Transcript Gene Expression and Protein Synthesi

A2
OCR F215-Control,
Genomes and Environment
This lesson you should be able to...
Grade B/A- Explain genetic control of
protein production in a prokaryote using
the lac operon.
Key terms you need to know.......
•Operon – length of DNA made from
structural genes and control sites
•Structural genes – code for protein
•Control sites – operator and promoter
region of the DNA
Starter
Exam Question Jan 2011
idea of change to , DNA / base(s) / nucleotide(s)
natural / directional , selection ;
regulatory
idea that makes , repressor protein / transcription factor
idea that product switches (structural / another) gene , on / off
structural -idea that makes , enzyme / polypeptide / protein ;
relationship between the 2
idea that regulatory gene , controls / affects , the expression of structural
gene ;
lactose has been , removed / digested / respired /
broken down (by bacteria) ;
to , lactic acid / lactate / other sugars ;
There are ~20,000 genes in each of our cells.
But a particular cell will not make use of each of
them.
For example – white blood cell and skin cell•A white blood cell will contain the gene coding
for the protein melanin, but will not be
expressed.
•A skin cell will contain the gene to make
antibodies, but it will not be expressed “not
switched on.”
• We know that.......
• mRNA – codes for a particular protein
• The code on the mRNA is complementary to
the base sequence on the DNA template
strand
• Therefore that code on the mRNA is a copy of
the base sequence on the DNA coding strand
• Proteins are specified by mRNA
Background information
• Bacteria can synthesise different
enzymes (proteins) depending on what
food substrate they are growing on.
• E.Coli can synthesise over 3000 different
enzymes.
• Enzymes involved in basic cell functions
are synthesised at a constant rate.
• Inducible enzymes are synthesised as
and when they are needed.
Background information
• E.Coli can adapt to its environment by
producing enzymes to metabolise
certain nutrients..........
• but only when those particular nutrients
are present
• E.Coli normally respires using glucose
but can also respire using lactose (sugar
found in milk/disaccharide)
• Needs to synthesise ß-galactosidase &
lactose permease
Background information
• ß-galactosidase – hydrolyses lactose
into glucose & galactose.
• lactose permease – transport protein
that becomes embedded in the e.coli
membrane – helps transport more
lactose into the cell.
• When placed in a lactose substrate,
e.coli increases the synthesis of these
two proteins by 1000x.
• Lactose triggers the enzymes production
– inducer molecule.
The lac operon
Lac system genes (in the bacterial DNA) form
the operon – consists of structural genes and
control genes.
Z - ß-galactosidase
Y - lactose permease
O – operator region – switches Z & Y on and off
P – promoter region-Binding site for RNA
polymerase for transcription of Z & Y
I- Regulator:-Codes for the repressor protein
If lactose is absent
Regulator gene is expressed and produces repressor
protein
Repressor protein binds to the operator region (P &
O)
• Partially covers the promoter region (P)
• RNA polymerase can’t bind – Z & Y genes can’t be
translated
• Z & Y are switched off
•
If lactose is present
Inducer molecule (lactose) binds to the repressor
protein
• Repressor protein dissociates from the operator
region
• Promoter is now unblocked
• RNA polymerase can now bind promoter region
• Z &Y can now be transcribed – mRNA produced
• ß-galactosidase /lactose permease can now be
synthesised
The lac operon
As a result of the 2 enzymes being made, e.coli
can now.....
Take up lactose from its environment because
lactose permease acts a transport protein.
Using ß-galactosidase, lactose (disaccharide) can
be hydrolysed into glucose & galactose
• E.coli can use these sugars for respiration
• E.coli is gaining energy from the lactose
Background information
ß-galactosidase
(lactase) –
hydrolyses
lactose to
glucose and
galactose
http://www.youtube.com/watch?gl=GB&v
=oBwtxdI1zvk
Protein synthesis in prokaryotes
The Lac Operon – Summary
Genes responsible for lactose digestion and their regulation.
Physical presence of lactose results in transcription of genes
to produce lactose permease and Beta-galactosidase.
A regulatory gene produces a repressor protein. When no
lactose is present, it joins with the operator region.
RNA polymerase joins to the promoter region. If the repressor
protein is attached to the operator region, the RNA
polymerase cannot move further along and transcript
structural genes to produce enzymes for lactose
digestion.
If lactose is present, it binds with the repressor protein and
causes a conformational change, so the repressor protein
can’t bind with the operator region.
