AP Biology : Transcription

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Post Transcription Modification

Which of the following statements about post-transcriptional processing is true?

Possible Answers:

Exons are spliced out of hnRNA and are not part of the mature mRNA

A methylguanine cap is added to the 3' end of the mature mRNA

A poly-A tail is added to the 5' end of the mature mRNA

Post-transcriptional hnRNA processing prevents RNA from degrading as it leaves the nucleus

Correct answer:

Post-transcriptional hnRNA processing prevents RNA from degrading as it leaves the nucleus

Explanation:

The hnRNA (heterogeneous nuclear RNA) produced from transcription must be processed by several enzymes to create an mRNA (messenger RNA) product that can pass from the nucleus to the cytoplasm without degrading. This involves cutting out introns, which remain in the nucleus, and splicing exons together. Furthermore, a methylguanine cap is added to the 5' end and a poly-A tail is added to the 3' end. 

Example Question #1 : Transcription

Which of the following is not an example of a post-transcriptional modification of mRNA molecules?

Possible Answers:

Splicing of anticodons 

Splicing of introns

Addition of a poly-A tail

Addition of a 5-methyl guanosine cap

Correct answer:

Splicing of anticodons 

Explanation:

Addition of a 5-methyl guanosine cap, the splicing out and removal of introns, and the addition of a poly-A tail are all processes that are essential to making stable, mature mRNA.

Codons are contained on the exons of mature mRNA and are matched to appropriate anticodons during translation. Anticodons are found on tRNA molecules, and are not involved in mRNA transcription.

Example Question #1 : Understanding Post Transcription Modification

Which biotechnology method will allow you to distinguish between identical twins?

Possible Answers:

Restriction fragment length polymorphism (RFLP) Analysis

Standard fingerprinting

Southern blot

DNA fingerprinting

Correct answer:

Standard fingerprinting

Explanation:

Fingerprints are different in all individuals, even identical twins. Due to RNA processing, or post-transcription modification, the grooves of a finger are different even in individuals with identical DNA.

The other techniques are used for DNA analysis between individuals with different DNA. Identical twins will be indistinguishable under these techniques, because their DNA is the same.

Example Question #4 : Understanding Post Transcription Modification

Which of the following statements is false concerning post-transcriptional modification?

Possible Answers:

A poly A tail is attached to the 3' end of the transcript

Spliceosomes remove exons from the transcript and splice together the introns

A 5' cap is added in order to provide an attachment site for ribosomes during translation

Methylation is not part of post-transcriptional modification

Both the 5' cap and poly A tail protect the transcript from degradation

Correct answer:

Spliceosomes remove exons from the transcript and splice together the introns

Explanation:

During post-transcriptional modification, spliceosomes can remove introns from the transcript and splice together exons. Introns are the parts that are removed from the transcript, meaning they are not translated. Exons are the portions of mRNA that code for the correct amino acids sequence for the desired gene.

Other modifications involve the poly-A tail, which is added to the 3' end, and the 5' cap. Both structures protect the transcript from damage during transport and provide binding sites for various proteins, as well as the ribosome itself. Methylation is a part of epigenetic DNA modification, and is not involved with transcription processes.

Example Question #1 : Understanding Post Transcription Modification

snRNPs assist in what post-transcriptional modification?

Possible Answers:

The removal of introns

Exporting the transcript to the ribosome

Addition of the poly-A tail

Addition of the 5' cap

Correct answer:

The removal of introns

Explanation:

snRNPs, or small nuclear ribonucleoproteins, are an essential part of the spliceosome complex. The spliceosome is responsible for the removal of introns from the primary transcript.

Example Question #1 : Understanding Post Transcription Modification

Which of the following is not an example of eukaryotic post-transcriptional modification?

