Accepted Answer

This is the most current and widely accepted definition of a gene. It acknowledges that genes can code for either RNA or polypeptides, and that gene expression can be influenced by a variety of factors beyond the DNA sequence itself. The other answer choices are outdated or incomplete definitions of a gene.

Question 6

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

Accepted Answer

Hershey and Chase used the fact that DNA contains phosphorus (in the form of phosphate groups), while proteins do not. They labeled bacteriophages with either radioactive sulfur (which is found in the protein component of the virus) or radioactive phosphorus (which is found in the DNA component of the virus). They found that only the phages labeled with radioactive phosphorus were able to transfer genetic information to host cells, indicating that DNA, not protein, is the genetic material.

Question 7

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have ________.

Accepted Answer

The new phages produced would have T4 protein and T4 DNA because the phage's protein coat does not dictate the DNA it carries or produces; the DNA inside the phage does. Since the bacteriophage was constructed with T4 phage DNA, the new phages produced would reflect the genetic material of T4, including both its DNA and the protein coat specified by that DNA.

Question 8

Which of the following characteristics, structures, or processes is common to bacteria and viruses?

Accepted Answer

Both bacteria and viruses have genetic material composed of nucleic acid, which can either be DNA or RNA.

Question 9

Use the following information to answer the question below.
The herpes viruses are important enveloped DNA viruses that cause disease in vertebrates and in some invertebrates such as oysters. Some of the human forms are herpes simplex virus (HSV) types I and II, causing facial and genital lesions, and the varicella zoster virus (VSV), causing chicken pox and shingles. Each of these three actively infects nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can later reactivate, replicate again, and infect others.
In electron micrographs of HSV infection, it can be seen that the intact virus initially reacts with cell surface proteoglycans, then with specific receptors. This is later followed by viral capsids docking with nuclear pores. Afterward, the capsids go from being full to being "empty." Which of the following best fits these observations?

Accepted Answer

The observation that viral capsids dock with nuclear pores and subsequently become empty suggests that the viral envelope mediates entry into the cell, the capsid mediates entry into the nuclear membrane, and the genome is all that enters the nucleus. Therefore, choice D is the best fit.

Question 10

In the figure associated with this question, which of the three types of viruses shown would you expect to include a capsid(s)?

Accepted Answer

The answer of  In the figure associated with this...

Question 11

Semiconservative replication involves a template. What is the template?

Accepted Answer

The template is the strand of the DNA molecule that is used as a guide for the synthesis of a complementary strand during replication. In semiconservative replication, one strand of the original DNA molecule is conserved (hence "semiconservative"), and used as a template for the synthesis of a new complementary strand. Therefore, option C is the correct answer.

Question 12

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

Accepted Answer

The nucleotide composition of DNA follows Chargaff's rule, where A is always equal to T and C is always equal to G. Therefore, A + C = G + T.

Question 13

Who performed classic experiments that supported the semiconservative model of DNA replication?

Accepted Answer

Meselson and Stahl performed an experiment in which they labeled DNA with heavy nitrogen (15N) and then switched to light nitrogen (14N) and analyzed the density of the resulting DNA. Their results supported the idea that DNA replication occurs in a semiconservative manner, with each new strand of DNA containing one original (parental) strand and one newly synthesized strand. Watson and Crick proposed the structure of DNA, Hershey and Chase conducted experiments suggesting that DNA, not protein, is the genetic material, and Franklin and Wilkins contributed to the understanding of DNA structure through X-ray crystallography.

Question 14

Within a double-stranded DNA molecule, adenine forms hydrogen bonds with thymine and cytosine forms hydrogen bonds with guanine. This arrangement ________.

Accepted Answer

The complementary base pairing between adenine and thymine, and cytosine and guanine, allows DNA replication and the accurate transmission of genetic information during cell division.

Question 15

In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules?

Accepted Answer

The base-pairing rules in DNA state that adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Therefore, the total amount of A and T will be equal to the total amount of C and G, making option B correct.

Question 16

DNA is synthesized through a process known as ________.

Accepted Answer

DNA is synthesized through semiconservative replication, which involves the separation of the two strands of the original DNA molecule, each serving as a template for a new complementary strand to be synthesized by the incorporation of nucleotides according to base-pairing rules. Conservative replication and translation are not involved in the synthesis of DNA, while transcription produces RNA rather than DNA.

Question 17

DNA contains the template needed to copy itself, but it has no catalytic activity in cells. What catalyzes the formation of phosphodiester bonds between adjacent nucleotides in the DNA polymer being formed?

Accepted Answer

DNA polymerase is the enzyme responsible for catalyzing the formation of phosphodiester bonds between adjacent nucleotides in the DNA polymer being formed. Ribozymes catalyze chemical reactions in RNA molecules, but not in DNA. ATP and deoxyribonucleotide triphosphates are involved in energy transfer and building blocks of DNA, respectively, but they do not directly catalyze the formation of phosphodiester bonds.

Question 18

Refer to the figure associated with this question. Which structure is responsible for stabilizing DNA in its single-stranded form?

Accepted Answer

Structure C is the single-stranded DNA binding protein (SSB), which helps to stabilize single-stranded DNA and prevent it from re-forming double-stranded DNA.

Question 19

Suppose you are provided with an actively dividing culture of E. coli bacteria to which radioactive thymine has been added. What would happen if a cell replicates once in the presence of this radioactive base?

Accepted Answer

During DNA replication, each strand serves as a template for the synthesis of a complementary strand. In the presence of radioactive thymine, one strand of the parental DNA would incorporate the radioactive thymine into its new complementary strand during replication. This would result in both daughter cells having radioactive DNA.

Question 20

Viruses ________.

Accepted Answer

Viruses do not have the ability to manufacture their own ATP, proteins, and nucleic acids, nor do they metabolize food to produce their own ATP. Instead, viruses rely on the host cell to provide the resources needed for replication and protein synthesis. They use the host cell machinery to copy their genetic material and to make the viral proteins needed for assembly into new virus particles.

Quiz 15: Dna and the Gene: Synthesis and Repair

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication) . DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?