Regulation of Gene Expressionin Prokaryotes I Flashcards
so far we know…
- DNA is arranged into genes.
- Genes provide for the storage of information.
- This information is expressed through the processes of:
Transcription
Translation
How is the expression of genes regulated such that we obtain a coordinated expression of the genetic material at the right time (and place) to obtain the desired effect?
- Very NB question
- Remember, not all genes are expressed at all times
Efficient expression of genetic information
is dependant on control mechanisms that promote or suppress gene activities.
How is the promotion/ suppression of gene expression achieved
In transcription:
The expression of genes relies on the presence of a cis-element termed a promoter and which is usually found upstream of the start codon of the gene.
a promoter
Determines when and what quantities a gene will be transcribed
Many of the early studies of gene expression where performed on bacteria and yeast
Why?
- They are easy to culture and have short generation times
- they can be easily mutated and pure mutated cultures can be
obtained for separate studies
Bacteria are ideal models for studies involving
- the induction of gene expression in response to changes in
environmental conditions - Bacteria regulate their gene expression in response to
environmental changes as well as a variety of non-environmentally
regulated cellular activities (such as cell division)
Adaptive hypothesis
Refers to the ability of organisms to adapt to their environment
E.G:
lactose in growth medium induces expression of enzymes specific for lactose metabolism
Various forms of gene expression
Constitutive, inducible, repressible systems
Which are under negative or positive control
Constitutive genes
- permanently expressed, regardless of the environmental conditions
- Call these housekeeping genes - expressed all the time- keep cell
alive
Inducible genes
- expressed in response to a particular condition
E.g: An inducer, such as lactose.
repressible systems
- genes may be repressed due to the presence of a particular
molecule - These molecules are often end products of a specific biosynthetic
pathway
positive control
- needs a substrate to directly stimulate transcription
Negative control
- occurs when a molecule turns off for transcription
Glucose
is the primary molecule used as a source of energy in all cellular metabolism
E. coli has the ability to grow on ______ as an alternative source of ______
lactose
carbon
Lactose is a disaccharide of
Galactose and glucose
Which enzyme digests lactose to produce galactose and glucose?
Beta-galactosidase
In the absence of lactose
- There are only a few copies of the Beta-galactosidase enzyme
present in the cell
In the presence of lactose
- Gene expression is induced and the number of enzymes available
rapidly increases to several thousand enzymes per cell - The enzymes = inducable
- lactose = inducer molecule
Lactose - the inducer
- technically, the inducer is allolactose - an isomer of lactose
- When lactose first enters the cell, a very small portion of it is
converted to allolactose by the Beta-galactosidase enzyme
operon
Prokaryotic genes with related functions are organised in groups and are expressed in a coordinated fashion
Watch slide 11 of prokGenRegnarrated1
The lac operon
- lac operon consists of 3 structural genes and
- 2 regulatory regions upstream (cis & trans action) - it is found 5’ to
the operon - The discovery of a regulatory gene and regulatory site which are
not part of the gene cluster - NB in understanding how the operon
was controlled - neither of these regions encode enzymes needed for lactose
metabolism = function of the 3 structural genes
Structural genes (related functions):
lacZ – b-galactosidase
lacY – b-galactoside permease
lacA – transacetylase
However, together, this entire gene cluster operates to provide a rapid response to the lactose status of the environment