Lecture 20 - RNA interference and microRNAs Flashcards
What is RNAi
RNA interference
What is a dicer
Dicer generates the small double stranded siRNAs from the double-stranded viral RNA genome
The small siRNAs need to be in a very precise molecular configuration in order to accepted by the RISC complex
What is RISC
RNA induced Silencing Complex
What does RISC do
mediates potent enzymatic destruction of the viral mRNA copies
What is the major component of RISC
Argonuate (Ago/Ago2)
What do microRNAs do
Higher organisms such as vertebrates have evolved alternative more effective and comprehensive means of viral defence
However we surprisingly find that the siRNA pathway machinery is still ubiquitously produced in vertebrate/mammalian cells
We now know that the pathway was essentially hijacked by endogenously produced microRNAs (miRNAs) expressed from vertebrate genomes
Several thousand miRNA genes have now been identified in our genome
These genes encode larger miRNA-precursor RNAs which are then processed be cellular enzymes into the much shorter mature miRNAs
The mature ~22nt miRNAs are used by the cell to exert control over mRNA translation
What are piRNAs
Piwi-intercting RNAs
How are piRNAs expressed
almost exclusively in germ line cells
What do piRNAs do
In mammalian germ line cells, bursts of retrotransposon activity can be observed, and piRNAs are employed to control/supress this activity
Increases complexity
Which strand of siRNA is retained
The guide strand
Parts of the dicer
RNase IIIa
RNase IIIb
Ruler helix - rigid
PAZ
What is the displacement of the dicer enzyme domains
2 - leaving overhangs
What does the N-terminal do
Unzips and unwinds 3’ end of the target RNA
What does the PIWI domain do
Has enzymatic activity when complementary is achieved
What does a DROSHA complex do when it senses mature miRNA
Creates incisions to help release the pre-miRNA to form pre-miRNA
forming a duplex - which then searches the cytoplasm for microRNA
What happens when miRNA has complementary base pairings
A seed is formed, usually a few mismatches,
What does the Ago protein do if the sequence of dsRNA doesn’t 100% match
Inhibits translation for a short amount of time
What are the genes involved in a piRNA
piRNA Cluster - creates piRNA precursors which has sequence complementarity to LINE-1 RNA
Piwi then degrades piRNA precursor to form smaller piRNA
transcribed into the LINE-1 gene in a transposon
Binding during transcription occurs almost immediately
What does DNMT do
Methylates any piRNA in the area
What does the histone complex condensation protein do
in the title
