Question 1

Which of the following statements is true regarding heterochromatin?

Accepted Answer

Specialized chromatin structures (scs) can act as insulators to protect genes from the repressive effects of nearby heterochromatin. Heterochromatin is tightly packed and transcriptionally inactive, so it would not represent transcriptionally active regions of the chromosome (choice B). Heterochromatin stains darkly when stained with Feulgen reagent, not lightly (choice A). Euchromatic genes that are artificially placed next to heterochromatin are transcribed at abnormally low levels, not high levels (choice D). Heterochromatin is resistant to DNAse digestion, not hypersensitive (choice E).

Question 2

The temporal sequence of puffing in polytene chromosomes is controlled by

Accepted Answer

The temporal sequence of puffing in polytene chromosomes is controlled by the hormone ecdysone. This hormone is secreted during the molting stage of insects and triggers the transformation of the larva into a pupa or an adult. Ecdysone regulates the expression of genes involved in cell differentiation, development, and metamorphosis, including those that induce the appearance of puffs in the polytene chromosomes. Therefore, A is the best choice. None of the other hormones mentioned have been directly linked to the regulation of polytene chromosome puffing.

Question 3

The use of small RNA molecules to interfere with gene expression is known as

Accepted Answer

The use of small RNA molecules to interfere with gene expression is known as RNA interference (RNAi). Through this process, small RNA molecules can bind to messenger RNAs (mRNAs) and prevent them from being translated into proteins, effectively silencing the expression of the targeted gene.

Question 4

Which of the following is true regarding eukaryotic transcription factors?

Accepted Answer

Many transcription factors have dimerization motifs, which allow them to form complexes with other transcription factors and bind DNA sequences with greater affinity and specificity. This enhances the efficiency and regulation of gene expression. Options A, C, and D are incorrect as they are not true statements. Option E is partially true, as homeodomains are a specific class of transcription factors, but not all transcription factors belong to this class.

Question 5

RISC-associated RNAs that result in mRNA cleavage are usually termed

Accepted Answer

The RISC (RNA-induced silencing complex) is composed of small RNAs that guide the complex to complementary target RNAs, resulting in mRNA cleavage. These small RNAs are typically called siRNAs (short interfering RNAs).

Question 6

An interaction mediated by base pairing between the single strand of RNA in the RNA-protein complex and a complementary sequence in the messenger RNA molecule prevents the expression of the gene that produced the mRNA.This is known as

Accepted Answer

RNA Induced Silencing Complex (RISC) is involved in gene silencing through the mechanism of RNA interference (RNAi), where it mediates the interaction between small RNA molecules (such as siRNA or miRNA) and their target mRNA, leading to the mRNA's degradation or translational repression.

Question 7

In the human genes for b-globin,a locus control region is characterized by

Accepted Answer

A locus control region (LCR) is a regulatory region that is responsible for controlling the expression of genes within a given locus. In the case of the human b-globin genes, the LCR is located upstream of the genes and is about 15 kb in length. The LCR contains several DNAse I hypersensitive sites, which are regions of chromatin that are more accessible to cleavage by DNAse I than other regions. These hypersensitive sites are thought to reflect regions of the chromatin that are actively involved in controlling gene expression. Therefore, all of the given choices are correct.

Question 8

Enhancers are

Accepted Answer

Enhancers are specialized cis-regulatory regions located near a gene to which transcription factors bind, leading to an increase in gene expression. They can act over long distances and can also work in conjunction with other enhancers to fine-tune gene expression levels. Enhancers are not domains in proteins and do not act as binding sites for basal transcription factors, although they can work together with these factors to regulate gene expression. While enhancers are generally associated with enhancing gene expression, they can also act to inhibit transcription in some cases.

Question 9

In transcribed DNA,nucleosomes are altered by multiprotein complexes that ultimately facilitate the action of RNA polymerase in a process known as

Accepted Answer

The process described of alterations to nucleosomes that facilitate the action of RNA polymerase is known as chromatin remodeling. Choice A (chromosome reworking) and choice D (chromosome altering) use terminology that is not specific to the described process. Choice C (chromatin editing) is not a commonly used term in molecular biology. Therefore, the best choice is B, chromatin remodeling.

Question 10

Which of the following could be a source for a siRNA or miRNA molecule?

Accepted Answer

All of these options could potentially be sources for siRNA or miRNA. 
- The Mir gene produces precursor miRNA molecules which are processed into mature miRNA. 
- Transposons have been found to produce siRNA which can silence transposon activity. 
- RNA viruses can produce both siRNA and miRNA as part of their replication strategy. 
- Transgenes can be designed to produce siRNA or miRNA molecules that target specific genes.

