Transcript Chapter 18 - Regulation of Gene Expression - Bio-Guru

Regulation of Gene Expression
Chapter 18
How do Organisms Control the
Level of Gene Expression?
• Cells must only express genes when
needed
• Gene expression (transcription,
translation) takes up large amounts of
cellular energy and resources
• Cells live frugal lifestyles – they conserve
energy and resources
• So genes will only be expressed when
their products are needed.
How Prokaryotes Control Gene Expression
• Bacteria control genes at the transcriptional
level
• In other words, the gene is either transcribed
or not, based on certain external stimuli
The Operon
• An operon is a segment of DNA that
consists of the following:
– A gene or group of genes
– A promoter of the gene/group of genes
– A region upstream of the gene called the an
operator
Operons
Lac Operon – Inducible operon
The lac operon is always off and is turned on by an inducer molecule
(allolactose). This is why it is called an inducible operon
The trp Operon - repressible operon
The trp operon is always on and is turned off only by end product – tryptophan.
This is why it is called a repressible operon.
trp Operon – repressible operon
How do Eukaryotes Control the
Level of Gene Expression?
• Cells of more complex organisms turn on and
turn off genes based on the functions of the cells
– hence cells differentiate
• Eukaryotes control genes at almost every level:
–
–
–
–
–
Regulation of Chromatin Structure
Regulation at the transcriptional level
Regulation at a post-transcriptional level
Regulation at a translational level
Regulation at a post-translational level
Regulation of Chromatin Structure
• Histone acetylation prevents DNA from
winding tightly around histones, allowing
easy access to promoter sites
(Deacetylation does the opposite)
• DNA methylation causes DNA to wind
tightly around histones, preventing easy
access to gene promoters (Demethylation
does the opposite)
Histones
• Histone subunits are:
–
–
–
–
Linker DNA links one
nucleosome to the next
2 units of H2A
2 units of H2B
2 units of H3
2 units of H4
• Histone H1 is not in the
core, but acts as a clamp
and keeps the linker DNA
in place
• Histones are positively
charged, so DNA which is
negatively charged,
wraps around them
Bacteria lack histones, although some
archea have them
Histone
• Histones have
tails that get
actylated by HAT
enzymes (Histone
Acetyl
Transferases)
Acetyl group
• Histone
acetylation makes
their (+) charge
more neutral, so
DNA interaction is
reduced
DNA Methylation in
conjunction with Histone
deacetylation
Epigenetic Inheritance
• Modification of chromatin does not change the
DNA, only its expression.
• However, this modification pattern IS inherited
(remember genomic imprinting?)
• Scientists now believe that certain environmental
factors may play a part in promoting chromatin
modification that causes expression or
suppression of certain genes – e.g. one twin
gets schizophrenia and another doesn’t. Certain
cancers may also be caused that way
Regulation at the transcriptional level
• Enhancers (proximal and distal)
• Silencers
• Transcription factors at promoters
– General transcription factors
– Specific transcription factors
– All these play a role in regulating gene expression.
– Enhancers increase the rate of a gene’s expression and
silencers decrease it.
– Transcription factors are needed if the gene is to be expressed
at all.
Organization of a typical Eukaryotic Gene
Regulation at a post-transcriptional level
• RNA processing – alternative splicing allows
certain proteins to be made instead of others (all
from the same gene)
• mRNA Degradation – cytoplasmic nucleases
degrade mRNAs so polypeptide synthesis stops.
More mRNA is made later, if necessary
• 5’ caps and 3’ tails can be removed or changed
and this will prevent translation
Pre-translational Regulation
• Certain proteins in the cytoplasm can bind
to the mRNA’s 5’ UTR and prevent
ribosomes from binding
• Any change in mRNA shape will prevent
ribosome binding
• Decreased length of poly-A tail will prevent
translation
Ribosome stalls
due to hairpin loop
in mRNA
• mRNA is then
degraded by a cluster
of exonucleases
(exosomes)
Post-translational Regulation
• Proteins can be ubiquitinzed and
degraded in a proteasome
Non-protein-coding RNAs and Gene Regulation
• MicroRNAs or miRNAs are small noncoding RNAs that were
– Transcribed from DNA
– Complexed with a number of proteins
– These miRNAs have several bases that are
complementary to some protein-coding
mRNAs
– The miRNA-protein complex can bind to these
protein-coding mRNAs and prevent them from
being translated
– Nucleases eventually degrade the dsRNA
RNA Interference
• A relatively new technology uses the
concept of miRNAs to stop the expression
of certain genes
• This is done by creating small RNAs that
have a corresponding sequence
(antisense RNA) to mRNAs that will give
rise to unwanted proteins
• The small interfering RNAs (siRNAs) then
bind to these mRNAs and block their
translation
Sense and Antisense RNA
• Messenger RNA (mRNA) is single-stranded. Its sequence of
nucleotides is called "sense" because it results in a gene product
(protein). Normally, its unpaired nucleotides are "read" by transfer
RNA anticodons as the ribosome proceeds to translate the
message.
RNAi
• However, RNA can form duplexes just as DNA does. All that is
needed is a second strand of RNA whose sequence of bases is
complementary to the first strand; e.g.
• 5´ C A U G 3´ mRNA
3´ G U A C 5´ Antisense RNA The second strand is called the
antisense strand because its sequence of nucleotides is the
complement of message sense. When mRNA forms a duplex with a
complementary antisense RNA sequence, translation is blocked.
This may occur because the ribosome cannot gain access to the
nucleotides in the mRNA or
• duplex RNA is quickly degraded by ribonucleases in the cell
• synthetic genes (DNA) encoding antisense RNA molecules can be
introduced into the organism.
Flavr Savr tomato
• Most tomatoes that have to be shipped to market are harvested
before they are ripe. Otherwise, ethylene synthesized by the tomato
causes them to ripen and spoil before they reach the customer.
• Transgenic tomatoes have been constructed that carry in their
genome an artificial gene (DNA) that is transcribed into an antisense
RNA complementary to the mRNA for an enzyme involved in
ethylene production. These tomatoes make only 10% of the normal
amount of the enzyme.
• The goal of this work was to provide supermarket tomatoes with
something closer to the appearance and taste of tomatoes
harvested when ripe. However, these tomatoes often became
damaged during shipment and handling and have been taken off the
market.
Zygote  Embryo  Adult organism
• How does a single egg or zygote become
a complete organism with many different
tissues and differentiated cells?
• How can this happen, when the zygote
undergoes many rounds of mitosis –
mitosis is supposed to produce identical
daughter cells?
Cytoplasmic Determinants
• Certain molecules such as maternal mRNAs, transcription
factors and other proteins are localized in specific cytoplasmic
regions of the unfertilized egg or zygote
• These molecules affect cell fate decisions by segregating into
different embryonic cells and controlling distinct gene
activities in these cells (specialized transcription factors will
only turn on certain genes).
• Cytoplasmic determinants are also found in some postembryonic cells, where they produce cytoplasmic asymmetry.
• In dividing cells, this leads to asymmetric cell division in which
each of the daughter cells differentiates into a different cell
type. Also called localized cytoplasmic determinants or
morphogenetic determinants.
Cytoplasmic Determinants and Cell Fate
THE END