Micro rn as gene silencing prez

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

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)

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

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

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

(A )Competition for the 5′-cap structure by Ago protein of the RISC complex

(B )Inhibition of mRNA circularization through deadenylation

(C )Inhibition of ribosomal subunit joining by Ago protein

(D )Premature ribosome drop-off(dissociation of ribosomes)

(E )Co-translational protein degradation

(F )mRNA decay, accelerated by deadenylation and decapping

(G )mRNA sequestration into processing bodies

miRNAs control developmental timing spatiotemporal expression of LIN-14 protein (encoded by lin-14 gene) in C. elegans

miRNAs control tissue patterning/segmentationlsy-6 and miR-273, identified in C. elegans

miRNAs control cellular proliferation and differentiationmiR-290 cluster in embryonic stem (ES) cells

miRNAs control diverse physiological processeslet-7b, miR-375, and miR-124 all regulate insulin secretion , liver-specific miR-122 has been linked to cholesterol metabolism

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 spongesmiRNA sponges are artificial mRNA targets with

multiple miRNA binding sites engineered into the 3′ UTRs

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 off-target transgene expression