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Gene Expression

Every cell has the same DNA

Yet, each cell is different, specialized

Differences due to gene expression
 Which
genes are turned on
 When
the genes are turned on
 How

much product they make
Different factors control all this
1
Gene Expression is Influenced by
the Organization of DNA

Prokaryotes have:
 Several
million base pairs in one chromosome
 Related genes are grouped together
 DNA that is mostly coding DNA

Eukaryotes have:
 Billions
of base pairs in several chromosomes
 Genes that are not grouped together
 DNA that is mostly non-coding DNA
2
The Organization of DNA is
Influenced by the Genome Size
Genome: total component of DNA
 Prokaryotes

 0.6
to 30 million base pairs
 Approximately 2,000 genes

Eukaryotes
 12
million to 1 trillion base pairs
 Humans have ~25,000 genes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
3
Gene Expression is Controlled by
DNA Packaging
Eukaryotic chromosomes are very large
 Must be packaged

 Unavailable

for transcription
Unpacking must occur before transcription
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
4
Levels of DNA
Packaging





Fully condensed DNA,
seen at metaphase
Tightly packed loops
30 nm fibers
Histone spool
Double helix, even this
has to be unpacked for
transcription
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
5
Patterns of Gene Expression
Bacteria directly exposed to environment
 Respond to changes in nutrient availability

 Make
enzymes for nutrients when they are
present (turn on the gene)
 Turn genes off when they are not
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
6
Patterns of Gene Expression



Eukaryotic cells
Tissue specific
expression
Housekeeping
genes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
7
Gene Expression: Development
Embryo development depends on gene
expression
 Timing of expression is complex, yet vital
 Controlled by cascades of gene
expression

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
8
Homeotic genes
Master switches
 Start gene cascades
 Coordinate development of specific
structures
 Cell and timing specific expression

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
9
Homeotic genes

Similar effects in
different organisms
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
10
Master Gene Switches: SRY

During reproduction, males determine sex
 If
the sperm has an X, it’s a girl
 If the sperm has a Y, boy
Y chromosome has the SRY gene
 SRY is a master switch that controls the
expression of other genes at other loci
 SRY changes the embryo to develop into
a boy

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
11
Control of Gene Expression

Based on two essential elements:
 Regulatory
DNA: switches gene on/off
 Regulatory Protein: binds to regulatory DNA
Control gene expression together
 Respond to environment or cell signals

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
12
Control of Gene Expression
E. coli
 Tryptophan
genes
 Repressor
 Operator

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
13
Eukaryotic Gene Expression
More complex than prokaryotic
 Many different types of regulatory proteins
 Many DNA elements controlling each gene
 In summary, the organization of DNA
controls gene expression in complex ways

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
14
Levels of Gene Control
Tightly packed DNA is not expressed
 Transcription regulation
 Regulation of mRNA breakdown
 Inhibition of translation
 Regulation of proteins after translation
 Destruction of completed proteins

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
15
Levels of
Gene Control
1.
2.
3.
4.
5.
6.
Packaging
Transcription
mRNA breakdown
Translation
Protein Regulation
Protein Degradation
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
16
Gene Control and Phenotype

Can have powerful effects
 Homeotic

genes
Can result from environmental conditions
 Genes
turn on and off
 Phenotype changes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
17
Process Animation 14.2
The Tryptophan Operon
you tube link
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
18
Process Animation 14.3
Control of Gene Expression
You tube link
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
19