lecture 7- regulation of gene expression

gene expression in eukaryotes

DNA → transcription→ RNA transcript → RNA processing → mRNA→ RNA transport→ mRNA → translation → protein

differentiation

long term control of gene expression in tissue

regulatory genes

→ regulatory proteins that bind to DNA blocking or stimulating transcription (depending on RNA polymerase interaction)

types of gene expression

constitutive expression

• when genes are essential they are continuously expressed, they are called housekeeping genes (actin)

induction and repression

• expression levels depend on external signals

example of whole-chromosome regulation

barr bodies (X chromosomes in female cells)

regulation in eukaryotes

1. transcription DNA→ mRNA

• activator proteins

• promoters

2. splicing of mRNA

• exons - expressed

regulation in prokaryotes

1. transcription DNA → mRNA

• operators

• promoters

• repressor protein

negative control

• active regulatory protein that turns transcription OFF

• induction: turns operon on

• repression: turns operon off

positive control

• active regulatory protein enhances rate of transcription

• induction: makes regulatory protein active

• repression: makes regulatory protein inactive

regulated gene expression

how a cell controls the speed/amount of a gene being expressed as a protein

how are eukaryotes different in transcription regulation?

• genes are almost always transcribed individually

• 3 RNA polymerases occur

transcription factors

molecules that allow RNA polymerase to bind to DNA and begin transcription

promoter in eukaryotes

sequence on DNA before the gene where activator proteins and transcription factors can bind to

enhancer

control element far from a gene or intron in transcription regulation, binds to activator proteins

activator

binds to enhancers to turn on transcription of a gene

equation for transcription

transcription factors + enhancer + activator + RNA polymerase II

= transcription

control elements

DNA sequences that react with proteins to help the binding of RNA polymerase

regulator proteins

bind to regulator sequences to activate transcription, found before the promoter

silencer sequences

stop transcription if they bind with repressor proteins

gene regulation in the third dimension

1. assembly of functional expression units

2. coordinated expression

3. functional organization of the nucleus

post transcription regulation

• mRNA variation (alternative splicing: shuffling exons)

• change lifespan of mRNA (making micro RNA, damages mRNA)

• edit RNA (alter genetic code)

splicing RNA

• after transcription, parts of transcribed mRNA are spliced out

• all exons are cut, translated and expressed in proteins- the spliced exons will code for a protein!

alternative splicing RNA

mix and match thats very specific

domains

units of discrete regions in proteins

exon shuffling

can result in evolution of new proteins

mRNA degradation in prokaryotes

• short life span

• degraded in seconds

• allows rapid response to environmental changes

mRNA degradation in eukaryotes

• survive from hours to weeks

• internal conditions constant, no need for rapid response

how to modify the lifespan of proteins?

attach ubiquitin to target it for breakdown via proteasome (woodchipper)

nucleosome packing

DNA wraps around histone proteins to form a structure called a nucleosome. Nucleosomes help pack DNA into eukaryotic chromosomes.

operon

grouped genes that are transcribed together, code for similar proteins

promoter in prokaryotes

section of DNA where RNA polymerase binds

operator

controls activation of transcription (on off switch)

repressor protein

binds to operator to block RNA polymerase and shut down transcription (turns off operon)

compressor vs inducer

compressor- keeps repressor protein on operon

inducer- pulls repressor off operon, turns on operon

regulatory gene

makes repressor proteins

structural gene

codes for proteins

lac operon

• inducible

• only turned on if lactose is “eaten”

• lactose is not necessary for cell to function

when lactose is present…

• repressor is NOT bound to operator

• RNA polymerase can bind to promoter and transcription occurs

• expression of lactase proteins occurs

when lactose is absent…

• repressor is BOUND to operator

• RNA polymerase cannot bind to promoter and transcription does NOT occur

• expression of lactase protein is repressed and the switch is off

trp operon

• repressible

• always making tryptophan

• repressed if tryptophan is “eaten”

• tryptophan is necessary for cell function

feedback inhibition

• enough product is made, system shuts down

• more product is made when needed, the product shuts down the process

gene expression

genes are only expressed when needed, often regulated at transcription