Gene transcription and RNA modification - Transcription in eukaryotes

19 important questions on Gene transcription and RNA modification - Transcription in eukaryotes

Which two things make eukaryotic transcription more complex than prokaryotic transcription?

- The cells of eukaryotes are more complex, which requires that eukaryotes make more proteins and therefore have more protein-coding genes;
- Most eukaryotes have different celltypes, and therefore the eukaryotic transcription requires more appropriate timing and coordination.

What are the differences between the three types of RNA polymerase in eukaryotes?

- RNA polymerase I transcribes all the genes for rRNA;
- RNA polymerase II transcribes all the mRNA's and most of the snRNA's and other types of RNA's;
- RNA polymerase III transcribes all tRNA's and one rRNA, and some other types of RNA's.

Which two features do eukaryotic protein-encoding genes have to make sure that transcription occurs at an appropriate rate?

- A core promoter
- Regulatory elements.
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What is a core promoter?

A relatively short DNA sequence that is necessary for transcription to take place, which consists of the TATA-box and the transcriptional start site (the TATA-box is about 25 bp upstream from the transcriptional start site).

What is basal transcription?

The most basic set of elements needed for transcription to occur start. In itself, the core promoter is enough for transcription to take place. However, the core promoter produces a low level of transcription (regulatory elements are needed for a higher level).

What are regulatory elements?

Short DNA sequences that affect the ability of RNA polymerase to recognize the core promoter and begin the process of transcription.

Which two categories of regulatory elements are there?

- Enhancers, which stimulate transcription (to give it a higher rate);
- Silencers, which inhibit transcription (used when it's necessary to prevent transcription of a given gene).

What is the location of regulatory elements (relatively to the transcriptional start site)

Around the -50 to -100 region.

What are cis-acting elements?

DNA sequences such as the TATA-box, enhancers and silencers, which are always found within the same chromosome as the genes they regulate, and they only have effect on one particular gene.

What are trans-acting elements?

The regulatory transcription factors that bind to cis-acting elements. They are themselves encoded by proteins, but they have effect on genes that can be even on another chromosome.

Which 3 categories of proteins are needed for basal transcription at the core promoter?

RNA polymerase II, general transcription factors (GTF's) and a mediator.

Which five general transcription factors (GTF's) are needed for RNA polymerase II to initiate transcription?

TFIID, TFIIB, TFIIF, TFIIE and TFIIH.

What is the role of TFIID and how does its parts fulfill this function?

It first binds to the TATA-box (recognizes the core promoter). It's composed of several subunits, including TATA-binding protein (TBP), which directly binds to the TATA-box. It also has proteins called TBP-associated factors (TAFs).

What is the function of TFIIH?

It has subunits of which some break the hydrogen bonds between the two strands of DNA and form an open complex.
Another subunit hydrolyzes ATP and phosphorylates a domain in RNA polymerase called the CTD, which breaks down the contact between RNA polymerase II and TFIIB.

What happens after RNA polymerase II lost the contact with TFIIB?

TFIIB, TFIIE and TFIIH dissociate and RNA polymerase II is going to proceed to the elongation stage of transcription.

What is the basal transcription apparatus?

The needed products for transcription to occur (when you put it together, transcription will occur). It consists of:
- TFIID, TFIIB, TFIIF, TFIIE and TFIIH;
- RNA polymerase II;
- A DNA sequence containing a TATA-box and transcriptional start site.

In one sentence, how does termination in eukaryotes begin?

After RNA polymerase II has transcribed the polyadenylation signal sequence, the RNA is cleaved just downstream from this sequence.

How does transcriptional termination occur, occording to the allosteric model?

- After RNA polymerase II has transcribed the polyadenylation signal sequence, it becomes destabilized, and eventually dissociates from the DNA.
- This destabilization is probably caused by the release of proteins that function as elongation factors or by the binding of proteins that function as termination factors.

How does transcriptional termination occur, according to the torpedo model?

RNA polymerase II is physically removed from the DNA.
-  The region of RNA downstream from the polyadenylation signal sequence is cleaved by an exonuclease (5' to 3').
- When the exonuclease catches up to RNA polymerase II, this causes RNA polymerase II to dissociate from the DNA.

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