Transcript chapt13_image

Chapter 13
Regulation of
Gene Activity
• Humans and nemotodes have about the same number
of genes roughly 20,500
• So how can a complex organism produce the proteins
they require?
• By regulation of pre-mRNA splicing to produce many
proteins from a single gene
• In 1961, Jocob and Monod showed that the bacteria
Escherichia coli could regulate the expression of
genes
• They received the Nobel prize for the “operon model”
to express gene regulation in prokaryotes
• Operon includes:
• Promoter which is a short sequence of DNA where
RNA polymerase first attaches to begin transcription
• Operator which is a short portion of DNA where an
active repressor binds
• When active repressor binds to operator, RNA
polymerase cannot attach to promoter and no
transcription
• Structural genes are one to several genes coding for
primary structure of enzymes in metabolic pathway
transcribed as a unit
• Regulator gene, usually located outside operon and
controlled by own promoter, codes for a repressor
that controls whether the operator is active or not
-Some operons in E. coli are usually in “on” rather than
“off” condition
-Trp operon, the regulator codes a repressor that
ordinarily is unable to attach to operator
-RNA polymerase is able to bind to promoter and
structural genes are expressed
-Then five enzymes promote anabolic pathway for
synthesis of amino acid tryptophan
-If tryptophan is present, it binds to the repressor,
changes its shape and binds to operon
Structural genes are not expressed
• Entire unit is called a repressor operon
• Tryptophan is the corepressor
• Repressible operons are usually involved in anobolic
pathways
Fig. 13.1
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regulator gene
promoter
operator
structural genes
DNA
RNA polymerase
5
3
mRNA
inactive repressor
enzymes
a. Tryptophan absent. Enzymes needed to synthesize tryptophan are produced.
RNA polymerase cannot bind to promoter.
DNA
active repressor
tryptophan
inactive repressor
b. Tryptophan present. Presence of tryptophan prevents production of enzymes used to synthesize tryptophan.
• Bacteria metabolism is efficient
• If a protein or enzyme is not needed, genes to make
them are inactive
• If lactose is not present, enzymes for lactose
catabolism are not active
• If E coli are denied glucose and given lactose, it
immediately makes the three enzymes needed to
metabolize lactose
• The three structural genes needed are adjacent to one
another and under control of a single promoter and
operon
• Lac operator repression usually binds to operator and
prevents transcription
• Lactose binds to repressor, changes its shape that
prevents its binding to promoter
• RNA polymerase binds to promoter and carries out
transcription of enzymes for lactose metabolism
• Presence of lactose brings about expression of genes
and is called inducer
• Entire unit is called inducible operon
• Inducible operons are usually necessary for catabolic
pathways
Fig. 13.2
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RNA polymerase cannot bind to promoter.
regulator gene
promoter operator
structural genes
DNA
active repressor
active repressor
a. Lactose absent. Enzymes needed to take up and use lactose are not produced.
RNA polymerase can bind to promoter.
DNA
inactive repressor
5
mRNA
active repressor
lactose
enzymes
b. Lactose present. Enzymes needed to take up and use lactose are produced only when lactose is present.
3
• E coli prefers using glucose
• A molecule called cyclic AMP (cAMP) accumulates
when glucose is absent
• cAMP binds to a molecule called catabolite activator
protein (CAP) and the complex attaches to site next to
lac promoter
• This bends DNA exposing the promoter to RNA
polymerase
Page 236
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adenine
5
P
O
CH2
3
OH
cyclic AMP
(cAMP)
Fig. 13.3
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CAP binding site
promoter
operator
DNA
RNA polymerase binds
fully with promoter.
cAMP
active CAP
inactive CAP
a. Lactose present, glucose absent (cAMP level high)
CAP binding site
promoter
operator
DNA
RNA polymerase does
not bind fully with promoter.
