AP Biology › Understand regulation of transcription
In prokaryotic transcription, which of the following is recruited to facilitate the binding of the synthesis enzyme to the DNA template?
The sigma factor
RNA polymerase
A promoter sequence
Messenger RNA
Inactive RNA polymerase must bind to a specific sigma factor in order to become active in gene transcription. Sigma factors are specialized transcription factors involved in recruiting and activating RNA polymerase. Only once RNA polymerase has bound the sigma factor can it identify promoter sequences and initiate transcription.
mRNA is the product of transcription and is not involved in prokaryotic RNA polymerase recruitment. An RNA primer is essential to recruiting DNA polymerase for DNA replication.
Which of the following is not true about prokaryotic transcription?
RNA polymerase binds directly to the Shine-Delgarno sequence for some promoters
Transcription occurs in the cytoplasm
RNA polymerase interacts directly with the DNA
mRNA is not subject to post-transcriptional modifications
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.
How does prokaryotic transcription differ from eukaryotic transcription?
Prokaryotic transcripts are polycistronic
Only prokaryotes use promoters
Prokaryotic transcripts are translated directly in the nucleus
Prokaryotic RNA polymerase transcribes the coding strand, rather than the template strand
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.
What is an important distinction between the process of translation in prokaryotes versus eukaryotes?
Translation may occur at the same time as transcription in prokaryotes; they always occur separately in eukaryotes
Only eukaryotes perform translation
Prokaryotes produce a different set of amino acids from translation than eukaryotes
Eukaryotes perform translation before transcription, while prokaryotes perform transcription first
Prokaryotes do not use RNA; eukaryotes do
The lack of a nuclear membrane in prokaryotes has the advantage of allowing the cell to translate RNA as it is transcribed from DNA. This means that even before the full RNA is produced, the protein coded by that RNA can start being made. Eukaryotes produce RNA inside the nucleus, so it must first be fully transcribed and undergo modifications before it can be moved to the cytoplasm, where translation occurs.
A segment of prokaryotic DNA that binds transcription factors, usually as repressors that prevent transcription, are best known as __________.
Operators
Promoters
Exons
Enhancers
Untranslated regions
The correct answer is operator. In most operons, repressors bind operators to prevent transcription of downstream genes.
Promoters are sequences of DNA upstream of genes that usually promote transcription by recruiting polymerases and other transcription factors. Enhancers are distant DNA sequences that promote transcription, whereas exons are the coding segments of a gene.
Which of the following binds to the repressor of an operon to prevent the repressor from binding to the operator region?
corepressor
activator
inducer
unrepressor
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.
Which of the following correctly pairs the part of an operator with its function?
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
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.
Which of the following is true of Rho-dependent transcription termination in prokaryotes?
Rho unwinds the DNA-RNA helix at the transcription bubble
Rho is a nuclease that terminates transcription by cutting template DNA
Rho uses GTP as a source of energy
Rho attaches to a Rho recognition site on the DNA template
None of the other answers
Rho attaches to a Rho recognition site on the mRNA strand and uses ATP to move along the mRNA strand towards RNA polymerase. When RNA polymerase pauses at the terminator, Rho unwinds the DNA-RNA hybrid. RNA polymerase, Rho, and the newly synthesized mRNA are released.
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
III only
I and III
II only
I, II, and IV
I and IV
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.
Prokaryotic genes are often grouped together based on function. What are these groups of genes called?
Operons
Operator sequences
Promoter sequences
Clusters
Linked genes
Bacteria organize some of their genes into operons. Operons contain genes of a similar function grouped together, and these genes are all transcribed together. For example, the lac operon involves the three genes required for breaking down lactose. There is no point in only transcribing one or two of the three genes since they are all required to break down lactose. Thus, they are under the control of a single operator and are all transcribed when the operator is active.