Question
Erwin Chargaff investigated the nucleotide composition of DNA. He analyzed DNA from various organisms and measured the relative amounts of adenine (A), guanine (G), cytosine (C), and thymine (T) present in the DNA of each organism. Table 1 contains a selected data set of his results.
Which of the following statements best explains the data set?
A. Since the %A and the %G add up to approximately 50 percent in each sample, adenine and guanine molecules must pair up in a double-stranded DNA molecule.
B. Since the %A and the %T are approximately the same in each sample, adenine and thymine molecules must pair up in a double-stranded DNA molecule.
C. Since the %(A+T) is greater than the %(G+C) in each sample, DNA molecules must have a poly-A tail at one end.
D. Since the %C and the %T add up to approximately 50 percent in each sample, cytosine and thymine molecules must both contain a single ring.
▶️Answer/Explanation
Ans: B
The percentages of adenine and thymine are approximately the same because adenine aligns with thymine in double-stranded DNA and they are therefore present in each organism in the same amounts.
Question
Which of the following statements best explains the structure and importance of plasmids to prokaryotes?
A. Plasmids are circular, single-stranded RNA molecules that transfer information from the prokaryotic chromosome to the ribosomes during protein synthesis.
B. Plasmids are circular, double-stranded DNA molecules that provide genes that may aid in survival of the prokaryotic cell.
C. Plasmids are single-stranded DNA molecules, which are replicated from the prokaryotic chromosome, that prevent viral reproduction within the prokaryotic cell.
D. Plasmids are double-stranded RNA molecules that are transmitted by conjugation that enable other prokaryotic cells to acquire useful genes.
▶️Answer/Explanation
Ans: B
Plasmids are small, circular, double-stranded DNA molecules. Plasmids can be acquired and can be replicated by prokaryotes. Plasmids may carry genes, such as antibiotic resistance genes, that may help the prokaryote survive.
Question
Antibiotics can be used to kill the specific pathogenic bacterium, Mycobacterium tuberculosis, that causes tuberculosis. The appearance of antibiotic-resistant strains has made it more difficult to cure M. tuberculosis infections. These antibiotic-resistant bacteria survive and pass on the genes to their offspring, making the resistant phenotype more common in the population. DNA analysis indicates that the genes for antibiotic resistance are not normally present in bacterial chromosomal DNA.
Which of the following statements best explains how the genes for antibiotic resistance can be transmitted between bacteria without the exchange of bacterial chromosomal DNA?
A. The antibiotic-resistant bacteria release a hormone that signals neighboring bacteria to become resistant.
B. The genes for antibiotic resistance are located on a plasmid that can be passed to neighboring bacteria.
C. The antibiotic-resistant bacteria are the result of bacteria that specifically modify their own chromosomal DNA to neutralize the antibiotics.
D. The antibiotic alters the bacterial genome of each bacterium, which results in an antibiotic-resistant population.
▶️Answer/Explanation
Ans: B
Bacteria can exchange and express the genes found on plasmids, which are foreign, extrachromosomal loops of DNA that they pick up. Genes for antibiotic resistance are located on these plasmids.
Question
The genome of a newly discovered virus has the following nucleotide composition: 22% guanine, 16% cytosine, 34% adenine, and 28%
uracil. Based on the nucleotide composition, the genome of this virus is most likely made of which of the following?
(A) single-stranded DNA
(B) double-stranded DNA
(C) single-stranded RNA
(D) double-stranded RNA
▶️Answer/Explanation
Ans:
(C) Since the question states that the virus’s genome contains uracil,
choices (C) and (D) are possibilities since RNA contains uracil and
DNA does not; thus, rule out choices (A) and (B). However, since the
percentage of guanine does not equal the percentage of cytosine and
the percentage of adenine does not equal the percentage of uracil, it
cannot be a double-stranded virus, so choice (C) is the best answer.
Question
Prokaryotic genomes are packaged into _________ chromosomes in the _________.
(A) circular; nucleoid region
(B) circular; nucleus
(C) linear; nucleoid region
(D) linear; nucleus
▶️Answer/Explanation
Ans:
(A) Prokaryotes have circular chromosomes in the nucleoid region of
their cells. Choice (B) is incorrect because prokaryotes do not have a
nucleus. Choices (C) and (D) are incorrect because prokaryotes do not
have linear chromosomes; eukaryotes have linear chromosomes.