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