Question 1

What evidence suggests that while the XIST gene may be required to initiate X chromosome inactivation, it may not be required to maintain it from one generation to another?
A)Tumor cells do not maintain X chromosome inactivation from generation to generation.
B)Some tumor cells in women contain inactivated X chromosomes whose XIST gene has been deleted.
C)Some tumor cells in women contain inactivated X chromosomes whose XIST gene has been duplicated.
D)Some women have patches of cells in their retinas that are color blind and others that are not.
E)Some tumor cells in women have no X chromosomes.

Accepted Answer

Tumor cells in women contain inactivated X chromosomes whose XIST gene has been deleted, suggesting that while the XIST gene may be required to initiate X chromosome inactivation, it may not be required to maintain it from one generation to another.

Question 2

What evidence suggests that RNAi plays a role in heterochromatization?
A)Deletion of RNAi components impairs both methylation of histone H3K9 and heterochromatization.
B)Addition of RNAi components impairs both methylation of histone H3K9 and heterochromatization.
C)Deletion of RNAi components enhances both methylation of histone H3K9 and heterochromatization.
D)Mutation of RNAi components denatures both methylation of histone H3K9 and heterochromatization.

Accepted Answer

Deletion of RNAi components impairs both methylation of histone H3K9 and heterochromatization, providing evidence that RNAi plays a role in heterochromatization. This suggests that RNAi is involved in the formation of heterochromatin through its role in regulating histone modifications.

Question 3

What energy source is used to maintain the Ran-GTP gradient across the nuclear envelope? A)energy released by ATP hydrolysis B)a proton gradient C)a H⁺ ion gradient D)energy released by GTP hydrolysis E)an electron gradient

Accepted Answer

Ran-GTP is generated in the nucleus by the RanGEF protein and hydrolyzed into Ran-GDP by the RanGAP protein in the cytoplasm. Therefore, the energy source used to maintain the Ran-GTP gradient across the nuclear envelope is the energy released by GTP hydrolysis.

Question 4

In a rare disease, individuals suffer from bone marrow failure due to the inability of this blood-forming tissue to produce a sufficient number of blood cells over a human lifetime. What is NOT a cause of this condition?
A)Individuals have greatly elevated telomerase levels.
B)Individuals have greatly reduced telomerase levels.
C)Individuals are heterozygous for the gene that encodes the telomerase RNA.
D)Individuals are heterozygous for the gene that encodes the telomerase protein subunit.

Accepted Answer

Elevated telomerase levels would actually promote longer telomeres and cell division, which could potentially delay or prevent bone marrow failure. Reduced telomerase levels or mutations in the telomerase gene would be more likely causes. Heterozygosity for the gene encoding the telomerase RNA or protein subunit could potentially lead to reduced telomerase activity and contribute to bone marrow failure.

Question 5

What is thought to maintain X chromosome inactivation?
A)repressive histone modifications
B)RNA inactivation
C)DNA methylation
D)localized DNA denaturation
E)both repressive histone modifications and DNA methylation

Accepted Answer

X chromosome inactivation is maintained through a combination of repressive histone modifications and DNA methylation. The histone modifications prevent the transcription machinery from accessing the genes on the inactive X chromosome, while the DNA methylation marks the genes as inactive so future generations of cells also maintain the same inactivation pattern. RNA inactivation and localized DNA denaturation are not known to play a significant role in X chromosome inactivation.

Question 6

What experimental evidence suggests that the N-terminal tails of histones participate in the formation of higher-order chromatin structure?
A)The tails cause histones to denature.
B)The tails when acetylated cause the dissolution of the 30-nm fibers.
C)Chromatin fibers with histones lacking their tails cannot fold into higher-order fibers.
D)Chromatin fibers with histones possessing longer tails cannot fold into higher-order fibers.
E)Chromatin fibers lacking tails are shorter.

Accepted Answer

Experimental evidence suggests that chromatin fibers with histones lacking their tails cannot fold into higher-order fibers. For example, studies with reconstituted chromatin fibers have shown that histone tails are required for the formation of the 30-nm fiber and higher-order chromatin structure. In addition, deletion of the N-terminal tails of histones in vivo leads to alterations in chromatin structure and gene expression.

Question 7

Genes are physically moved to sites within the nucleus called _________, where the transcription machinery is concentrated and within which genes involved in the same response tend to become colocalized.
A)transcription homes
B)transcription sites
C)transcription factories
D)translation factories
E)nucleosynthetic locales

Accepted Answer

The term "transcription factories" refers to sites within the nucleus where the transcription machinery is concentrated and where genes involved in the same response tend to become colocalized. Therefore, choice C is the best answer. The other options - A, B, D, and E - are not accurate terms for describing this phenomenon.

Question 8

Why was it initially expected that cloned animals might have a shorter life span?
A)The chromosomes of the donor nuclei had shortened telomeres.
B)The chromosomes of the donor nuclei had lengthened telomeres.
C)The egg cell had elevated telomerase.
D)The donor cell had elevated telomerase.
E)After transplant of the nucleus, telomerase levels climbed precipitously.