Question 11

Whenever the RNA within the RISC pairs imperfectly with its target sequence,the mRNA is usually not cleaved; instead,translation of the mRNA is inhibited.RISC-associated RNAs that have this effect are usually termed

Accepted Answer

The statement describes the function of miRNAs, which inhibit translation of mRNA when they pair imperfectly with the target sequence. SiRNAs, on the other hand, usually cleave mRNA that fully matches with the target sequence. SNRPs are not involved in RNA interference, mtDNAs encode mitochondrial genes, and STRs are short tandem repeats in DNA.

Question 12

The nuclease sensitivity of transcriptionally active genes depends on at least two small nonhistone proteins known as

Accepted Answer

The question specifically mentions two proteins and among the choices given, only option D includes both HMG 14 and HMG 17, which are known to be involved in the nuclease sensitivity of transcriptionally active genes. Therefore, option D is the correct answer.

Question 13

Which of the following molecules participates in RNA interference?

Accepted Answer

RNA interference involves the use of short interfering RNAs (siRNAs) that are typically 20-25 nucleotides in length and function to degrade or inhibit the expression of specific target messenger RNAs (mRNAs). tRNA and rRNA are involved in translation, while dsDNA is not an RNA molecule and does not participate directly in RNA interference.

Question 14

Which of the following motifs is not characteristic of a eukaryotic transcription factor?

Accepted Answer

Transmembrane domains are not typically found in transcription factors, as they are usually located in the cytoplasm or nucleus of the cell where they bind to DNA to regulate gene expression. Helix-turn-helix, leucine zippers, and helix-loop-helix motifs are all commonly found in eukaryotic transcription factors.

Question 15

Which type of enzyme produces siRNA molecules in eukaryotes?

Accepted Answer

Dicer enzymes are responsible for producing small interfering RNA (siRNA) molecules in eukaryotes. These enzymes cleave double-stranded RNA into small fragments, which are then incorporated into the RNA-induced silencing complex (RISC) to target complementary mRNA molecules for degradation.

Question 16

DNA regions which are locally unwound because transcription has begun,and which are sensitive to low doses of DNAse 1 are known as

Accepted Answer

DNase 1 hypersensitive sites are regions of DNA that are sensitive to the enzyme DNase 1, which cleaves DNA at locations where the DNA is not tightly packaged. These hypersensitive sites occur where the DNA is locally unwound due to transcription initiation or other factors, and can be used to identify regulatory elements such as promoters and enhancers. RNase hypersensitive sites do not exist, and RNase hyposensitive sites are not sensitive to the RNase enzyme. DNase 1 hyposensitive sites do exist but are not relevant to this question.

Question 17

Which of the following gives support to the idea that transcription takes place in "open" regions of the chromosome?
1)The fact that 3H-uridine incorporated into newly synthesized RNA is localized around the lateral loops of the lampbrush chromosomes rather than around the condensed axes in amphibian oocytes.
2)The study of polytene chromosomes in Drosophila and other Dipteran insects
3)The study of lambrush chromosomes in human oocytes

Accepted Answer

Option 1 provides evidence that newly synthesized RNA is localized in the "open" regions of the chromosome because the lateral loops are regions of the chromosome that are not tightly condensed and allow for easier access for transcription machinery. Option 2 highlights the use of polytene chromosomes in studying gene expression and further supports the idea that transcription takes place in "open" regions of the chromosome. Therefore, the correct choice is D as both options support the idea that transcription occurs in open chromatin regions. Option 3 (study of lampbrush chromosomes in human oocytes) does not provide as clear evidence for open chromatin regions and can be eliminated as a choice.

Question 18

Which of the following is a component of eukaryotic and prokaryotic gene regulation?

Accepted Answer

Heat shock proteins are a component of both eukaryotic and prokaryotic gene regulation, acting as molecular chaperones that help in protein folding and protection against stress conditions in both cell types.

Question 19

Which of the following is true about signal transduction by hormones?

Accepted Answer

Steroid hormone-receptor complexes can indeed enter the cell nucleus. This is because steroid hormones are lipid-soluble and can pass through the cell membrane to bind to their receptors inside the cell. The hormone-receptor complex then moves into the nucleus to act on specific genes, influencing transcription.

Question 20

Which of the following is true of enhancer sequences?

Accepted Answer

Enhancer sequences are regulatory DNA sequences that can act over very large distances, sometimes even affecting genes located on different chromosomes.

Answer: B and C are incorrect
 Enhancer sequences are orientation independent and can be of varying lengths, ranging from a few base pairs to several kilobases.

Answer: D and E are incorrect
 Enhancer sequences can function both upstream and downstream of a gene.

Quiz 19: Regulation Of Gene Expression In Eukaryotes