Lecture 8 Flashcards
(17 cards)
What is Euchromatin?
Lightly packed form of chromatin. Diffuse and spread out. In interphase.
What is heterochromatin?
Condensed DNA, inaccessible for transcription.
How are expressed genes different to ones that are not expressed?
Expressed genes have a more open structure -shown by greater sensitivity to DNase. The only packaging is into nucleosomes.
E/g beta globin gene
When are globin genes transcribed?
Only in erythroid cells -the precursors of red blood cells.
What are the globin genes like in the liver?
Tightly packed, not transcribed here and insensitive to DNase
What are the 2 processes involved in opening up of chromatin?
Histone modification and Chromatin remodelling complexes
What is involved with histone modification?
Acetylation by histone acetylates.
The addition of acetyl groups to lysine residues.
(the histone has 20 amino acid tail that sticks out, lysine is positively charged and os a target for enzymes to add acetyl groups).
This reduces the positive charge of histone -reduces the affinity of the histones for the DNA -opens up the compact nucleosome -reduced electrostatic attraction.
What do chromatin remodelling complexes do?
Increases the accessibility of DNA
Moves nucleosomes around on the DNA -gives access to other DNA binding proteins.
Requires ATP
Give some examples of chromatin remodelling:
Nucleosomes can be partly unwrapped
They can move around- create spaces for proteins to bind
Remove -fewer nucleosomes -leaves space for other proteins
This allows access for other DNA-bonding proteins.
Which RNA polymerase in eukaryotes transcribes genes coding for proteins?
RNA polymerase II
What does specific initiation require in eukaryotes?
Enzymes that later chromatin structure
Specific activator proteins
Basal or general transcription factors -help RNA polymerase to bind promoters and initiate transcription
What are the basal factors for transcription by RNA polymerase II?
TFIIA
TFIIB
What is the key sequence for the invitation of transcription in promoter of many genes?
TATA box -in the promoter!
Consensus: TATAA -directs RNA polymerase to correct starting point
25bp upstream of tranicription starting point
How is the start site identified?
TATA box binding protein binds to the TATA box. (part of TFIID protein)
Puts a wedge in the minor groove of DNA -twisting the DNA so it bends at 70 degrees,
Presents a new surface that attracts the binding of other transcription factors to form a transcription complex
What is TFIID made up of?
TATA box binding protein and TAFs (TATA box associated factors)
Assembly of the transcription complex:
TFIID binds to the TATA box
RNA polymerase II holoenzyme binds -contains core enzyme and additional polypeptides
Mediator component interacts with Transcription activator proteins
What happens once the transcription complex has formed?
Transcription factor TFIIH phosphorylates RNA polymerase II.
Introduces negative charges -changes interaction with their proteins -releases RNA polymerase
Can now initiate transcription