Possible Answers:

A poly-A tail is added to the 3' end

After splicing, the exons are rejoined to form a final mRNA transcript

The 3' end of the growing strand is cleaved

A methylguanosine cap is added to the 5' end

Exons, or non-coding regions, are removed via splicing

Correct answer:

Exons, or non-coding regions, are removed via splicing

Explanation:

Post-transcriptional modifications are changes that are made to the mRNA transcript before it is translated into a protein. The first of these changes is cleavage at the 3' end to separate the new strand, or "primary transcript," from the transcription machinery. Next, a protective 5' cap is added, as is a string of adenine nucleotides at the 3' end. Finally, noncoding regions, called introns, are spliced out and the exons, or coding regions, are reconnected.

The incorrect statement confuses exons and introns.

Example Question #1 : Understanding Post Transcription Modification

Which of the following choices is not one of the processes associated with post-transcriptional modification?

Possible Answers:

Protein folding

Polyadenylation

Splicing

5' capping

Correct answer:

Protein folding

Explanation:

Post-transcriptional modification is the stage where the recently synthesized primary RNA transcript undergoes changes to become a mature RNA molecule. Post-transcriptional modifications ensure that the correct RNA transcripts are produced and that the correct proteins are translated. Post-transcriptional modification includes the processes of polyadenylation, 5’ capping, and splicing. Polyadenylation is the addition of adenine bases (the poly-A tail) to the 3’ end of the RNA primary transcript. The poly-A tail is important in export, stability, and translation of the transcript. 5’ capping is the addition of guanine bases to the 5’ end of the primary transcript. The 5’ cap aids in export and translation of the transcript and also protects it from degradation. Splicing is the removal of non-coding regions, or introns, from the primary transcript.

Example Question #1 : Understanding Post Transcription Modification

Which of the following choices is not a true characteristic of spliceosomes?

Possible Answers:

Most spliceosomes are found in the nucleus

They are composed of small nuclear RNA molecules (snRNAs)

Both prokaryotes and eukaryotes have spliceosomes

They splice introns from primary RNA transcripts

Correct answer:

Both prokaryotes and eukaryotes have spliceosomes

Explanation:

The spliceosome is a complex structure composed of small nuclear RNA molecules (snRNAs) and other proteins. They are often located in the nucleus. The spliceosome splices introns from the primary RNA transcript. This process occurs through splicing at 5’ and 3’ sites that are identified by particular nucleotide sequences. Prokaryotic cells do not contain spliceosomes. 

Example Question #9 : Understanding Post Transcription Modification

Which statement correctly describes the process of alternative splicing?

Possible Answers:

Alternative splicing can create a variety of proteins from the same primary transcript

There are different methods of alternative splicing (spliceosomes, self-splicing, etc.)

In alternative splicing, some primary RNA transcripts are spliced and others are not

There are different alternative splicing processes between prokaryotes and eukaryotes

Correct answer:

Alternative splicing can create a variety of proteins from the same primary transcript

Explanation:

Alternative splicing is the process where many different proteins can be formed from a primary transcript. This can be done in a number of ways, including using different splice sites, maintaining introns, and splicing out exons. Alternative splicing is an important process because it increases cellular efficiency; if many proteins can result from the same primary transcript, then the genome doesn’t need to code for as many unique transcripts.

Example Question #1 : Transcription

What is the main difference between prokaryotic and eukaryotic genes?

Possible Answers:

Prokaryotes have one large gene, but eukaryotes have many smaller genes

Eukaryotes stores genes together in operons, but prokaryotes do not

Prokaryotic genes only have exons, but eukaryotic genes have exons and introns

Prokaryotic genes are not replicated, and only transcribed to RNA to then become protein

Prokaryotes do not have gene regulatory sequences, but eukaryotes do

Correct answer:

Prokaryotic genes only have exons, but eukaryotic genes have exons and introns

Explanation:

The correct answer is that prokaryotes only have exons, whereas eukaryotes have exons and introns. As a result, in eukaryotes, when mRNA is transcribed from DNA, the introns have to be cut out of the newly synthesized mRNA strand. The exons, or coding sequences, are then joined together. Prokaryotes do not have to process their mRNA to this extent. 

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