Download Slide 8- Phase variation is one of the mechanisms by which bacteria

Simulating phase variation in Neisseria
VGEC: Teachers notes
The aim of this lesson and activity is to learn the nature
of mutation and natural selection in bacteria, whilst also
forming a link between mutation and the ability to cause
disease.
Slide 2- DNA is the genetic material encoding all of the
proteins needs to form a complete cell. DNA is
comprised of 4 nucleotide bases (adenine, thymine,
cytosine and guanine) which are arranged into sets of
three. These ‘codons’ are assembled into genes. Genes
are ‘transcribed’ into the intermediate molecule, RNA.
RNA is then read by the bacterial ribosome, with each
three bases coding for an amino acid. These amino
acids are built up by the ribosome into protein
molecules.
Slide 3+4- Some proteins (among other molecules) on
the surface of bacteria can be recognised as ‘foreign’ by
the immune system. We refer to these foreign molecules
as antigens. B lymphocytes (B cells) are specialised in
producing antibodies against these antigens. Antibodies
bind to the target antigen, labelling it to be destroyed by
specialised immune cells (phagocytes, T-killer cells etc).
Simulating phase variation in Neisseria
Slide 5- Meningitis is a very dangerous disease caused
by infection of the membranes around the brain and
spinal cord (meninges). Meningitis can be caused by
many microbes, but the most common (particularly in
babies) is the bacteria Neisseria meningitidis.
Slide 6- The early symptoms of meningitis include fever,
headache and neck stiffness, but if left untreated can
progress to death.
Slide 7- N. meningitidis is found in the throats of a large
proportion of the healthy population. Despite this, only a
comparative few people are diagnosed with
meningococcal disease. This implies that the bacteria
must have a way to remain unseen by the immune
system.
Slide 8- Phase variation is one of the mechanisms by
which bacteria achieve this. Phase variation is the
switching on/off of gene expression. If a phase variable
gene is switched off, the gene still remains, but the
protein molecule it encodes is not expressed. Phase
variation is caused by a reversible mutation in the gene.
The result is that genes can rapidly be switched on and
off interchangeably.
Slide 9- In a population then, theoretically you will begin
with a single bacterial cell. This cell will grow and divide
by binary fission as with most cells. Every time this cell
divides however, there is a small chance it will get a
mutation that will cause a gene to be switched on or off
Simulating phase variation in Neisseria
by phase variation. This occurs at a given rate, known
as the mutation rate which is affected by many different
factors. These mutations (and gene switching) will be
passed down to the daughter cells.
Slide 10- This process is reversible and as long as cells
divide, genes will continue to switch on and off.
Slide 11- N. meningitidis have many different phase
variable genes. These genes mutate independent of
each other, meaning you can get different combinations
of genes switched on or off. With two genes for
example, there are 4 combinations, with 3 there are 9
and so on.
Slide 12- The result is a population that essentially all
have very similar DNA, but have different combinations
of proteins expressed on their surface.
Slide 13- We refer to the mechanism by which these
mutations lead to phase variation as ‘slippery DNA’
(hypermutatable DNA). Repeats of a single nucleotide
(G in this case) are far more prone to mutation than
other pieces of DNA, meaning they are more likely to
have bases inserted or deleted. (See ‘genetics, mutation
and cancer’ page of the VGEC for more information). An
insertion of a base in the poly G tract then will cause a
frame shift mutation. This will change the coding
sequence of DNA, and can even introduce a stop
codon, halting protein production all together. This is
known as the off phase. Conversely, another deletion of
Simulating phase variation in Neisseria
a base from the poly-G tract will reverse the frame shift
and switch expression of the gene back on again.
Slide 14 + 15- This can be beneficial for bacteria. As we
recapped on at the start, surface proteins are often
recognised by the immune system, leading to the
bacteria being killed. If however, bacteria have a way to
switch the expression of these proteins on and off, then
there is always a chance that at least a few cells in the
population will survive.
Slide 16- Some examples of phase variable genes in N.
meningitidis are those involved in sticking to the host
surface, and taking up nutrients for example.
The simulator activity will allow students to see phase
variation occurring in real time, and hopefully draw a
link between mutation and disease.
Pupils should be given a copy of the information sheet,
question sheet and the simulator.