Biochemistry : Nucleic Acid Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #75 : Anabolic Pathways And Synthesis

During which of the following phase(s) of the cell cycle does transcription occur?

Possible Answers:

S phase

G2 phase

G1 phase

More than one of these are true

Correct answer:

More than one of these are true

Explanation:

Transcription is the process of transcribing RNA molecules from DNA. This is a normal cellular process that is required for cells to grow and function properly (because these RNA molecules are eventually converted to proteins, the building blocks of cells). Growth of cells occurs in G1 and G2 phases; therefore, transcription occurs during both of these phases.

Note that DNA replication occurs during S phase; therefore, no DNA molecules will be available for transcription during S phase and transcription will be halted.

Example Question #5 : Regulating Transcription

A bacteria is known to have a defect in a protein that codes for the sigma factor. What will you most likely observe in this bacteria?

Possible Answers:

Complete halt of transcription because RNA polymerase stays as a holoenzyme

Increased post-transcriptional modifications

Complete halt of transcription because there is an increased degradation of RNA polymerase

Complete halt of DNA replication and transcription because there is an increased degradation of both DNA and RNA polymerase

Correct answer:

Complete halt of transcription because RNA polymerase stays as a holoenzyme

Explanation:

Sigma factor is a special molecule in bacteria that is used to initiate transcription. In a bacterial cell, RNA polymerase is typically kept in its inactive form, called holoenzyme. When it is needed for transcription, RNA polymerase is converted to its active form by sigma factor. Sigma factor facilitates the binding of RNA polymerase to the gene sequence on the corresponding DNA molecule. Upon binding, RNA polymerase will carry out transcription and generate a new mRNA strand.

Example Question #76 : Anabolic Pathways And Synthesis

Promoter regions on DNA templates bind RNA polyermase and determine where transcription will begin.  Which of the following could be part of a promoter region in bacteria?

Possible Answers:

3' - TTCGTAGCATAACG - 5'

5' - CCGGTTAACCGG - 3'

5' - TACGTGCGAATAG - 3'

3' - CTAGCGTAGCAGCA - 5'

5' - CGCTATAATGCT - 3'

Correct answer:

5' - CGCTATAATGCT - 3'

Explanation:

A Pribnow box is a type of promoter region in bacteria that contains a sequence similar to the eukaryotic TATA box.  5'- TATAAT -3' will be found in the Pribnow box and signifies that the particular section of DNA is a promoter region.  The eukaryotic TATA box typically contains the sequence 5'- TATAAA -3' and also serves as a promoter region, typically found upstream of a gene.

Example Question #77 : Anabolic Pathways And Synthesis

Genetic variety is accomplished in eukaryotes via which of the following mechanisms?

I. Pieces of DNA can move around spontaneously within the genome

II. Multiple, distinct proteins can be translated from a single coding region of mRNA

III. Segments of DNA can spontaneously switch to become new DNA coding regions

Possible Answers:

I, II, and III

I and II

I and III

II and III

I only

Correct answer:

I, II, and III

Explanation:

Transposable elements are those that can move around within the genome, which increases genetic diversity. Also, due to alternative splicing of introns, multiple distinct proteins can be synthesized from the same exact mRNA transcript. One example of this is antibody production. Segments of DNA can spontaneously switch to become new DNA coding regions (mutation). This also increases genetic diversity, if this occurs in the germ-line cells.

Example Question #81 : Anabolic Pathways And Synthesis

Inhibition of RNA polymerase II would disrupt which of the following processes?

Possible Answers:

Synthesis of mRNA

Synthesis of protein

Synthesis of rRNA

Synthesis of DNA

Synthesis of tRNA

Correct answer:

Synthesis of mRNA

Explanation:

RNA polymerase II is the polymerase that catalyzes the synthesis of mRNA from a coding strand of DNA. Therefore, mRNA synthesis would be greatly affected by an inhibition of RNA polymerase II.

Example Question #7 : Regulating Transcription

How does the action of histone acetyltransferases affect transcription?

