RNAiRNA interference

MECHANISM OF RNAi

The process by which RNAi occurs in different organisms varies slightly ; This is usually because different proteins are used in the process of RNAi .

The Mechanism of RNA Interference

In Interference• RNA

– siRNA– miRNA

• RISC • Enzymes

– Dicer : – Drosha

[ Types of small RNA's]

4

Differencesbetween siRNA and

microRNA miRNA: ssRNA 19-25nt. In nucleus MicroRNAs

predominantly suppress the translation of mRNA targets,

siRNA:• dsRNA 21-22 nt.• In sytoplasm• siRNAs prevent

mRNA translation and lead to mRNA degradation, also involved in the regulation of gene expression at the transcriptional level

RISC

• RISC:RNA-inducing silencing complex • The siRNAs are incorporated into the (RISC) which consists of an Argonaute (Ago) protein components. • Argonaute protein is considered catalytic engine • Ago cuts mRNA targets guided by siRNA via its endonuclease nicknamed“ slicer”.

DICER & Drosha

dsRNAs are cleaved into 21-23 nt segments siRNAs by an enzyme called "Dicer" Drosha : cleaves base hairpin in to form pre miRNA; which is later processed by Dicer

RNAi Mechanism 4 major steps:• dsRNA is cleaved into siRNA• The siRNA association with RISC,

unwinding & activation of RISC• Association with target mRNA• Degradation of mRNA

IN nucleus

STEP 1• dsRNA is

processed into sense and antisense RNAs • 21-25 nucleotides in

length • have 2-3 nt 3’

overhanging ends • Done by Dicer (an

RNase III-type enzyme)

Step 2

• The siRNAs associate

with RISC (RNA- induced silencing complex) and unwind

Step 3

• the antisense siRNAs act as guides for RISC to associate with complimentary single-stranded mRNAs.

Step 4

• RISC cuts the mRNA approximately in the middle of the region paired with the siRNA

• The mRNA is degraded further

RNAi Mechanism of action excerpt Science.mp4