inactive CAP
b. Lactose present, glucose present (cAMP level low)
• Each cell in multicellular eukaryote, has a copy of all
genes
• Different genes are actively expressed in different
cells
• Types of control in eukaryotic cells:
• 1) Chromatin structure- Chromatin packing is used to
keep genes turned off by preventing access to RNA
polymerase
• In nucleus, loosely condensed chromatin is available
for transcription
• Part of epigenetic inheritance, the transcription of
genetic information outside coding sequence of a gene
• 2) Transcriptional control is the degree to which a
gene is transcribed into mRNA determines amount of
gene product
• Transcription factors may promote or repress
transcription
• 3) Posttranscriptional control involves mRNA
processing and how fast mRNA leaves the nucleus
• Can determine type of protein product made and
amount of gene product made in a given time
• 4) Translational control occurs in cytoplasm and
affects when translation begins and how long it
continues
• Any influence on the persistence of 5’ cap and 3’
poly-A tail affect length of translation
• Excised introns are involved in regulatory system and
affect life span of mRNA
• 5) Posttranslational control occurs in cytoplasm after
protein synthesis
• Only functional protein is an active gene product
Fig. 13.4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
histones
Chromatin
structure
Transcriptional control
3
premRNA
5
intron
exon
Posttranscriptional control
mRNA
5
3
nuclear pore
nuclear envelope
Translational
control
polypeptide chain
Posttranslational
control
plasma
membrane
functional protein
• Highly condensed heterochromatin is inaccessible to
RNA polymerase
• It appears as darkly stained portions within nucleus in
electron micrographs
• Example of heterochromatin is the Barr body in
mammalian females
• It is an inactive X chromosome that does not produce
gene products
• In females one X chromosome transcribes genes and
the other becomes a Barr body
• Which X is inactive depends on which X chromosome
that cell received
• One X comes from father and the other from the
mother
• Conditions in human females include: ocular
albinism, Duchanne muscular distrophy, X-linked
hereditory absence of sweat glands
Fig. 13.6
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Coats of tortoiseshell
cats have patches
of orange and black.
active X chromosome
allele for
orange color
inactive X
Barr bodies
cell division
inactive X
allele for
black color
active X chromosome
Females have two
X chromosomes.
One X chromosome is inactivated in
each cell. Which one is by chance.
© Chanan Photo 2004
• Term euchromatin is used for the more loosely packed
active chromatin
• In herterochromatin, the histone tails tend to bear
methyl groups (-CH3)
• In euchromatins, the histone tails tend to be acetylated
and have attached acetyl groups (-COCH3)
• When euchromatin is transcribed, chromatin
remodeling complex pushes aside the histone portion
of nucleosome so access to DNA is not barred
• Also affects gene expression by adding acetyl or
methyl groups to histone tails
• Epigenetic inheritance concerns the pattern of
inheritance that does not depend on only the genes
• If a histone is methylated, the DNA may also be
methylated
• Genomic imprinting occurs when either the mother’s
or father’s allele is methylated during gamete
formation
• If inherited, the gene is not expressed
• Transcriptional control is the most critical of all
controls
• No operons like those in prokaryotic cells have been
found in eukaryotes
• Every cell contains transcriptional factors, proteins
that help regulate transcription
• In eukaryotes, transcription activators are DNA
binding proteins that speed transcription
• They bind to a region of DNA called enhancer that
can be far away from promoter
• A hairpin loop in the DNA brings the transcription
activators attached to enhancers into contact with
transcriptional factor complex
• Transcription factors, activators,and repressors are
always present in nucleus, but have to be activated
before they bind to DNA
Fig. 13.7
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DNA
promoter
enhancer
gene
transcription
activator
mediator proteins
transcription
factor complex
RNA polymerase
mRNA transcription
• During pre-mRNA splicing, introns (noncoding
regions) are excised, and exons (expressed regions)
are joined together to form mRNA
• Sometimes an exon is skipped or an intron is included
• Results in mature mRNA that has an altered sequence,
and protein encoded differs
Fig. 13.8
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intron
5
cap
A
exon
B
C
D
pre-mRNA
RNA
intron
E
3
5
poly-A
tail
cap
B
C
D
pre-mRNA
splicing
RNA
intron
mRNA
mRNA
protein product 1
protein product 2
3
poly-A
tail
intron
A B D E
b.
