Question 1

In the mismatch repair process, enzyme complexes replace bases that were incorrectly inserted into the newly synthesized DNA strand. To function, they must be able to distinguish between the parent DNA strand and the new strand. How is this accomplished?

Accepted Answer

The parent strand is methylated, which creates a small but detectable difference between the parent and newly synthesized strands. This allows the enzyme complexes to distinguish between the two and properly repair any mismatched bases.

Question 2

What are thymine dimers?

Accepted Answer

Thymine dimers are formed when two adjacent thymine bases on the same DNA strand become covalently bonded due to damage, often caused by ultraviolet (UV) radiation. This bonding can lead to errors in DNA replication and transcription.

Question 3

Telomere shortening is a problem in which types of cells?

Accepted Answer

Telomere shortening occurs in eukaryotic cells because they have linear chromosomes with telomeres at the ends that protect DNA from degradation and fusion with other chromosomes during replication. Prokaryotic cells, on the other hand, have circular chromosomes with no telomeres and different mechanisms to ensure the integrity of their genetic material.

Question 4

Which one of the following is least likely to cause mutations in DNA?

Accepted Answer

Light from an incandescent bulb does not have enough energy to cause mutations in DNA. The other options either directly or indirectly cause damage to DNA, which can result in mutations.

Question 5

Recent studies have shown that xeroderma pigmentosum an error in the nucleotide excision repair process) can result from mutations in one of seven genes. What can you infer from this finding?

Accepted Answer

The fact that mutations in one of seven genes can result in xeroderma pigmentosum indicates that there are several enzymes involved in the nucleotide excision repair process.

Question 6

Telomere shortening puts a limit on the number of times a cell can divide. Research has shown that telomerase can extend the life span of cultured human cells. How might adding telomerase affect cellular aging?

Accepted Answer

Telomerase is an enzyme that adds telomeric DNA sequences to telomeres. The addition of telomerase would eliminate telomere shortening and allow for continued cell division, ultimately leading to a retardation of aging. Studies have shown that telomerase expression can lead to an extension of the lifespan of cells in culture.

Question 7

Hereditary nonpolyposis colorectal cancer HNPCC) is an inherited disorder. The genetic defect identified is an error in the mismatch repair mechanism. Which of the following would be an expected result of this mutation?

Accepted Answer

The mutation in the mismatch repair mechanism would result in a decreased ability to repair certain DNA mutations, including those that could lead to the development of HNPCC.

Question 8

The epsilon s) subunit of DNA polymerase III of E. coli has exonuclease activity. How does it function in the proofreading process?

Accepted Answer

The epsilon subunit of DNA polymerase III functions in proofreading by having exonuclease activity that can remove a mismatched nucleotide. This helps to ensure that the correct nucleotide is incorporated into the growing DNA strand, increasing the accuracy of DNA replication.

Question 9

DNA replication is highly accurate. It results in about one mistake per billion nucleotides. For the human genome, how often would errors occur?

Accepted Answer

The human genome contains approximately 3 billion base pairs. Given the error rate of one mistake per billion nucleotides, this would result in about 3 mistakes each time the entire genome is replicated. Since each base pair consists of two nucleotides (one from each strand of the DNA double helix), and replication involves copying both strands, this effectively doubles the number of errors to 6 per genome replication.

Question 10

What is a telomere?

Accepted Answer

A telomere is a region of repetitive nucleotide sequences found at the ends of linear chromosomes. It serves to protect the coding regions of DNA from degradation and fusion with neighboring chromosomes.

Question 11

Which of the following agents can cause mutations in DNA?

Accepted Answer

All three agents listed are known to cause mutations in DNA. Ultraviolet radiation from sunlight can cause thymine dimers to form in DNA, which can lead to errors in replication or transcription. Aflatoxins are potent carcinogens produced by certain species of Aspergillus molds that can alter the DNA and RNA in cells. Free radicals, which are highly reactive molecules with unpaired electrons, can damage DNA and other biomolecules through oxidation reactions, resulting in mutations. Therefore, all three agents are mutagenic and can cause mutations in DNA.

Question 12

If during the process of DNA replication, the enzyme topoisomerase was not functioning i.e. it was mutated) what do you expect will happen to the replication process?

Accepted Answer

Topoisomerase plays a crucial role in removing the supercoils/torsion generated during DNA unwinding. The absence of topoisomerase will lead to over-twisting of the DNA, which will ultimately lead to the termination of DNA unwinding prematurely. Therefore, the correct choice is C.

Question 13

If DNA repair mechanisms fail, what is the result?

Accepted Answer

When DNA repair mechanisms fail, it can lead to a variety of consequences such as cancer, mutations, and the production of defective enzymes. Therefore, choice D is the correct answer.

Question 14

What appears to be a dark side to telomerase activity with regard to human health?

Accepted Answer

Telomerase is active in most cancer cells, allowing them to divide indefinitely and potentially become malignant. This highlights the dark side of telomerase activity with regard to human health.

Question 15

Which of the following cells do not have active telomerase activity?

Accepted Answer

Most normal somatic cells do not have active telomerase activity, while most cancer cells and some germ cells do have active telomerase activity. This is because telomerase activity is responsible for the maintenance of telomere length, which is important for cell proliferation and immortality. Most somatic cells do not need to divide indefinitely and therefore do not require active telomerase, while cancer cells and germ cells have a high rate of cell division and therefore require active telomerase to maintain their telomere length.

Question 16

Researchers found E. coli that had mutation rates 100 times higher than normal. What is a possible explanation for these results?

Accepted Answer

A higher mutation rate could be due to a problem with the proofreading mechanism of DNA polymerase, which normally corrects errors during DNA replication.

Question 17

Bodnar et al. 1998) used telomerase to extend the life span of normal human cells. Telomere shortening puts a limit on the number of times a cell can divide. How might adding telomerase affect cellular aging?

Accepted Answer

Adding telomerase would prevent telomere shortening, which is a hallmark of cellular aging. This would allow cells to divide more times before reaching the Hayflick limit and undergoing senescence. Therefore, telomerase would extend the life span of cells and potentially delay the onset of cellular aging.

Question 18

Which of the following are important in reducing the errors in DNA replication in E. coli organisms?

Accepted Answer

All of the options listed are important in reducing errors in DNA replication in E. coli. Mismatch repair and nucleotide excision repair help to correct errors after they have occurred, while the proofreading activity of the epsilon subunit of DNA polymerase III helps to prevent errors during replication. Together, these mechanisms work to minimize errors in DNA replication in E. coli.

Question 19

Which of the following would you expect of a eukaryote lacking telomerase?

Accepted Answer

Telomerase is responsible for adding repetitive DNA sequences to the ends of chromosomes, called telomeres, which maintain chromosome length during DNA replication. Without telomerase, as cells divide, telomeres gradually shorten until they become too short, leading to cell death or senescence. In gametes, the chromosome length must be maintained to ensure proper fertilization and development of the offspring. Therefore, a lack of telomerase would result in a reduction in chromosome length in gametes.

Quiz 15: Dna and the Gene: Synthesis and Repair