Accepted Answer

Telomeres are the protective ends of chromosomes that shorten with each cell division, eventually leading to cell death. Since donor cells used in cloning are usually already differentiated and have gone through many cell divisions, their telomeres are already shorter than those in embryonic cells. Thus, clones formed from mature cells were initially expected to have shorter telomeres and a shorter lifespan.

Question 9

What distinguishes the regulatory, noncoding RNA that is suspected of being responsible for X chromosome inactivation from other noncoding RNA molecules?
A)It is much larger than the other noncoding RNAs.
B)It is much smaller than the other noncoding RNAs.
C)It is more tightly coiled than the other noncoding RNAs.
D)It contains a higher content of uracil than the other noncoding RNAs.
E)It contains a higher content of adenine than the other noncoding RNAs.

Accepted Answer

The regulatory, noncoding RNA responsible for X chromosome inactivation, known as Xist RNA, is distinguished by its large size compared to other noncoding RNAs.

Question 10

Which is NOT an example of an epigenetic phenomenon?
A)X chromosome inactivation
B)histone methylation patterns
C)sequence of distal promoter elements
D)localization of histone variant CENP-A
E)covalent modification of DNA

Accepted Answer

The sequence of distal promoter elements is a genetic phenomenon, not an epigenetic one. Epigenetic phenomena involve modifications to DNA or associated proteins that can alter gene expression without changing the underlying DNA sequence. X chromosome inactivation, histone methylation patterns, localization of histone variant CENP-A, and covalent modification of DNA are all examples of epigenetic phenomena.

Question 11

Which of the following would be the most acceptable explanation for why cloned animals actually have telomeres of normal length on their chromosomes?
A)The telomeres were elongated before transfer to the egg by telomerase in egg cell cytoplasm.
B)The telomeres were elongated after transfer to the egg by telomerase in egg cell cytoplasm.
C)The telomeres were shortened before transfer to the egg by telomerase in egg cell cytoplasm.
D)The telomeres were shortened after transfer to the egg by telomerase in egg cell cytoplasm.
E)The telomeres were replaced prior to mitosis by DNA repair.

Accepted Answer

Cloned animals have telomeres of normal length on their chromosomes because the telomeres were elongated after transfer to the egg by telomerase in egg cell cytoplasm. This is because the somatic cells used for cloning have shortened telomeres due to repeated cell division, but the egg cell used for cloning still has telomerase activity. When the somatic cell nucleus is transferred to the egg cell, the telomerase in the egg cell adds telomeres to the transferred nucleus, resulting in normal-length telomeres in the cloned animal.

Question 12

What is the name of the gene responsible for the inactivation of the X chromosome?
A)INAC
B)XIST
C)1STX
D)IRS
E)XCHR

Accepted Answer

The gene responsible for the inactivation of the X chromosome is called XIST (X-inactive specific transcript). It is located on the X chromosome and produces an RNA molecule that coats one of the two X chromosomes in female cells and leads to its inactivation. Choices A, C, D, and E are not related to this process.

Question 13

When an importin α/β complex carries a protein destined to move into the nucleus, what does it bind to when it comes to the outer surface of the nucleus near the nuclear pore complex?
A)the torus
B)the outer ring of the nuclear pore complex
C)the inner ring of the nuclear pore complex
D)cytoplasmic filaments that extend from the outer ring of the nuclear pore complex
E)the transporter

Accepted Answer

The cytoplasmic filaments that extend from the outer ring of the nuclear pore complex are the binding site for the importin α/β complex when it reaches the outer surface of the nucleus. This binding allows the complex to be directed towards the central channel of the nuclear pore complex for nuclear import.

Question 14

A stretch of repeated sequences at the tip of a DNA molecule forming a cap at the end of the chromosome is called a ________.
A)telomere
B)chromomere
C)centromere
D)telophase
E)karyomere

Accepted Answer

The stretch of repeated sequences at the end of a DNA molecule that forms a cap is called a telomere. Chromomeres are distinct regions of condensed chromosome when viewed under a microscope. Centromeres are the region of a chromosome where the two sister chromatids are joined. Telophase is the final stage of mitosis where chromosomes have reached opposite poles and the nuclear envelope starts to reform. Karyomere is not a recognized term in biology.

Question 15

What happens if histone H1 is selectively extracted from compacted chromatin?
A)chromatin uncoils to form a more extended beaded filament
B)chromatin uncoils to form a less extended cylindrical filament
C)chromatin completely disassembles to its component nucleotides
D)chromatin breaks into small fragments
E)chromatin breaks into large fragments

Accepted Answer

Histone H1 is responsible for producing compacted chromatin by linking the nucleosomes together through the DNA linker region. If histone H1 is selectively extracted, the linker DNA becomes more accessible to nucleases which will break the DNA and result in the uncoiling of chromatin to form a more extended beaded filament.