RNA polymerase can travel along and the enzymes for lactose
digestion can be produced.
lactose binds to repressor protein ;
changes , shape / structure (of protein) ;
removes it from / stops it binding to , operator ;
RNA polymerase binds to promoter ;
idea that (so that Z and Y) are , transcribed / mRNA
made
Exam Question Jan 2011
Mrs Funk
A2
OCR F215-Control,
Genomes and Environment
This lesson you should be able to...
Grade B/A- Explain that the genes that
control development of body plans are
similar in plants, animals and fungi, with
reference to homeobox sequences.
Embryo development
Human
Frog
Chicken
Controlling development
All organisms begin life as a
single cell. This cell divides
and the new cells produced
start to differentiate and
specialize.
‘Switching on’ the expression
of a gene or keeping it
switched off determines the
development of features.
Many organisms contain similar genes that control
development of body plans. For example groups of
genes called the homeobox genes play an important role
in the development of many multicellular organisms.
Homeobox genes
The genome of the fruit fly contains one ‘set’ or cluster
of homeobox genes. These control development,
including the polarity of the embryo, polarity of each
segment and the identity of each segment.
Homeobox genes code for
transcriptional factors.
These regulate the
expression of other genes
important in development.
Mutations in homeobox genes can cause changes in
the body plan. For example a mutation in the gene
controlling leg placement can cause legs to grow
where the antennae are normally found.
Drosophilia Fruit development pg
114
Drosophilia Fruit fly body plan pg
114
Maternal effect genes – determine embryo
polarity (which end head, which end tail)
Segmentation genes – specify polarity of
each segment (which part of head is brain
which part is mouth piece)
Mutation Example 1:
•Has a homeobox gene called Ubx, which
prevents formation of wings in T3 ( a part
of the body).
•Mutation in both forms of Ubx = wings
grow in T3
•Fruit fly has two sets of wings – cannot
fly
Mutation Example 1
•Antennapedia – where the antennae look
more like legs
http://www.youtube.com/watch?v=LFG-aLidT8s
Homeobox genes
Homeobox genes are present in
the genomes of most
organisms. They control
development of body parts in
similar ways.
There is little variation in many
regions of the homeobox genes
in different organisms. This
suggests that these have been
highly conserved throughout
evolutionary history. They are
thought to be especially
important to the basic
development of organisms.
Genetic Control
The homeobox regulatory gene is found in the genome
of segmented animals from annelids to humans. The
genes contain a 180 bp homeobox sequence that codes
for polypeptides including transcription factors, which
bind to genes upstream initiating transcription switching
genes on and off.
This regulates the expression of other genes.
Homeobox genes work in similar ways in vertebrates
plants and fungi.
Homeobox genes summary
• Homeobox genes are genes whose activity switches a
whole set of other genes on or off, affecting an
organism’s body plan (overall design of an organism’s
body).
• They are found in clusters called hox clusters
• Most animals have very similar homeobox genes.
• Genes are highly conserved (have not evolved much)
• Code for production of transcription factors. These
can bind to certain sections of DNA and cause it to be
transcribed.
• Thalidomide disrupted the homeobox genes in
developing foetuses, so arms and legs did not develop
properly.
Retinoic Acid
Many of the effects of retinoic acid on
development occur through the ability of retinoic
acid to regulate gene expression in various
specific cell types. Retinoic acid binds to
proteins in the steroid receptor family, and it is
this RA receptor complex which mediates gene
expression.
Retinoic Acid
The activation of Hox genes also explains the
teratogenenicity of Retinoic Acid (Isoretinoin,
Accutane), a vitamin A derivative used for the
treatment of cystic acne.
Despite label warnings, Accutane has been taken by at
least 160,000 women of childbearing age. The most
common abnormality found is cleft palate and other
head and neck malformations. Formation of the palate
roof through fusion of the palate is a complex process,
and the cells in this region of the anterior head do not
express genes of the Hox complexes.
Plenary
Exam Question Jan 2012
homeotic / regulatory, (gene) ; contains, 180 bp / homeobox,
sequence ; that codes for homeodomain (on protein) ; (gene
product) binds to DNA ; initiates transcription / switch genes, on /
off ; control of, development / body plan ;
these genes very important ;
mutation would, have big effects / alter body plan ;
many other genes would be affected / knock-on effects ;
mutation likely to be, lethal / selected against ;
fungi / plants
protein synthesis / transcription and translation ;
respiration ;
DNA replication ;
mitosis ;
cytokinesis ;
apoptosis ;
differentiation / gene switching ;