Transcript What is a gene?

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Gene regulation in prokaryotes and
eukaryotes
• How do organisms respond correctly to their
environment?
• How do they control their growth and
development?
• How do they control which genes are
expressed?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The process by which genetic
information flows from genes to
proteins is called gene expression
Prokaryotes
Proteins interacting with
DNA turn prokaryotic genes
on or off in response to
environmental changes
Figure 11.1A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
genes for related enzymes are often controlled
in groups called operons
lac operon
regulator gene
promotor
operator
i gene
codes for
repressor
protein
p
o
z gene
y gene
binding site codes for
of RNA
b-galactosidase,
polymerase which clips
lactose molecules
a gene
codes for
permease enzyme
that transports
lactose into cells
DNA
lac operon
regulator gene
i gene
RNA polymerase
p
operator
o
z gene
promotor
repressor protein
blocks binding of
RNA polymerase
y gene
a gene
DNA
no transcription
repressor
protein
Regulatory proteins bind to control sequences in
DNA and turn operons on or off
2 RNA polymerase
binds to promoter
p
i gene
o
z gene
y gene
a gene
DNA
3 transcription proceeds
repressor
mRNA transcript
lactose
b-galactosidase
1 lactose the (inducer)
inactivates the repressor
so that it cannot bind
to the operator
permease
galactose
glucose
lactose
• Two types of repressor-controlled operons
Promoter
Operator
Genes
DNA
Active
repressor
Active
repressor
Tryptophan
Inactive
repressor
Inactive
repressor
Lactose
lac OPERON
Figure 11.1C
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trp OPERON
Eukaryotic gene control
• Transcription is a major control point
• Individual genes are regulated, rather than
operons
• Activation is more important than repression
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Activators bind to enhancer regions of DNA and to
other transcription factors
– These interactions
turn the
DNA
transcription
Transcription
of eukaryotic
factors
genes
on or off
Enhancers
Promoter
Gene
Activator
proteins
Other
proteins
RNA polymerase
Bending
of DNA
Figure 11.8
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Transcription
Control of transcription
• Control access of enzymes to DNA:
1. Methylation of DNA - inhibits
2. Acetylation of histones - promotes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
How do eukaryotes control genes of a single
metabolic pathway?
• genes for individual enzymes of one pathway
are often located far apart in the genome
• same enhancer region(s) common to all genes
of a pathway.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Eukaryotic RNA may be spliced in more than one
way
• After transcription, alternative splicing
may generate two or more types of mRNA
from the same transcript
Exons
DNA
RNA
transcript
RNA splicing
mRNA
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
or
Figure 11.9
Translation and later stages of gene expression are
also subject to regulation
• The lifetime of an mRNA molecule helps
determine how much protein is made
• Initiation of translation
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The protein may need to be activated in
some way
• Rate of protein degradation
Folding of polypeptide and
formation of S–S linkages
Initial polypeptide
(inactive)
Folded polypeptide
(inactive)
Cleavage
Active form
of insulin
Figure 11.10
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
DNA
double
helix
(2-nm
diameter)
multiple
levels of
DNA
packing
Histones
“Beads on
a string”
Nucleosome
(10-nm diameter)
Tight helical fiber
(30-nm diameter)
Supercoil
(200-nm diameter)
•DNA packing
tends to block
gene expression
700
nm
Metaphase chromosome
Figure 11.6
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
In female mammals, one X chromosome is inactive
in each cell
EARLY EMBRYO
TWO CELL POPULATIONS
IN ADULT
Active X
Allele for
orange fur
X chromosomes
Orange fur
Inactive X
Cell division
and
X chromosome
inactivation
Inactive X
Active X
Black fur
Allele for
black fur
Figure 11.7
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosome
DNA unpacking
Other changes
to DNA
GENE
GENE
TRANSCRIPTION
Exon
RNA transcript
Intron
Addition of
cap and tail
Splicing
Tail
Cap
mRNA in nucleus
NUCLEUS
Flow
through
nuclear envelope
mRNA in cytoplasm
CYTOPLASM
Breakdown of mRNA
Translation
Brokendown
mRNA
Polypeptide
Cleavage/modification/
activation
ACTIVE PROTEIN
Breakdown
of protein
Brokendown
protein
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 11.11
Review: Multiple mechanisms regulate gene
expression in eukaryotes
• Each step of gene expression can be
turned on or off, speeded up, or slowed down
• most important control is usually the start of
transcription
• DNA can be mobile; position will affect
transcription. Transposons
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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What is a gene?
• First version: one gene, one polypeptide
• Now: DNA segment that is transcribed to
RNA
• Proteins and RNA function in cells
Nature vs nurture
• Do environments or genes determine phenotype?
• Himalayan rabbits and fur color
• Twin studies - identical cp. fraternal
Complex human traits
• Language - FoxP2
• Depression
• Social bonds
Do 460 bp determine our life
partner?
Genome imprinting
• Same gene is expressed differently, depending
on whether it was inherited from the male or
female parent
• Ex. Corn kernel color
• Ex. Xsome 15 deletion
• Gene is “reset” during gamete formation