E
splicing
C
A B CDE
a.
A
exon
• Translation control begins when processed mRNA
reaches cytoplasm and before there is a protein
product
• Includes presence or absence of 5’ cap and length of
poly-A tail at 3’ end
• Micro RNAs (miRNAs) can regulate translation by
causing the destruction of mRNAs before they can be
translated
• Much like a dimmer switch on a light, miRNAs can
fine-tune the expression of genes
Fig. 13.9
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pre-mRNA
5
3
microRNA
(miRNA)
MicroRNA is cut from
a pre-mRNA and binds with
proteins to form RISC.
3
5
RISC
(RNA-induced
silencing complex)
proteins
Complementary base pairing
between RNAs allows RISC
to bind to mRNA.
mRNA
5
3
RISC
Translation
is inhibited.
or
The mRNA
is degraded.
• A gene mutation is a permanent change in the
sequence of bases in DNA
• Can range from no effect to complete inactivation
• Germ-line mutations occur in sex cells and can be
passed to subsequent generations
• Somatic mutations occur in body cells and affect only
a small number of cells in a tissue
• Somatic mutations are not passed on to future
generations, but can lead to cancer
• Spontaneous mutations are associated with any
number of normal processes
• The movement of transposons from one chromosomal
• location to another can disrupt a gene and lead to an
abnormal product
• A base in DNA may undergo a chemical change that
leads to a miss pairing during replication
• These mutations are rare because DNA polymerase
proofreads the new strand against the old strand,
detects most mismatched nucleotides, and usually
replaces them with correct nucleotides
• Induced mutations are caused by mutagens,
environmental factors that can alter base composition
of DNA
• Includes radiation and organic chemicals
• Many mutations are also carcinogens (cancer-causing)
• Chemical mutagens are present in some food we eat
and many industrial chemicals
• Tobacco smoke contains a number of carcinogenic
organic chemicals
• One-third of all cancer deaths can be attributed to
smoking
• Lung cancer is most frequent lethal cancer in the
United States
• Ames test is used for mutagenicity of a chemical to be
carcinogenic
• A histidine-requiring strain of bacteria is exposed to
• the chemical
• If the chemical is mutagenic, bacteria can grow
without histidine
Fig. 13.10
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Suspected
chemical
mutagen
Control
bacterial
strain
(requires
histidine)
Plate onto petri plates
that lack histidine.
bacterial
growth
bacterial
strain
(requires
histidine)
Incubate overnight
Mutation occurred
Mutation did not occur
• Point mutations involve a change in a single DNA
nucleotide with a possible change in a specific amino
acid
• Frameshift mutations occur most often because one or
more nucleotides are either inserted or deleted from
DNA
• May form a completely new sequence of codons and
nonfunctioning protein
• A single nonfunctioning protein can have a dramatic
effect on the phenotype, because enzymes are often
part of metabolic pathways
Page 244
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A
(phenylalanine)
EA
B
(tyrosine)
EB
C
(melanin)
Fig. 13.12
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5
3
No mutation
C A C G T GG AGT G AG G T C T C C T C
Val
His
His
Leu
Thr
Pro
Glu
Glu
His
(normal protein)
C A C G T A G AG T G A G G T C T C C T C
Val
His
Leu
Thr
Pro
Glu
Glu
b. Normal red blood cell
Glu
Val
(abnormal protein)
C A C G T GG AGT G AG G T C A C C T C
Val
Glu
Stop
(incomplete protein)
a.
His
Leu
Thr
Pro
Val
Glu
C A C G T GG AGT G AG G T A T C C T C
Val
His
Leu
Thr
Pro
Stop
b, c: © Stan Flegler/Visuals Unlimited.
c. Sickled red blood cell
• The development of cancer involves a series of
accumulating mutations that can be different for each
type of cancer
• The cell cycle occurs inappropriately when protooncogenes become oncogenes and tumor suppressor
genes are no longer effective