Gene Silencing
Antisense RNA
and
RNA Interference
How antisense RNA works
• During protein synthesis
a single strand of mRNA
is produced by
transcribing the DNA
code inside the nucleus
• This mRNA contains a
“sense” code; it is sent
to the ribosome to
facilitate making a
specific protein
How antisense RNA works
• If we want to stop that
protein from being
made we can disable
the sense strand of RNA
• RNA can become
double stranded
• What is needed is an
“antisense” strand of
RNA to bind with the
“sense” strand
How antisense RNA works
• To get the antisense
strand we can insert a
gene into the DNA that
codes for the antisense
chain of RNA
• When this gene is read an
antisense RNA chain is
produced
• The antisense chain binds
with the sense chain
disabling it
• No protein is produced –
the gene is silenced
Applications of this process
• Scientists used this process
to silence a gene in the
tomato that produced
ethylene and caused the
tomato to ripen
• Scientists used this process
to silence a gene in the
potato that produced either
amylose or amylopectin so
that these two starches did
not have to be separated
later
Modeling this process
• We will modify our protein
synthesis model activity to role
play how this process works
• Two additional roles will need to
be added:
– We will need another mRNA
molecule to read the antisense
gene
– We will need another mRNA
polymerase to prepare the
antisense chain and deliver it to
the sense chain to stop it from
entering the ribosome for
translation
• We will not need the amino acid
beads or the tRNAs because no
proteins will be made
How RNA interference works
• mRNA is found in the
cytoplasm as a single
stranded molecule
• If any double stranded
RNA is found in the
cytoplasm it is attacked
by an enzyme called
dicer
How RNA interference works
• Dicer cuts the double
stranded RNA into small
fragments
• The fragments then
separate into a sense
and antisense strand.
The fragment is called
siRNA
How RNA interference works
• The antisense strand
combines with another
protein (argonaute) - this
combo molecule is called
RISC
(RNA induced silencing
complex)
• With the aid of this second
protein the antisense chain
can combine with the
mRNA on its way to the
ribosome
• The mRNA is disabled and
the gene is silenced
How RNA interference works
Double stranded RNA +
Dicer = siRNA
siRNA + Argonaute =
RISC
RISC degrades mRNA
No Protein
Applications of this process
• Scientists are studying
how to use RNAi
technology to remove
caffeine from coffee
• Scientists used this
process to save the
papaya from a virus
• Scientists used this
process to enable
soybeans to make a
healthier oil
Animation
Let’s watch how this works in the following animations
• RNAi animation
(http://www.youtube.com/watch?v=cK-OGB1_ELE)
• RNAi animation second link
(http://blogs.nature.com/news/2011/12/videoanimation-rna-interference.html)
• There are other methods of gene silencing being
studied, like the micro RNA described in the animation.