Biochemistry : Transcription

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Transcription

Which of the following are true regarding the product of transcription?

I. The product has more uracil than the parent strand

II. The product has both hydrophilic and hydrophobic amino acids

III. The nucleotides on the RNA polymerase binds to complementary nucleotides on the parent strand

Possible Answers:

I only

II only

I and II

II and III

Correct answer:

I only

Explanation:

Transcription is the second process involved in the production of proteins from a gene. The three processes are DNA replication, transcription, and translation. DNA replication involves replication of DNA from a parent strand, transcription involves the synthesis of a RNA molecule from a DNA molecule, and translation involves the conversion of the mRNA molecule to a polypeptide.

As mentioned, transcription produces an RNA molecule from a DNA molecule (parent strand). Recall that RNA molecules have uracil, whereas DNA molecules have thymine; therefore, the product will contain more uracil.

Amino acids are found in proteins. Since the products of transcription are nucleic acids (RNA molecules) they won’t contain any amino acids. Recall that a nucleic acid consists of pentose sugar molecules (ribose in RNA and deoxyribose in DNA), nitrogenous bases (adenine, guanine, cytosine, thymine (in DNA), and uracil (in RNA)), and phosphate groups.

RNA polymerase is an important enzyme involved in transcription. Its function is to add nucleotides to the growing mRNA chain. Although it adds complementary nucleotides to the DNA, RNA polymerase itself doesn’t bind to complementary DNA sequences, rather it binds at promoters.

Example Question #2 : Transcription

A researcher is analyzing an enzyme. Results reveal that the enzyme is found in eukaryotes, and is involved in synthesis of ribosomes. Which of the following could be the identity of the enzyme?

Possible Answers:

RNA polymerase III

More than one of these could be correct

RNA polymerase I

RNA polymerase II

Correct answer:

RNA polymerase I

Explanation:

There are three types of RNA molecules. First, mRNA molecules are the main products of transcription that undergo translation to produce most of the proteins found in a cell. Second, tRNA molecules are special RNA molecules that facilitate the addition of amino acids to a growing polypeptide chain during translation. Third, rRNA molecules are components of ribosomes and are synthesized in the nucleolus (location of assembly of ribosomes). The enzyme in this question is involved in the production of rRNA molecules. RNA polymerase I is used in production of rRNA molecules. RNA polymerase II is used for mRNA molecules and RNA polymerase III is used for tRNA molecules.

Example Question #3 : Transcription

Which of the following is true regarding both prokaryotic and eukaryotic transcription?

Possible Answers:

None of the these are true

Polyadenylation and addition of methyl cap occurs only in prokaryotic transcription

Both involve polyadenylation and addition of methyl cap at the 5’ end

Both involve polyadenylation and addition of methyl cap at the 3’ end

Correct answer:

None of the these are true

Explanation:

Prokaryotic and eukaryotic transcription are similar in many ways; however, they are also different from one another. One main difference is that in eukaryotic transcription there are several events that occur after the completion of transcription. These events are called post-transcriptional modifications.

There are three main post-transcriptional modifications: polyadenylation, capping, and splicing. Polyadenylation involves the addition of multiple adenine molecules at the 3’ end of the newly synthesized RNA molecule. This segment of RNA with multiple adenine molecules is called the poly A tail. Capping involves the addition of a methyl cap to the 5’ end of the RNA molecule. Splicing involves the excision of segments of the mRNA molecule that aren’t used in translation. These segments are called introns and the usable segments are called exons. The exons are ligated back together and the end product is released into the cytoplasm where it can undergo translation.

Products of prokaryotic transcription don’t undergo any of these post-transcriptional modifications and are immediately translated into proteins.

Example Question #4 : Transcription

Consider the following events in transcription:

1. RNA polymerase adds new nucleotides

2. Sigma factor binds to inactive RNA polymerase

3. Transcription bubble forms

Which of the following is the correct order of these events?

Possible Answers:

3, 1, 2

3, 2, 1

2, 1, 3

2, 3, 1

Correct answer:

2, 3, 1

Explanation:

Transcription is the process of producing RNA molecules from a parent strand of DNA. The first step in transcription involves the binding of a sigma factor to inactive RNA polymerase, which in turn binds to the promoter region on the DNA molecule. After the binding of RNA polymerase to the DNA promoter, RNA polymerase gets activated and removes the hydrogen bonds between nucleotides of the double stranded DNA. Finally, RNA polymerase adds complementary nucleotides to the growing RNA molecule.

