Download Gene Expression

Gene Expression
Open to Chapter 18
Dilemma
• All the DNA in an organism’s cells is
basically the same.
• We have many of the same genes as a fruit
fly (abt 60%).
• What accounts for the differentiation in the
cells?
…Which genes are expressed.
Regulation of Gene Expression
• Regulation can occur at many levels in the
DNA RNA PROTEIN
–
–
–
–
Transcription
Post-Transcription
Translation
Final Folding and Refolding of Proteins
Overview
• Prokaryotes have operons
• Eukaryotes
–
–
–
–
Regulatory sequences
Selective transcription
Homeobox Genes (HOX)
Epigenome
PROKARYOTE OPERON
– Promotor- Where RNA polymerase attaches
– Operator- switch that turns on or off. If
repressor attaches, it is “off”. Transcription is
disabled.
– Gene
– Examples:
• Lac Operon- If Lactose is present, repressor
inactive, gene turned on.
• Trp Operon - If Tryptophan is present, repressor
active, gene turned off.
Figure 18.3b-1
DNA
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Lac and Trp Operons
• No matter which type of operon, when the
repressor is active, the gene is switched___.
• When lactose is present, the gene is
switched _____
• When tryptophan is present, the gene is
switched ______
Lac and Trp Operons
• No matter which type of operon, when the
repressor is active, the gene is switched off.
• When lactose is present, the lac gene is
switched on
• When tryptophan is present, the trp gene is
switched off
What would happen if the
repressor of an operon were
mutated so that it could not bind
to the operator?
a) irreversible binding of the repressor to the
promotor.
b) reduced transcription of the operon’s genes
c) continuous transcription of the operon’s
genes
EUKARYOTE REGULATORY
SEQUENCES
• Proteins (transcription factors) can bind to
enhancer sequences on gene. Depending
upon cellular conditions, this may enable
gene to turn on (promote) or off (repress).
Eukaryotes have multiple switches.
– Induction- If proteins from neighboring cells
are present, gene may turn on (ex: retina)
– Hormones and other molecules may attach to
enhancer sequence to turn on genes.
Signal
NUCLEUS
Chromatin
PAGE 356
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethylation
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing, such
as cleavage and
chemical modification
Degradation
of protein
Active protein
Transport to cellular
destination
Cellular function (such
as enzymatic activity,
structural support)
EPIGENOME
• Environmental effects can impact gene
expression.
• Histone Acetylation- Adding acetyl group
to histone proteins makes DNA more
accessible, promoting transcription
• Methylation seems to prevent genes from
loosening from histones, repressing
transcription.
– Twins can have different diseases, and their
gene expression becomes more divergent as
they age.
HOMEOBOX GENES
• HOX genes (Homeotic genes)
– Master control genes
• Segmented organisms have HOX genes
associated with genes for body parts. For
example, since the same genes form antenna
and leg, the structure that forms depends
upon its HOX master control gene.
Normal Head
Mutant Head
Transcriptional control
• Name an epigenetic factor that in represses
transcription.
• Name an epigenetic factor that promotes
transcription.
• The reason that the same gene can code for
either an antenna or a leg is that this gene is
controlled by ______
SELECTIVE TRANSCRIPTION
• During transcription, intervening sequences
of mRNA are removed (introns).
• Exons are spliced together.
– Males and females have the same set of genes,
the fact that they are spliced differently
accounts for the difference in gender.
– Splicing and DNA rearrangement account for
millions of different antibodies from the same
genes.
Post transcriptional Modification
RNAi
• Interference RNA- RNAi
• Regulates gene expression at transcription
level, by attaching to complementary
mRNA
• Often inhibits or silences.
• Forms may be called microRNA, or siRNA
• See Video RNAi on Teachers Domain
Figure 18.15
Hairpin
Hydrogen
bond
miRNA
Dicer
5 3
(a) Primary miRNA transcript
miRNA
miRNAprotein
complex
mRNA degraded Translation blocked
(b) Generation and function of miRNAs
Figure 18.25
Cancer is a multistep process
-Mutations of tumor suppressor
- Mutation of proto-oncogene
Colon
1 Loss
of tumorsuppressor
gene APC
(or other)
Colon wall
Normal colon
epithelial cells
2 Activation
of ras
oncogene
4 Loss
of tumorsuppressor
gene p53
3 Loss
5 Additional
mutations
of tumorSmall benign suppressor Larger
Malignant
growth
tumor
gene DCC benign growth
(polyp)
(adenoma)
(carcinoma)
• See HMMI Click and Learn Genetic
Switches
•
http://www.hhmi.org/biointeractive/geneswitch
• See Controlling
Protein Synthesis AP
Boardworks, Slide 4-7
Muscle cells differ from nerve
cells mainly because they
a) express different genes
b) contain different genes
c) have different chromosomes