AP Biology : AP Biology

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #14 : Understand Regulation Of Transcription

Which of the following is not true about prokaryotic transcription?

Possible Answers:

RNA polymerase interacts directly with the DNA

Transcription occurs in the cytoplasm

mRNA is not subject to post-transcriptional modifications

RNA polymerase binds directly to the Shine-Delgarno sequence for some promoters

Correct answer:

RNA polymerase binds directly to the Shine-Delgarno sequence for some promoters

Explanation:

RNA polymerase does not interact with the Shine-Delgarno sequence. The Shine-Delgarno sequence is present on some prokaryotic mRNAs and serves as a ribosomal binding site for the initiation of translation. RNA polymerase is only involved in transcription and will bind to DNA, not RNA.

The other answers are all true and unique to prokaryotic transcription. Eukaryotic transcription is much more tightly regulated by transcription factors and DNA packaging (chromatin), and is confined to the nucleus.  

Example Question #15 : Understand Regulation Of Transcription

Which of these are characteristic of transcription in eukaryotes? 

I.  Transcription can occur at the same time as translation

II.  The resulting RNA molecule is functionally mature

III.  The resulting RNA molecule must undergo splicing and other modifications before it is functional

IV.  Transcription occurs in the cytoplasm

Possible Answers:

I and III

II only

I and IV

III only

I, II, and IV

Correct answer:

III only

Explanation:

Prokaryotic transcription occurs in the cytosol, since prokaryotes lack a nucleus. This allows ribosomes to interact with RNA even while it is still be synthesized.

In contrast, eukaryotic transcription occurs in the nucleus. Once RNA has been synthesized it must be transported from the nucleus to the cytoplasm before it can interact with ribosomes. The newly-synthesized RNA undergoes splicing to remove introns, addition of a 5'-cap, and addition of a poly-A tail before it can exit the nucleus. These modifications help prevent degradation of the RNA. Only after these modifications can the RNA leave the nucleus and becomes functionally active.

Example Question #21 : Transcription

What is a Shine-Delgarno sequence?

Possible Answers:

A polymerase binding site in prokaryotic DNA

A polymerase binding site in eukaryotic RNA primers

A ribosomal binding site in eukaryotic mRNA

A ribosomal binding site in prokaryotic mRNA

A site for restriction endonuclease digestion

Correct answer:

A ribosomal binding site in prokaryotic mRNA

Explanation:

Shine-Delgarno sequences are ribosomal binding sites slightly upstream of start codons on prokaryotic mRNA. Eukaryotic mRNA is more complicated (it does not contain Shine-Delgarno sequences) and contains promoter regions that are responsible for recruiting translation factors and ribosomal subunits. 

Example Question #17 : Understand Regulation Of Transcription

How does prokaryotic transcription differ from eukaryotic transcription?

Possible Answers:

Prokaryotic transcripts are translated directly in the nucleus

Only prokaryotes use promoters

Prokaryotic RNA polymerase transcribes the coding strand, rather than the template strand

Prokaryotic transcripts are polycistronic

Correct answer:

Prokaryotic transcripts are polycistronic

Explanation:

While prokaryotic and eukaryotic transcription processes are quite similar, there are some key differences. One significant difference is that prokaryotic transcripts can contain multiple genes, which will then transition as a single RNA strand to the ribosome. This is referred to as a polycistronic transcript. Eukaryotes have only one gene per transcript.

Both prokaryotes and eukaryotes use promoters. Prokaryotes do not have nuclei, though transcription and translation can occur simultaneously and in close proximity in these cells.

Example Question #18 : Understand Regulation Of Transcription

In a bacterial cell, RNA polymerase binds to a promoter. What needs to bind to RNA polymerase for transcription to initiate?

Possible Answers:

A metal cofactor

An alpha factor

A prosthetic group

A sigma factor

A gamma factor

Correct answer:

A sigma factor

Explanation:

In bacterial cells, binding of a sigma factor to RNA polymerase is required for the initiation of transcription. Once the sigma factor binds, RNA polymerase is referred to as a holoenzyme and begins to make the RNA transcript.

Example Question #19 : Understand Regulation Of Transcription

Which of the following correctly pairs the part of an operator with its function?

Possible Answers:

Promoter; contains the genes to be transcribed

Operator; region where the repressor binds

Corepressor; signals for RNA polymerase to binds and transcribe genes

Inducer; decreases RNA polymerases’ transcription of genes

Correct answer:

Operator; region where the repressor binds

Explanation:

The operator region is the location where the repressor binds. Other parts of an operon include the promoter (where RNA polymerase binds), and structural genes.

Example Question #20 : Understand Regulation Of Transcription

How is transcription controlled in prokaryotes?

Possible Answers:

Addition of 5’ cap

Repressor and activators

Addition of 3’ polyadenylated tail

Intron splicing

Correct answer:

Repressor and activators

Explanation:

Addition of 5’ cap, 3’ tail, and intron splicing occur in eukaryotes, not prokaryotes. Repressors bind to the operator region of the gene and prevent RNA polymerase from transcribing the gene, while activators bind to the promoter and increase transcription of the gene.

Example Question #21 : Understand Regulation Of Transcription

Which of the following binds to the repressor of an operon to prevent the repressor from binding to the operator region?

Possible Answers:

corepressor

unrepressor

activator

inducer

Correct answer:

inducer

Explanation:

A repressor is a molecule that binds to the operator region of a gene and prevents RNA polymerase from transcribing the genes. An inducer can bind to a repressor, preventing the repressor from binding to the operator region, and thus allowing RNA polymerase to transcribe the genes.

Example Question #31 : Transcription

The lac operon contains the lac repressor, which binds to the operator to inhibit transcription. What type of operon is this, and how can transcription of the genes proceed?

Possible Answers:

Repressible; a repressor binding to the repressor will allow transcription

Repressible; a repressor binding to the repressor will allow transcription

Inducible; an inducer binding to the operator will allow transcription

Inducible; an inducer binding to the repressor will allow transcription

Correct answer:

Inducible; an inducer binding to the repressor will allow transcription

Explanation:

The lac operon is inducible, meaning that it is normally turned off or not transcribing genes (due to the repressor binding to the operator region). However, transcription can proceed if an inducer molecule (in the case of the lac operon this will be allolactose) binds to the repressor, preventing the repressor from binding to the operator region and thus allow RNA polymerase to transcribe the genes.

 

Example Question #32 : Transcription

The trp operon is normally active or turned “on.” However, when tryptophan is present, it binds to a repressor molecule, activating it, and causing inhibition of transcription of the genes. What type of molecule is tryptophan, and what type of operon is the trp operon?

Possible Answers:

corepressor; inducible

corepressor; repressible

repressor; repressible

inducer; inducible

Correct answer:

corepressor; repressible

Explanation:

The trp operon is normally on, meaning that the genes are normally being transcribed by RNA polymerase. Thus, this is a repressible operon. The operon can be turned off or repressed if another molecule (called a corepressor) binds to a repressor and causes the repressor to bind to the operator region (in the case of the trp operon this molecule is tryptophan; it binds to a repressor causing the repressor to bind to the operator region and prevents RNA polymerase from transcribing the genes).

 

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