Chapter 11 – Gene Expression
11.1 – 11.3
Skip 11.4
AP Biology Radjewski 2014
Some rats are genetically programmed
to prefer alcohol to water
Review
• DNA  RNA  Protein
Examples
1. When an extracellular signal binds to its
receptor, it sets in motion a signal
transduction pathway that may end in some
genes being activated and some being
repressed.
2. During cell cycle, cyclins are synthesized only
at specific points.
In some cases:
• Gene expression is modified to counteract
changes in environment to help maintain
homeostasis
• In other cases, gene expression changes so that
the cell can perform a specific function.
– For example, all of our cells carry the genes to encode
keratin (protein in hair) and hemoglobin.
• But keratin is only made in specialized epithelial cells and
hemoglobin is only made by developing RBC’s
– In contrast, all cells express the genes that encode for
enzymes for metabolism
Two types of genes
• Constitutive genes
– Actively expressed all
the time
• Inducible genes
– Expressed only when
their proteins are
needed by the cell
Genes are subject to positive and
negative regulation
• Positive regulation – there is a binding of an
activator that stimulates transcription
• Negative regulation – there is a binding of a
repressor, that prevents transcription
What are viruses?
• Non living particle that can only
reproduce within a host cell
• Not cellular
• Can have ds DNA, ssDNA, dsRNA or
ssRNA
• Takes over the host cells protein
synthesis machinery within minutes
of entering the host
• 2 types of reproductive cycles
– Lytic
– Lysogenic
Lytic Cycle
• Lytic – “break”, meaning host cell is destroyed
afterwards
• 6 steps
1. Bacteriophage (virus infecting a bacteria cell) infects
a host cell – viral DNA enters
2. It uses the bacterium’s RNA polymerase to transcribe
early genes
3. One early protein shuts down host (bacterial) gene
transcription
4. Another protein stimulates viral genome replication
5. Another protein stimulates late gene transcription
6. New viral capsid proteins and a protein lyses the host
cell
HIV Review
• Human immunodeficiency virus
• Typically infects only cells of the immune
system that express a surface receptor called
CD4.
• Proteins on the membrane are involved in the
infection of new cells, which HIV enters by
direct fusion of the viral envelope with the
host plasma membrane
• It is a retrovirus
Retrovirus
• Genome is single stranded RNA
• Carries an enzyme, reverse transcriptase that makes a
DNA strand that is complementary to the RNA, while at
the same time degrading the RNA and making a second
DNA strand that is complementary to the first
• The resulting dsDNA becomes integrated into the
host’s chromosome, where it resides and the virus can
become dormant.
• Eventually cellular triggers result and stimulates
transcription of the viral DNA, resulting in mRNAs that
are translated into viral proteins, and in new copies of
the viral genome
Negative regulation of HIV
• Normally a host cell has a negative regulatory
system that can repress the expression of
invading viral genes.
• However HIV can counteract this with a virusencoded protein called Tat (Transactivator of
transcription)
• Tat binds to the viral mRNA along with
proteins that allow RNA polymerase to
transcribe the viral genome.
Without Tat (normal Human)
With Tat (HIV infected humans)
Operon
• Cluster of genes with a single promotor that
code for proteins in the DNA of bacteria
– Codes for 3 lactose-metabolizing enzymes in E.
Coli
• Called the lac operon
– Example of negative regulation
• 4 major components
Component #1
• Promotor region
– Region of DNA to which the RNA polymerase
attaches to begin transcription
Component #2
• Structural genes
– Contain DNA sequences that code for several
enzymes
• Operator
Component #3
– Located between the promotor and the structural
genes
– Region of DNA that is able to control RNA
polymerase’s access to structural genes
– It’s like a switch that can turn the operon on or off
Component #4
• Repressor protein
– A substance that can prevent gene expression by
binding to the operator and prevents RNA
polymerase from transcribing the structural genes
– Transcription would resume when the repressor is
removed by a molecule called an inducer
Background Information
• When you consume milk, the disaccharide
lactose is soon present in your intestinal tract
and available to the E. Coli living there.
• Before E. Coli can absorb lactose, it must first
make beta-galactosidase, the enzyme that
breaks down lactose into glucose and
galactose.
• 3 enzymes are needed to metabolize lactose:
1. B-galactoside permease – moves sugar into
cell
2. B-galactosidase – hydrolyzes lactose to
glucose and galactose
3. B-galactoside transacetylase – transfers
acetyl groups from acetyl CoA to certain Bgalactosides. Role is unclear.
• It is in E. Coli’s best interest to focus its energy
on using available nutrients
• Therefore E. Coli should only make the
enzyme when lactose is present.
• How does E. Coli do this?
– By an operon!
So if lactose is absent:
Repressor binds to operator and it prevents RNA polymerase
from binding to promotor, so transcription is blocked.
No mRNA is produced, so no enzyme is produced.
And if lactose is present:
Lactose binds to repressor. The
repressor becomes inactive!
Binds to promotor
Operator is free so transcription takes
place and the enzymes are made!!
Inducible System
• Lac operon is called an inducible system
• Allolactose (alternative form of lactose) is the
inducer and it leads to the synthesis of
enzymes in the lactose-metabolizing pathway
by binding to the repressor protein and
preventing its binding to the operator.
Trp Operon
• Trp – tryptophan, an amino acid
• A co-repressor is involved. It is a molecule that binds
to the repressor, causing it to change shape and bind to
the operator, thereby inhibiting transcription.
• When tryptophan is adequately present in the cell, it is
energy efficient to stop making the enzymes for
tryptophan synthesis
• Therefore tryptophan functions as a co-repressor and
binds to the repressor of the trp operon!
– This causes the repressor to bind to the trp operator to
prevent transcription.
Tryptophan is inadequate/absent
Tryptophan is present/too much!
Repressible System
• Trp operon
• The product of a metabolic pathway (the corepressor) binds to the repressor protein,
which is then able to bind to the operator and
block transcription.
Eukaryotic Cells
• Can also regulate the transcription of large
stretches of DNA (containing many genes) by
reversible, non-sequence-specific alterations
to either the DNA or the chromosomal
proteins
• These alterations can be passed on to
daughter cells after mitosis or meiosis
• Are called Epigenetic changes (not mutations)
DNA Methylation
• 1-5% of cytosines in the
DNA are chemically
modified by the addition
of a methyl group to form
5-methyl-cytosine.
• Catalyzed by the enzyme
DNA methyltransferase
• Usually occurs when C’s
that are adjacent to G’s
• Areas rich in the
methylation are called
CpG islands, and are
abundant in promotors
DNA Methylation continued…
• This change in DNA is heritable
– When DNA is replicated, an enzyme called
maintenance methylase catalyzes the formation of 5methylcytosine in the new DNA strand
• But it is reversible by demethylase, which
catalyzes the removal of the methyl group from
cytosine.
• Methylated DNA binds specific proteins that are
involved in the repression of transcription; thus
heavily methylated genes tend to be inactive