Possible Answers:

It decreases the rate of transcription by adding positive charge to histones

It increases the rate of transcription by adding positive charge to histones

It decreases the rate of transcription by removing positive charge from histones

It increases the rate of transcription by removing positive charge from histones

Correct answer:

It increases the rate of transcription by removing positive charge from histones

Explanation:

For this question, we need to consider how histone acetyltransferases affect histones. Then, we need to determine how these modified histones affects the expression of genes.

First, it's important to note that histones are proteins that mostly contain positive charges. As a result of this, histones are able to associate with DNA very well, since DNA contains a negatively charged backbone. When histones associate with DNA in this way, the DNA molecule becomes tightly coiled around the histones. In this tightly bound conformation, the collection of DNA and proteins are referred to as hererochromatin. What's more is that when the DNA is tightly bound like this, the transcription machinery in the cell is physically blocked from associating with genes. Thus, gene expression is lowered.

Histone acetyltransferases are enzymes that attach acetyl groups to the positively charged lysine residues that are part of histones. Remember, the positive charge of these lysine residues is what allows the histones to associate with the DNA. When acetyl groups are added, the positive charge on these histones becomes neutralized. As a result, the histones are no longer able to associate with the DNA. What this means is that the transcription machinery in the cell is now able to physically access the genes, allowing gene expression to increase.

Example Question #82 : Anabolic Pathways And Synthesis

An inhibition of RNA polymerase III would directly affect which of the following processes?

Possible Answers:

Synthesis of DNA

Synthesis of mRNA

Synthesis of rRNA

Synthesis of protein

Synthesis of tRNA

Correct answer:

Synthesis of tRNA

Explanation:

RNA polymerase III catalyzes the synthesis of tRNA - RNA that is responsible for carrying amino acids during translation. So, synthesis of protein will be affected down the line, however the direct effect of an inhibition of RNA polymerase III would be the inability to create tRNA.

Example Question #51 : Nucleic Acid Synthesis

How does RNA polymerase know when to end transcription of a gene?

Possible Answers:

It reaches an uncodeable segment of the DNA

It reaches the Hogness box

It reaches the TATA box

It reaches a poly A tail

It synthesizes a termination sequence

Correct answer:

It synthesizes a termination sequence

Explanation:

RNA polymerase travels down DNA beginning at the promoter site (could be TATA box or Hogness box in eukaryotes). It reads the DNA and synthesizes mRNA along the way, until it reaches a point where it reads the DNA and synthesizes a termination sequence. This notifies the RNA polymerase that it should end transcription of the gene.

Example Question #52 : Nucleic Acid Synthesis

Spliceosomes must be able to recognize where to splice mRNA so that introns are correctly cut out. What sequence is nearly always conserved in introns to ensure proper splicing?

Possible Answers:

None of these

(Splice) GU-------pyrimidine--AG (splice)

(Splice) AG-----------purines--GU (splice)

(Splice) GU----------purines--AG (splice)

(Splice) AG----------pyrimidines--GU (splice)

Correct answer:

(Splice) GU-------pyrimidine--AG (splice)

Explanation:

Spliceosomes recognize the conserved sequence, GU, and splice just before those two nucleotides. They then continue onwards and when they recognize a pyrimidine followed by the nucleotides, AG, they splice again immediately after the AG. This is almost always conserved in introns to ensure proper splicing.

Example Question #53 : Nucleic Acid Synthesis

Which of the following is/are true regarding prokaryotic RNA polymerases?

I. RNA polymerase requires the sigma protein factor to initiate transcription.

II. Prokaryotes have multiple types of RNA polymerase.

III. RNA polymerase requires the rho protein factor to terminate transcription.

IV. Sigma protein is not required for RNA polymerase to initiate transcription in prokaryotes.

Possible Answers:

I and IV

I, II, and III

I and II

I and III

II, III, and IV

Correct answer:

I and III

Explanation:

There are few differences between prokaryotes and eukaryotes in what concerns transcription. In prokaryotes there is only one RNA polymerase, while in eukaryotes there are three: I , II and III. In prokaryotes, both sigma factor and rho factor are required for transcription to occur, but not in eukaryotes. 

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