Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Regulation
of
Gene Expression

Regulation of Gene Expression

Overview of Gene Expression
 The

control of gene expression is vital to the
proper and efficient functioning of an
organism
...


Gene expression occurs throughout an organism’s life, starting at the very
beginning
...

 At first, all the cells are exactly alike, but that characteristic quickly
changes
...

 After the tissue type is decided, certain genes in each cell become active,
and others get permanently turned off
...



 In

part, the tissue-specific nature of gene expression
is due to location —genes in cells respond to cues
from the cells around them
...


Prokaryotic Gene Regulation

Control of Gene Expression in Bacteria
 Bacteria

often respond to environmental change by
regulating transcription
...

 An operator site on the DNA switches the operon on
or off, resulting in coordinate regulation of the genes
...

 An operon consists of:




operator
promoter
and genes that they control

 All

together, the operator, the promoter, and
the genes they control – the entire stretch of
DNA required for enzyme production for the
pathway – is called an operon
...

Both the repressible and inducible operon are types of
NEGATIVE gene regulation because both are turned OFF
by the active form of the repressor protein
...





Trp operon – repressible operon is always in the on position until
it is not needed and becomes repressed or switched off
...


Figure 18
...
whfreeman
...
html

Figure 18
...
mcgraw-hill
...
swf

Figure 18
...
sumanasinc
...
html

Figure 18
...
mcgraw-hill
...
html

Prokaryotic Gene Regulation

Review: Structure/Function of Prokaryotic Chromosomes
1
...

3
...

5
...

7
...


shape (circular/nonlinear/loop)
less complex than eukaryotes (no histones/less elaborate
structure/folding)
size (smaller size/less genetic information/fewer genes)
replication method (single origin of replication/rolling circle
replication)
transcription/translation may be coupled
generally few or no introns (noncoding segments)
majority of genome expressed
operons are used for gene regulation and control

Chromosome Structure

The Structure of the Chromosome
 In



 In




Prokaryotes:
The bacterial chromosome is a double-stranded, circular
DNA molecule associated with a small amount of protein
In a bacterium, the DNA is “supercoiled” and found in a
region of the cell called the nucleoid

Eukaryotes:
Eukaryotic chromosomes have linear DNA molecules
associated with a large amount of protein
Chromatin is a complex of DNA and protein, and is found
in the nucleus of eukaryotic cells
Histones are proteins that are responsible for the first
level of DNA packing in chromatin

Eukaryotic Chromosomes

Chromosome Structure of Eukaryotes

Eukaryotic chromosomes contain DNA wrapped around proteins called
histones
...


Eukaryotic Chromosomes

Eukaryotic Gene Regulation

Control of Gene Expression in Eukaryotes
 Eukaryotic

gene expression can be regulated at

any stage
...

 Opportunities for the control of gene expression
in eukaryotes include:
1
...

3
...

5
...
6


THIS FIGURE IS
HIGHLIGHTING KEY
STAGES IN THE
EXPRESSION OF A
PROTEIN-CODING GENE
...




MAIN LESSON: each stage
is a potential control point
where gene expression can
be turned on or off, sped up,
or slowed down
...


both prokaryotes and eukaryotes, DNA associates
with proteins to form chromatin, but in the eukaryotic
cell, the chromatin is ordered into higher structural
levels
...


Eukaryotic chromatin is composed
mostly of DNA and histone proteins
that bind to the DNA to form
nucleosomes, the most basic units of
DNA packing
...


In interphase cells, most chromatin is in
a highly extended form, called
euchromatin
...

 In eukaryotic genomes, most of the DNA (97% in
humans) does NOT encode protein or RNA
...


Eukaryotic Gene Regulation

1
...

Genes of heterochromatin (highly condensed) are
usually not expressed because transcription proteins
cannot reach the DNA
...

 Histone acetylation seems to loosen nucleosome
structure and thereby enhance transcription
...










Methylation renders DNA inactive
...

Comparison of the same genes in different types of tissues
shows that the genes are usually more heavily methylated in
cells where they are not expressed
...


At least in some species, DNA methylation seems to be
essential for the long-term inactivation of genes that
occurs during cellular differentiation in the embryo
...





When the histones of nucleosome are acetylated,
they change shape so that they grip the DNA less
tightly
...


Eukaryotic Gene Regulation

Transcription Initiation
 Transcription

is controlled by the presence or
absence of particular transcription factors, which
bind to the DNA and affect the rate of transcription
...