Question 16

Scientists can stain chromosomes from mitotic cells and photograph the chromosomes using a microscope. Each chromosome can then be cut out of the photograph, the chromosomes matched up in homologous pairs and placed in order of decreasing size. This methodcan be used to screen individuals for chromosomal abnormalities, like extra, missing or grossly altered chromosomes. What is such a picture called?
A)karyoplan
B)eukaryote
C)karyotype
D)karyokinesis
E)prokaryote

Accepted Answer

The picture is called a karyotype, which shows the chromosomes arranged in homologous pairs and ordered by size. The other options are not applicable or do not describe the specific image produced by staining and examining chromosomes.

Question 17

The fact that chromosomes should become shorter and shorter with each round of cell division is referred to as the ________.
A)shortening contradiction
B)the replication paradox
C)the short end hypothesis
D)the truncated end problem
E)the end-replication problem

Accepted Answer

The fact that chromosomes become shorter and shorter with each round of cell division is known as the end-replication problem. This is because during DNA replication, the enzymes responsible for replicating the very end of the chromosome cannot complete the process, resulting in a shorter end.

Question 18

If cells cannot replicate the ends of their DNA, what should happen with each round of cell division?
A)The chromosomes should stay exactly the same length.
B)The chromosomes should get longer.
C)The mitotic spindle should shrink.
D)The chromosomes should get shorter.
E)The chromosomes should denature.

Accepted Answer

As cells divide, the DNA must be replicated to ensure that the daughter cells receive a complete copy of the genetic material. However, the replication machinery cannot copy the very ends of chromosomes, known as telomeres. Therefore, with each round of cell division, the telomeres will gradually get shorter. Over time, this can lead to cell senescence or death, as critical genetic information is lost.

Question 19

What is the advantage of the highly condensed state of the DNA of mitotic chromosomes?
A)This state allows gene expression needed during mitosis.
B)The highly condensed state favors delivery of an intact package of DNA to each daughter cell.
C)The condensed state allows chromosomes to fuse more easily.
D)The condensed state allows replication to occur more rapidly as the cell enters mitosis.
E)The condensed state favors DNA repair prior to mitosis.

Accepted Answer

The highly condensed state of DNA in mitotic chromosomes allows for a more efficient and intact delivery of chromosomes to each daughter cell during cell division. This ensures that each cell receives a complete and accurate set of genetic information.

Question 20

There are genes on the X chromosome that manage to escape inactivation. What particular genes are included in this group and what might be the reason that they remain activated?
A)genes that are missing from the X chromosome; so they will be expressed equally in both sexes
B)genes that are also present on the Y chromosome; so they will be expressed equally in both sexes
C)genes that are also found on Chromosome 1; so they will be expressed equally in both sexes
D)centromere genes; so that all cells will be able to attach to the spindle
E)centromere genes; so they will be expressed equally in both sexes

Accepted Answer

The genes on the X chromosome that escape inactivation are also present on the Y chromosome. This is because in males, who have only one X chromosome, these genes need to be expressed for proper development and function. This suggests that the reason these genes remain active on one X chromosome in females is due to their importance for normal cellular function.

Quiz 12: Control of Gene Expression

What evidence suggests that while the XIST gene may be required to initiate X chromosome inactivation, it may not be required to maintain it from one generation to another?

What evidence suggests that RNAi plays a role in heterochromatization?

What energy source is used to maintain the Ran-GTP gradient across the nuclear envelope?

In a rare disease, individuals suffer from bone marrow failure due to the inability of this blood-forming tissue to produce a sufficient number of blood cells over a human lifetime. What is NOT a cause of this condition?

What is thought to maintain X chromosome inactivation?

What experimental evidence suggests that the N-terminal tails of histones participate in the formation of higher-order chromatin structure?

Genes are physically moved to sites within the nucleus called _________, where the transcription machinery is concentrated and within which genes involved in the same response tend to become colocalized.

Why was it initially expected that cloned animals might have a shorter life span?

What distinguishes the regulatory, noncoding RNA that is suspected of being responsible for X chromosome inactivation from other noncoding RNA molecules?

Which is NOT an example of an epigenetic phenomenon?

Which of the following would be the most acceptable explanation for why cloned animals actually have telomeres of normal length on their chromosomes?

What is the name of the gene responsible for the inactivation of the X chromosome?

When an importin α/β complex carries a protein destined to move into the nucleus, what does it bind to when it comes to the outer surface of the nucleus near the nuclear pore complex?

A stretch of repeated sequences at the tip of a DNA molecule forming a cap at the end of the chromosome is called a ________.

What happens if histone H1 is selectively extracted from compacted chromatin?

The fact that chromosomes should become shorter and shorter with each round of cell division is referred to as the ________.

If cells cannot replicate the ends of their DNA, what should happen with each round of cell division?

What is the advantage of the highly condensed state of the DNA of mitotic chromosomes?

There are genes on the X chromosome that manage to escape inactivation. What particular genes are included in this group and what might be the reason that they remain activated?