L8 - Nucular Gene Transcription Flashcards
What is Euchromatin?
Uncondensed chromatin, found in cells during interphase
What is heterochromatin?
condensed chromatin, inaccessible for transcription
What is X-chromosome inactivation?
In female mammals, only 1 X chromosome is active, the other is always condensed and inactive
Why are expressed genes more sensitive to DNAase?
Expressed genes have a more open structure than others, the only packaging is into nucleosomes.
Where would the B-globin gene be most sensitive to DNAase?
Erythroid cells (precursors for RBCs)
What are the 2 main processes for opening up chromatin?
1) Histone modification
2) Chromatin remodelling complexes
What effect does acetylation have on histones?
Acetylation removes a positive charge from the histone, causing the interactions between histones and DNA to loosen, as the electrostatic attraction is reduced
How do Chromatin remodelling complexes work?
These increase the accessibility of DNA - this is an active process and requires ATP from hydrolysis.
It is to allow access by other DNA-binding proteins by partly unwrapping, and moving or removing histones
How many types of RNA polymerase do eukaryotes have and what are they?
3:
RNAP I: transcribes ribosomal RNA genes
RNAP II: transcribes genes coding for proteins
RNAP III: transcribes genes coding for small RNA such as tRNAs
A transcription complex of at least 50 polypeptides is required for transcription. Where does the transcription complex form?
At the TATA box or a similar key sequence. It is usually found around 25bp upstream from the transcription starting point
What does TBP stand for and what is its function?
TATA box binding protein (TBP) binds to the TATA box. This puts a wedge into the minor grove of the DNA molecule, twisting it so that it bends 70 degrees
THat fors the complex TFIID
TBP and around 8 other polypeptide subunits, sometimes called TAFs
Describe the steps of initation
1) When a gene becomes available to transcription, transcription activator proteins bind to specific sequences of DNA displacing histones. Some nucleosomes are acetylated causing the DNA to open up. Additional activator proteins bind to the DNA at specific sites when transcription is about to take place
2) The protein TFIID binds to the TATA box
3) RNAP holoenzyme binds, which contains RNAP core enzyme and some additional polypeptides. Mediator components interact with transcription activator proteins
4) Once the transcription complex has formed, TFIIH phosphorylates RNAPII. This releases the enzyme so in can initiate transcription
Some genes don’t have a TATA box but instead a different sequence, what happens in this situation?
Some genes have 1 or more other sequences that define where transcription starts and bond to TFIIB or other subunits of TFIID
(they can also have both these and TATA boxes all at once)
What happens when genes don’t have any kind of initiation sequences?
They tend to be for ‘housekeeping’ proteins - ones that are required in all or most cells in small amounts.
Transcription of these typically doesn’t start at a specific base, instead over a range of 50-100bp