After completion of transcription, RNA molecules undergo post-transcriptional modifications and exit the nucleus and enter cytoplasm, where it can undergo translation.

Example Question #5 : Transcription

Which RNA Polymerase is associated with the transcription of DNA to synthesize mRNA?

Possible Answers:

RNA polymerase III

RNA polymerase I

RNA polymerase II

None of these answers

All of these answers

Correct answer:

RNA polymerase II

Explanation:

RNA Polymerase II is used to catalyze the polymerization of mRNA during transcription. RNA polymerase I catalyzes the polymerization of rRNA, and RNA polymerase III catalyzes the polymerization of tRNA.

Example Question #6 : Transcription

If a given piece of coding DNA is 5’ AATGACGTC 3’, which of the following is true?

Possible Answers:

tRNA corresponding to this DNA would have the anticodons: 3’ AAU 5’ 3’ GAC 5’ 3’ GUC 5’

tRNA corresponding to this DNA would have the anticodons: 3’ UUA 5’ 3’ CUG 5’ 3’ CAG 5’

The amino acid chain matching this DNA sequence is Leu-Leu-Gln

tRNA corresponding to this DNA would have the anticodons: 5’ UUA 3’ 5’ CUG 3’ 3’ CAG 5’

Correct answer:

tRNA corresponding to this DNA would have the anticodons: 3’ UUA 5’ 3’ CUG 5’ 3’ CAG 5’

Explanation:

If the coding DNA reads 5’ AATGACGTC 3’, then the template strand would read 3’ UUACTGCAG 5’. The mRNA transcription would read 5’ AAUGACGUC 3’. The corresponding tRNA anti-codons would be 3’ UUA 5’ 3’ CUG 5’; 3’ CAG 5’, which produce the amino acids Asn-Asp-Val. To determine the amino acid sequence, you find the portion of the genetic code table corresponding to the DNA or mRNA, not the tRNA, nucleotides. (That's why ”Leu-Leu-Gln" is incorrect.)

Example Question #7 : Transcription

Which enzyme could be considered to violate the "central dogma" of biology?

Possible Answers:

Ligase

Reverse transcriptase

Carboxylase

Helicase

Catalase

Correct answer:

Reverse transcriptase

Explanation:

The "central dogma" of biology says that information goes from DNA via transcription to RNA via translation to proteins. Reverse transcriptases, however, employed by retroviruses, synthesize DNA from RNA. As for the other enzymes: one function of helicases (among others) is to pull apart double helix strands. Catalase breaks down hydrogen peroxide. Carboxylase adds a carboxyl group to a substrate, and a ligase creates a bond between two molecules, for example, via a phosphodiester bond.

Example Question #8 : Transcription

The post-transcriptional spliceosome reaction, which removes the intron in a lariat structure, takes place via __________.

Possible Answers:

two transesterifications

one transacetylation

two transacetylations

one transesterification and one transacetylation

two isomerizations

Correct answer:

two transesterifications

Explanation:

The spliceosome first releases the 5' exon by forming a lariat structure (2'-5' phosphodiester) bond between two introns in a transesterification reaction. Then, the exons are spliced together with another transesterifaction reaction, and the intron lariat is released.

Example Question #9 : Transcription

Which of the following is a ribozyme?

Possible Answers:

RNA polymerase II

All of these are ribozymes

RNase H

Guanyltransferase

RNase P

Correct answer:

RNase P

Explanation:

RNase H is involved in reverse transcriptase, but it is not a ribozyme. RNase P is a ribonuclease that cleaves/processes rRNA and generates 5' ends. The RNA component of RNase P is its catalytic subunit.

Example Question #10 : Transcription

Within a eukaryotic chromosome, which of the following pairs of processes can occur at the same time?

Possible Answers:

Translation and splicing

Transcription and translation

Replication and transcription

Transcription and splicing

Replication and splicing

Correct answer:

Transcription and splicing

Explanation:

In eukaryotes, transcription and splicing could occur simultaneously. Both of these processes take place in the nucleus of the cell, while translation takes place in the cytoplasm. Therefore, translation could not happen at the same time as either transcription or splicing. Replication occurs totally independently from all of the other processes listed.

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