 Once

a gene is “unpacked”, the initiation of
transcription is the most important and universally
used control point in gene expression
...
Transcriptional Control of
Gene Expression
 Most

gene control in eukaryotes, occurs
during transcription
...

When the gene is “off,” transcription is
suspended
...


 Transcription

produces the mRNAs used in translation;
 therefore, when transcription is happening, translation is
in motion, and gene expression is on
...
The timing of transcription can be controlled
by a number of factors, which include:
A
...
Regulation from other genes
C
...
Tightly wound: The effect
of DNA packaging
 DNA must

unwind a bit from its tight coils in order to
be available for transcription to occur
...

 You just can’t have every gene in every cell flipped
on and running amuck all the time; you want specific
genes acting only in the tissues where their actions
are needed
...


 Genes

are kept in the off position in two ways:
Tight packaging:
-DNA packaging is a highly effective mechanism to make sure
that most genes are off most of the time because it prevents
transcription from occurring by preventing transcription
factors from getting access to the genes
...


First, the
DNA is wrapped around special proteins called histones
...

When DNA’s wrapped up this way, it can’t be transcribed
because transcription factors can’t bind to the DNA to find
the template strand and copy it

Repressors:
=Some proteins act to block transcription and prevent it from occurring
...

=To turn genes on, the DNA must be removed from its packaging
...


B
...

 There are four types of genes that
micromanage the activities of others
 Divided into two groups based on how they
relate to one another
...
Micromanaging
transcription
Three types of genes act as regulatory agents
to turn transcription up:
(enhancers),
turn it down (silencers), or drown out the
effects of enhancing
silencing elements (insulators)
...

Unlike promoter sequences, which are always
located just a few bases “upstream” from the
genes they control ,
 Enhancers can be upstream, downstream, or
even smack in the middle of the transcription
unit
...


 Nonetheless,

enhancers are very tissuespecific in their activities — they only influence
genes that are normally activated in that
particular cell type
...

 These are gene sequences that hook up with
repressor proteins to slow or stop transcription
...

 Silencers work to keep the DNA tightly
packaged and unavailable for transcription
...

 Insulators

work to protect some genes
from the effects of silencers and enhancers, confining the activity of
those sequences to the right sets of genes
...


Jumping genes
Some genes like to travel
...





These wanderers are called transposable
elements (TEs), and they’re quite common —
50 percent of your DNA is made up of
transposable elements, also known as jumping
genes
...


 How

they pull off this trick isn’t completely clear
because TEs have several options when it comes to
travel
...


Signals sent to genes from other
cells
by way
of hormones
 Hormones
are complex
chemicals that control gene
expression
...

 Hormones circulate in the bloodstream and can affect
tissues far away from the hormones’ production sites
...

 Essentially, hormones act like a master switch for gene
regulation all over the body
...

 HREs sit close to the genes they regulate and
bind with the hormone-receptor complex
...
8

Eukaryotic Gene and its Transcript

Assembling of Transcription Factors
1) Activator proteins bind to
enhancer sequences in the
DNA and help position the
initiation complex on the
promoter
...
Other
transcription factors and
RNA polymerase are nearby
...


http://highered
...
com/
olc/dl/120080/bio28
...


Eukaryotic Gene Regulation

Post-Transcriptional Factors



Transcription alone DOES NOT constitute gene
expression
Post-transcriptional mechanisms play supporting roles in
the control of gene expression:






Alternative RNA splicing – where different mRNA molecules
are produced from the same primary transcript, depending on
which RNA segments are treated as exons and which as
introns
...

http://highered
...
com/olc/dl/120080/bio31
...


With alternative splicing, an
organism can get more than
one type of polypeptide from a
single gene
...





The life span of mRNA molecules in the cytoplasm is an
important factor in determining the pattern of protein
synthesis in a cell
...


Eukaryotic Gene Regulation

Protein Processing and Degradation
 The

final opportunities for controlling gene expression
occur after translation:





Protein processing – cleavage and the addition of
chemical groups required for function
...

Cells can also limit the lifetimes of normal proteins by
selective degradation – chopped up by proteasomes
...
6


THIS FIGURE IS
HIGHLIGHTING KEY
STAGES IN THE
EXPRESSION OF A
PROTEIN-CODING GENE
...




MAIN LESSON: each stage
is a potential control point
where gene expression can
be turned on or off, sped up,
or slowed down

---