Ch 17. Gene control in Eukaryotes

Characteristics of eukaryotic gene expression

1 promoter per structural gene

chromatin struct—histones must be unwound

nuclear envelope separates transcription and translation

Chromatin is transcriptionally active when

there is DNase hypersensitivity

histones are unwound so you can access the promoter

Increase in DNase=

increase in transcription

Chromatin remodeling complex

When histones shift to expose a transcription factor binding site to increase transcription

Histone modifications

Methylation

Ubiquitination

Phosphorylation

Acetylation

Methylation and histone modifications

methyl groups added to histone tails

closed/inaccessible for transcription

How does methylation occur?

CpG islands

C is methylated (adjacent to G)

results in no transcription

Acetylation, phosphorylation, and ubiquitination and histone modifications

open/accessible

stimulates transcription

Ubiquitination

ubiquitin degrades histone octamer—opens up space

more space=allows for transcription

Transcription factors

bind to regulatory promoter

specific sequences on regulatory promoter will be turned on by transcription factors

resulting in turning on/off gene expression

Different types of transcription factors

Activators or repressors

Activators

bind to enhancer sequences

enhance transcription

Repressors

bind to silencer sequences

inhibit transcription

Where are enhancers and silencers located

upstream—work from a distance

regulatory promoter

Insulators

DNA sequence that blocks effect of enhancers

normal: Enhancer—Promoter =transcription

insulated: Enhancer—Insulator—Promoter =blocked

Consequences of regulatory elements

could stall and elongate transcription

Multiple response elements

Enhancer DNA sequences on multiple regulatory promoters that respond to environmental stimuli

=stimulate transcription

Degradation of RNA

done 5’→3’

shortening of poly A tail

removal of cap

siRNA

cleavage, degradation

miRNA

inhibition of translation

Process of siRNA and miRNA 

RNA cut up by Dicer to produce si/mi

si/mi binds with proteins to make RISC

pair imperfectly, leading to cleavage/inhibition

Alternative splicing

different splice sites equals different gene expression

Ex. tra protein

availability can influence translation and impact gene regulation

ribosomes, charged tRNAs, initiation and elongation factors 

Ex. virus increases translation and results in lots of cells being transcribed—abnormal amount