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Small interfering RNA
Petunias
White pigment Purple pigment
Chalcone synthase
Producing a deep purple petunia
White pigment Purple pigment
Chalcone synthase
Insert gene encoding
chalcone synthase
More mRNA synthesised
More enzyme produced and more
pigment formed
Producing a deep purple petunia
White plant Deep purple plant
Instead of deep purple plants, many of the
plants produced were white
Genetically engineered
plant
Making double-stranded RNA
A U C A G U A C C C A G U A U C G
mRNA is single stranded
RNA-dependent RNA polymerase
U A G U C A U G G G U C A U A G C
Uses mRNA as a template to produce a complementary RNA strand
Two RNA strands held together by hydrogen bonds
Double-stranded RNA
(dsRNA)
What happens to double-stranded RNA?
Small interfering RNA (siRNA)• Usually 21 base pairs long• Two base overhang at each end
Double-stranded RNA is cut by Dicer enzyme
Stopping protein synthesis
We will start by simplifying the
diagram of the siRNA molecule
Stopping protein synthesis
We will start by simplifying the
diagram of the siRNA molecule
Stopping protein synthesissiRNA forms a complex
(RISC) with protein
One of the siRNA strands is destroyed
The siRNA–protein complex binds to mRNA
Stopping protein synthesisThe mRNA is cut by the siRNA–protein complex
The mRNA is then broken down. This
prevents further protein synthesis
White plant Deep purple plant
So why were white plants produced instead of deep purple plants?
Genetically engineered
plant
Use the information about making double-stranded
RNA and small interfering RNA to explain why.
The genetically engineered petunia plants had a higher concentration of mRNA
This resulted in RNA-dependent RNA polymerase producing double-stranded RNA from this mRNA
More siRNA molecules were formed that would bind to the mRNA coding for chalcone synthase
Less chalcone synthase was produced so flowers were white, not deep purple
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