control of euk gene exp (inc)

Question 1

what are the 2 processes of control at the chromatin level

Answer 1

1. histone modification – acetylation and deacetylation

2. dna methylation

Question 2

describe acetylation of dna

Answer 2

HISTONE ACETYLTRANSFERASE acetylates lysine residues in DNA, neutralising their positive charge and hence reducing their affinity of histone complex for DNA. Chromatin is thus less compact and control regions of genes are exposed to transcription factors

Question 3

describe deacetylation

Answer 3

HISTONE DEACETYLASES deacetylates lysine residues, allowing them to REGAIN their positive charge, hence increasing the affinity of histone complex for dna, making chromatin more compact. This prevents transcription factors and RNA polymerase from gaining access to control regions of the gene.

Question 4

Describe DNA methylation and explain the 2 possible mechanisms that it undergoes

Answer 4

DNA methylation is when methyl groups are added to specific nucleotides after DNA replication, and the process is catalysed by DNA METHYLTRANSFERASE

1) addition of methyl groups will change the 3D conformation of DNA, preventing transcription factors and RNA polymerase from binding to the promoter, hence preventing transcription initiation.
2) methylated DNA will be recognised by METHYL-CPG-BINDING PROTEINS which recruits Histone Deacetylases. (describe process of deacetylation)

Question 5

how do activators increase rate of transcription

Answer 5

1. activators facilitate the proper positioning of the transcription initiation complex
2. (edit) interact with mediator proteins to improve recruitment of general transcription factors and RNA polymerase, forming a more stable transcription initiation complex.

Question 6

what are the three post-transcriptional level modification

Answer 6

1. mRNA splicing
2. polyadenylation (adding of 3’ poly(A) tail)
3. alternative splicing (different splice sites produces different mature mRNAs from the same pre-mRNA, generating different proteins)

Question 7

how does stability of mRNA differ and what is the effect on rate of translation

Answer 7

• longer poly(A) tail increases half life of mRNA, stabilising it. This increases rate of translation and hence more polypeptide is made.