Small interfering RNA

Petunias White pigmentPurple pigment Chalcone synthase

Producing a deep purple petunia White pigmentPurple pigment Chalcone synthase Insert gene encoding chalcone synthase More mRNA synthesised More enzyme produced and more pigment formed

Producing a deep purple petunia White plantDeep 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 synthesis siRNA forms a complex (RISC) with protein One of the siRNA strands is destroyed The siRNA–protein complex binds to mRNA

Stopping protein synthesis The mRNA is cut by the siRNA–protein complex The mRNA is then broken down. This prevents further protein synthesis

White plantDeep 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