Question 1

In a healthy cell,the rate of DNA repair is equal to the rate of DNA mutation.When the rate of repair lags behind the rate of mutation,what is a possible fate of the cell?

Accepted Answer

If the rate of repair lags behind the rate of mutation, the cell will accumulate errors in its DNA, increasing the risk of developing mutations that may lead to uncontrolled growth and division, which is a hallmark of cancer cells. Thus, the possible fate of the cell is to transform into a cancerous cell. Options B and C are not relevant to the question, and D does not reflect the potential consequences of impaired DNA repair.

Question 2

Telomere shortening is a problem in which types of cells?

Accepted Answer

Telomere shortening is a problem that occurs in eukaryotic cells, specifically in cells that divide frequently such as stem cells and cancer cells. Prokaryotic cells do not have telomeres as their genetic material is arranged differently.

Question 3

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, meaning it has a chemical modification (addition of a methyl group) to certain bases. The enzyme complex can distinguish between the two strands by recognizing the presence of these methylated bases on the parent strand. This allows the complex to choose the new, unmethylated strand for repair.

Question 4

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

Accepted Answer

Telomerase activity is typically restricted to cells that have a high proliferative capacity, such as stem cells and cancer cells. In most normal somatic cells, telomerase activity is turned off, leading to a progressive shortening of telomeres over time. This limits the number of times a cell can divide before undergoing replicative senescence or entering a state of permanent growth arrest known as cellular senescence. In contrast, most normal germ cells and cancer cells have active telomerase activity, which allows them to maintain their telomere length and continue to proliferate.

Question 5

If DNA repair mechanisms fail,which of the following is not a direct result?

Accepted Answer

Failure of DNA repair mechanisms can lead to mutations and increase the risk of cancer, as well as lead to defective enzymes. However, DNA polymerase being slower at replicating DNA is not a direct result of failed DNA repair mechanisms.

Question 6

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 finding that mutations in one of seven genes can cause xeroderma pigmentosum indicates that there are multiple genes involved in the nucleotide excision repair process. Therefore, choice C is the correct answer. Choices A and B are not supported by the given information, and choice D is not relevant to the topic at hand.

Question 7

What is a telomere?

Accepted Answer

Telomeres are repetitive DNA sequences located at the ends of linear chromosomes that protect and stabilize the chromosome during cell division. They prevent the degradation of genetic material at the ends of chromosomes and also play a role in regulating the lifespan of cells. A, B, and C are unrelated to telomeres.

Question 8

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

Accepted Answer

The proofreading mechanism of DNA polymerase is responsible for detecting mismatches and repairing them during DNA replication. If this mechanism is not working properly, it can result in higher mutation rates.

Question 9

Which of the following would be least likely to cause DNA damage in an individual suffering from xeroderma pigmentosum?

Accepted Answer

Xeroderma pigmentosum is a genetic disorder that makes individuals extremely sensitive to ultraviolet (UV) radiation, which can cause DNA damage. Incandescent lightbulbs emit very little UV radiation, and are therefore less likely to cause DNA damage in individuals with xeroderma pigmentosum compared to natural lighting, low-level UV lights, and reflected sunlight.

Question 10

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 has approximately 6 billion base pairs. With an error rate of one mistake per billion nucleotides, there would be about 6 errors each time the entire genome is replicated.

Question 11

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

Accepted Answer

The proofreading activity of the epsilon subunit of DNA polymerase III, mismatch repair, and nucleotide excision repair all play important roles in reducing errors in DNA replication in E.coli. Proofreading activity allows for the correction of errors as they occur, mismatch repair fixes errors after replication, and nucleotide excision repair corrects errors caused by DNA damage. Therefore, all of the above options are important in minimizing errors in DNA replication in E.coli.

Question 12

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 genetic defect in HNPCC affects the mismatch repair mechanism, which normally helps to correct errors that occur during DNA replication. Without this mechanism, there is a decreased ability to repair certain types of DNA mutations, leading to an increased risk of developing colorectal cancer. Choices A, B, and C are not related to the mismatch repair mechanism and would not be expected outcomes of this mutation.

Question 13

The epsilon (ε)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 of E.coli has exonuclease activity, which means it can remove nucleotides from the 3' or 5' end of a DNA strand. During DNA replication, if the polymerase adds a mismatched nucleotide, the epsilon subunit can recognize and remove it. This proofreading process helps to increase the accuracy of DNA replication. Therefore, choice A is the correct answer.

Question 14

Which of the following agents are unlikely to cause mutations in DNA?

Accepted Answer

Radiation from an incandescent lightbulb is unlikely to cause mutations in DNA as the light produced by these bulbs is in the form of visible light, which does not have enough energy to break bonds in DNA. However, it is still recommended to use energy-efficient bulbs as they emit less heat and reduce the risk of fire hazards. The other options listed are known to cause mutations in DNA in different ways. Aflatoxins, which are produced by moldy grains, can intercalate with DNA and cause mispairings, while free radicals can damage DNA by oxidizing its bases. Ultraviolet radiation from sunlight can also cause direct damage to DNA by inducing thymine dimers.

Question 15

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 ensure that the ends of the chromosomes (telomeres) are accurately replicated, thus preventing the shortening of telomeres. This, in turn, would extend the life of the cell and delay cellular aging. Therefore, Option B is the correct answer. Option A is incorrect because the addition of telomerase would not necessarily speed up the rate of cell proliferation. Option C is incorrect because telomerase prevents telomere shortening, which actually delays cellular aging. Option D is incorrect because telomerase would have a significant effect on delaying cellular aging.

Question 16

In humans,xeroderma pigmentosum is a disorder of the nucleotide excision repair mechanism.These individuals are unable to repair DNA damage caused by ultraviolet light.Which of the following are the most prominent types of mutations in individuals suffering from xeroderma pigmentosum?

Accepted Answer

Xeroderma pigmentosum is caused by defects in nucleotide excision repair mechanism, which is responsible for removing DNA damage caused by thymine dimers induced by ultraviolet radiation. Therefore, the most prominent mutation in individuals suffering from xeroderma pigmentosum is the formation of pyrimidine dimers.

Question 17

What are pyrimidine dimers?

Accepted Answer

Pyrimidine dimers refer to adjacent pyrimidines (usually thymine) on the same DNA strand that join by covalent bonding, forming a structure that disrupts DNA replication and transcription. The most common type of pyrimidine dimer is the cyclobutane pyrimidine dimer (CPD), which forms when two adjacent thymines on the same strand of DNA are photochemically linked by a covalent bond. Pyrimidine dimers are considered a type of DNA damage and can be caused by exposure to UV radiation or certain chemical agents.

Quiz 14: Dna and the Gene: Synthesis and Repair