Download Regulation of Gene Expression

Ch. 16
Regulation of Gene
Expression
Learning objectives
After this lecture you should be able to…
•  Describe how gene expression is regulated in proand eukaryotes emphasizing the many different
levels this can be achieved on.
•  Account for the mechanisms for interbacterial
transfer of genetic material and how a virus
reproduces itself by the use of the host
reproductive system.
Disposition
Virus (including bacteriophages)
lytic <> lysogenic cell cycle
HIV - a retrovirus that infects humans
Bacteria
The lac operon
Eukaryotes, regulation
On chromosome level
At transcription initiation
By alternative splicing
By mRNA degradation
By protein degradation
Virus
Genome:
ssDNA
dsDNA
ssRNA
dsRNA
They have no cell
membranes and don t
regulate uptake / excretion of
substances.
They don t carry out
metabolic processes.
Capsule of protein
(rarely lipids)
They cannot reproduce
independently of a host cell
Can we claim that viruses are “life”?
Bacteriophages
virus that infects bacteria
Bakteriofag T2
E. coli
Bacteriophage T4 infecting E. coli cells
The lytic life cycle
The lysogenic life cycle
HIV
Retrovirus
•  The genome is ssRNA
• Surface glycoproteins recognize the CD4
receptor on host cells
•  Inside the host cell, reverse transcriptase
generates DNA (cDNA) based on the viral
RNA
•  Integrase integrates the cDNA in the host
cell DNA = provirus
•  Upon activation, the provirus genes are
transcribed and translated
•  Host terminator protein binds viral DNA
making the RNA polymerase fall of. HIV tat
protein binds the terminator protein and
inhibits the interaction
•  New virus particles are generated by
viral RNA and proteins and the host cell s
membrane.
HIV drugs
1) Co-receptor inhibitors
2) Reverse transcriptase inhibitors
3) Protease inhibitors
4) Integrase inhibitors
Loss of facial fat
Gain of abdominal fat
Regulating Gene
Expression in Bacteria
Adapting to changing environments
Example: The lac operon
The lac operon
RNA polymerase
Lactose
Lac genes
Adapted from Gonick og Wheelis The Cartoon Guide to Genetics
The lac operon
A more silly
variant of Figure
16.11…
Adapted from
Gonick and Wheelis
”The Cartoon Guide to Genetics”
More on the lac operon...
Glucose is also involved in the regulation of trancription from the lac
operon:
Low glucose level =>
High cAMP level =>
cAMP binds the cAMP receptor protein (CRP) =>
CRP binds the promoter of the lac operon =>
Increased RNA polymerase attraction to the
promoter => increased transcription of the lac
genes.
High glucose level = >
Low cAMP level =>
cAMP doesn t bind CRP =>
CRP doesn t bind promoter =>
RNA polymerase doesn t bind promoter =>
No transcription of the lac genes.
Imagine an E. coli in surroundings with plenty of glucose and
lactose. How is the level of transcription from the lac operon?
Is lactose degraded?
Regulating Gene Expression in
Multi-Cellular Organisms
Same genome – different
expression at different stages
Same genome – different
expression in different places
Chromatin
(DNA+histones)
vsrp.uhnres.utoronto.ca/research1.htm
Euchromatin: Lightly packed chromatin. The genes are
transcribed.
Heterochromatin: Tightly packed chromatin. The genes are
normally not transcribed.
Remodelling of chromatin structure before transcription
Gene expression can be regulated here
Barr body – one of the X
chromosomes of women are
inactivated as heterochromatin
During the development of a
female embryo, one of the two
X chromosomes are
inactivated.
The female body is a
mosaic, where some areas
contain cells in which one X
chromosome is inactivated,
while cells in other areas
have the other X
chromosome inactivated.
XX
XX
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XX
XX
X
X
X
X
X
X
X
X
X
X
X
X
XX
X
X
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X
X
© Sinauer Associations, Inc.
The timing of the decision on which X chromosome is
inactivated determines how large areas with orange /
black coat is ...
BREAK
Epigenetics
Epigenetics: The study of changes in the level of
expression of genes or sets of genes that are not caused
by changes in the DNA sequence (but, e.g., DNA
methylation).
Epigenetic changes can be inherited by the offspring!
Caused by:
- Methylation of cytosin
- Changes of the
chromosomal proteins
(histone proteins)
Cytosin
5 -Methylcytosin
Lamarck
Heritability of acquired characteristics.
Darwin and natural selection
Initiation of
trancription
Here gene expression can be regultated
More on initiation of transcription
RNA polymerase II
Activator
Enhancer
Repressor
Silencer
Repressor
Activator TFIID
Silencer Enhancer
Initiation of transcription
Promoter
The same transcription factor can bind/regulate several genes!
Gene expression can be regulated here
The MAOA gene (involved in aggression)
- Multiple repeats of 30 bp are present in the promoter of the MAOA gene
- Different alleles have different number of repeats
- The level of transcription from the MAOA gene varies depending on the
allele
Fra Sabol et al, Hum Genet (1998); 103(3):273-9.
- For men, the alleles with a low level of transcription is associated
with an aggressive and antisocial behavior, especially if the men had
a bad childhood or drink too much.
Structure of eukaryotic proteinencoding genes
Start codon
Exons: Code for amino acids
Stop codon
Introns: Code for nothing
Promoter – Technically not part of the
gene
Here RNA polymerase II binds
Terminator
Signals to the RNA
polymerase to stop
transcribing
© Sinauer Associations, Inc.
Animated Tutorial 14.3 RNA Splicing
Alternative Splicing
Exon1
Intron1
Exon2
Intron2
Exon3
Pre-mRNA
Mature mRNA
Gene expression can be regulated here
MicroRNA – regulation mRNA longevity
•  Small RNA molecules, ≈ 22 to 23 nucleotides
•  Bind via basepairing to the 3’ end of the mRNA
•  Binding inhibits translation and sometimes the mRNA is
even degraded
•  Computers predict that more than 1000 genes encode
microRNA
•  It is also predicted that microRNA molecules regulate the
expression of more than 1/3 of all human genes
Gene expression can be regulated here
Post translational regulation of expression
Ubiquitination - or
Gene expression can be regulated here
Examining gene expression
using micro arrays