Page 59 - Cell biology PDC 2024

It is short double-stranded RNA molecules, a few hundred nucleotides in
length that targets mRNA
Mechanism of action:
First, the double-stranded RNAi are cleaved by an endonuclease enzyme
called Dicer, which cleaves RNAi at the 3’ ends to give siRNA (short
interference RNA). This siRNA (20nt long) will form a complex with
several proteins (RISC, RNA-induced silencing complex).
The RISC will split the double-stranded fragment into single strands, one
of which is exactly complement to the miRNA. The mRNA/siRNA
hybrid is then destroyed by an endonuclease.

Figure 26: siRNA

(3) The ubiquitin-proteasome system :
It destroys the gene product (protein)
It occurs in 3 steps:
A- Ubiquitin activating enzyme (E1) activates the ubiquitin by combining
it with ATP to give ubiquitin-adenylate and then transfers the ubiquitin to
itself via a cysteine thioester bond.
B- The ubiquitin is then transferred to a cysteine in the E2 enzyme
(ubiquitin-conjugating enzyme) by means of esterification reaction

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