1. MicroRNAs (miRNAs) form a large class of ncRNAs
function in repression of gene expression in Eukaryotes
By recognizing short stretches of nucleotides within the
, untranslated regions of mRNAs
miRNAs recruit partner proteins to individual transcripts, leading
to mRNA cleavage or hindering of translation

2. Mainly involved in PTGS
Undergo complex biogenesis, ending in loading of a s.s 19–23
nucleotide sequence in miRNP or RISC
In RISC, miRNAs serve as probes for recognizing
nucleotide sequences on target RNAs
Resulting in either arrest of translation or mRNA cleavage
and degradation
Gene silencing achieved in this manner is called the RNA
(interference (RNAi

3. Located in introns of protein-coding genes and in both introns
and exons of non-protein-coding genes
In the nucleus, primiRNAs cleaved by a type III RNase called
Drosha and its d.s RNA-binding protein co-factor named
DGCR8 also called Pasha
Alternatively, a subclass of pre-miRNAs is produced directly
from introns (termed mirtrons) of mRNA precursors by
splicing
Pre-miRNAs are actively transported to cytoplasm by a nuclear
membrane protein complex exportin-5/RanGTPase

4. In the cytoplasm, the loop of pre-miRNA is cleaved off by
another RNase III called Dicer
The resulting RNA duplex consists of a functional miRNA
( guide strand) retained by an Ago protein and a passenger
*(strand (denoted as miRNA
Ago-bound miRNA constitutes the basis of miRNA–protein
effector complex miRNP or RISC
Within RISC, miRNA serves as an adaptor for orienting and
fastening specific mRNAs

6. (miRNAs bind to specific miRNA recognition elements (MREs
Complementarity between MREs and miRNAs differ from plants to
animals
The extent of miRNA–MRE complementarity is directing the way a
gene is silenced
miRNAs with seed region base pairing to mRNA targets, cause gene
silencing through interfering with translation or causing mRNA
decay rather than endonucleolytic mRNA cleavage
Non-cleavage repression and seems to proceed by a number of
distinct mechanisms

7. A) Competition for the 5′-cap)
structure by Ago protein of the
RISC complex
B) Inhibition of mRNA)
circularization through
deadenylation

8. C) Inhibition of ribosomal)
subunit joining by Ago protein
D) Premature ribosome drop-off)
)dissociation of ribosomes)

9. E) Co-translational protein)
degradation
F) mRNA decay, accelerated by)
deadenylation and decapping

10. G) mRNA sequestration into processing bodies)

11. miRNAs control developmental timing
spatiotemporal expression of LIN-14 protein (encoded by lin-14 gene) in C.
elegans
miRNAs control tissue patterning/segmentation
lsy-6 and miR-273, identified in C. elegans
miRNAs control cellular proliferation and differentiation
miR-290 cluster in embryonic stem (ES) cells
miRNAs control diverse physiological processes
let-7b, miR-375, and miR-124 all regulate insulin secretion ,
liver-specific miR-122 has been linked to cholesterol metabolism

12. Endogenous miRNAs can be suppressed using decoy targets
(antagomirs or AMOs (anti-miRNA oligonucleotides
synthetic antisense oligonucleotides ,which efficiently
compete with endogenous mRNA targets for binding to
specific miRNAs
microRNA sponges
miRNA sponges are artificial mRNA targets with multiple
miRNA binding sites engineered into the 3′ UTRs

13. To enhance gene silencing elicited by a certain miRNA
enzymatically generated or chemically synthesized miRNAs
can be introduced into cells
for longer lasting effects, expression of recombinant
miRNAs from plasmids or viral vectors is a better option
Combining the miRNA-based targeting with tissue-specific
promoters is an effective strategy of preventing offtarget transgene expression