Small interfering RNA

1
Small interfering RNA
2
Petunias
Chalcone synthase
White pigment
Purple pigment
3
Producing a deep purple petunia
Chalcone synthase
White pigment
Purple pigment
More enzyme produced and more pigment formed
Insert gene encoding chalcone synthase
More mRNA synthesised
4
Producing a deep purple petunia
Genetically engineered plant
White plant
Deep purple plant
?
Instead of deep purple plants, many of the plants
produced were white
5
Making double-stranded RNA
mRNA is single stranded
A U C A G U A C C C A G
U A U C G
RNA-dependent RNA polymerase
Uses mRNA as a template to produce a
complementary RNA strand
U A G U C A U G G G U C
A U A G C
Two RNA strands held together by hydrogen bonds
Double-stranded RNA (dsRNA)
6
What happens to double-stranded RNA?
Double-stranded RNA is cut by Dicer enzyme

• Small interfering RNA (siRNA)
• Usually 21 base pairs long
• Two base overhang at each end

7
Stopping protein synthesis
We will start by simplifying the diagram of the
siRNA molecule
8
Stopping protein synthesis
We will start by simplifying the diagram of the
siRNA molecule
9
Stopping protein synthesis
siRNA forms a complex (RISC) with protein
One of the siRNA strands is destroyed
The siRNAprotein complex binds to mRNA
10
Stopping protein synthesis
The mRNA is cut by the siRNAprotein complex
The mRNA is then broken down. This prevents
further protein synthesis
11
So why were white plants produced instead of
deep purple plants?
Genetically engineered plant
White plant
Deep purple plant
?
Use the information about making double-stranded
RNA and small interfering RNA to explain why.
12
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