MicroRNA (miRNA) is a non-coding RNA found in eukaryotes, involved in gene regulation and metabolic pathways. Discovered in 1993 during research on nematodes, miRNAs are processed through complexes in the nucleus and cytoplasm, playing key roles in gene silencing via the RNA-induced silencing complex (RISC). Applications of miRNA research include studies on cancer, viral infections, and metabolic processes.

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miRNA
BIOCHEMISTRY
PRESENTED BY-AANCHAL MANCHANDA
MSc.Zoology

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What is miRNA?
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• miRNA or micro-RNA is non coding RNA found only in eukaryotes.
• The microRNA often abbreviated as miRNA is made up of 20 to 25 nucleotide
short stretch of the single-stranded hairpin RNA, involved in the genetic
regulation.
• These are the conserve sequences present in all the eukaryotes since
evolution, although some miRNAs are also found in viruses too.

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Who discovered it and how?
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• In 1993, Lee et al., discovered the first miRNA in nematodes during the genetic
screening.
• While studying the postembryonic development in the model organism C. elegans, Lee
et al., discovered miRNA.
• At that time, it was believed that the miRNA was only present in the nematodes.
• Lee and co-workers observed that the lin-4 gene could transcribe into mRNA but could
not be translated into protein, instead, it transcribe a short RNA molecule of 22 to 60
nucleotides long undergo the regulation of lin-14 gene by the mechanism called
antisense RNA.

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The numbers of different miRNA in a different organism are
enlisted below,

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Types of miRNA:
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Four different types of miRNA are present in the genome of eukaryotes based on their location and
function:
• Intronic miRNA in coding transcription units
• Exonic miRNA in coding transcription units
• Intronic miRNA in non-coding transcription units
• Exonic miRNA in non-coding transcription units

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The function of miRNA:
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• The miRNA functions as translational repression by degrading the protein-coding mRNA thus
regulates the gene expression.
• The miRNA often called a smaller RNA are further involved in the regulation of numerous
metabolic pathways such as lipid metabolism, immune system, cell proliferation and apoptosis,
neuronal patterning and hematopoietic cells differentiation, also in plants, it is involved in leaf
and flower development.
• Using state of the art sequencing methods, computational tools and bioinformatics algorithms,
thousands of different miRNA and their function in gene regulation are now known to us.
• the miRNA is involved in a variety of biological regulatory function in animals and other
eukaryotes
.

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Some of the human miRNA and their function are enlisted
below,
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Processing of miRNA:
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• The processing of the miRNA is somehow similar as like the siRNA or the
shRNA, the process of the miRNA maturation and its action in translation
repression completes in the nucleus as well as in the cytoplasm.

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Processing in the nucleus:
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• The pre-miRNA transcript is first formed in the nucleus by the transcription.
• A large number of miRNAs are transcribed using the RNA polymerase II, though some are transcribed by the
RNA polymerase III too.
• The RNA pol II or RNA pol III mediated transcribed product is known as the pri-miRNA or primary-miRNA
which is several kb in length having a hairpin-like stem-loop like structure.
• The Drosha in conjugation with the DGCR8 or Pash forms a large microprocessor complex which cleaves
the Pol II transcribed pri-miRNA hairpin at the junction of loop and stem.
• The precursor miRNA hairpin is formed by this large microprocessor complex.
• Two different RNase III enzymes involved in the process of maturation of miRNA, first, the Drosha binds to
pri-miRNA(primary miRNA) and creates the precursor miRNA.
• The Drosha cleaves the dsRNA hairpin duplex and generates the 5’ phosphate and 2 nucleotide overhang at
the 3’ OH end (as like the siRNA).

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• These precursor-miRNA transfer to the cytoplasm by the protein called the transportin 5 and in the
cytoplasm, it is converted into the mature miRNA or miRNA by the another RNase III called the Dicer.
• Contrary to this, in plants, the entire process of the miRNA maturation occurs only in the nucleus and
instead of Dicer, the protein is known as DCL1 (Dicer-like 1 protein) govern the entire process.

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Processing in the cytoplasm:
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• Now the pre-miRNA is transported through the nuclear pores to the cytoplasm using the exportin5, the
EXP5 or exportin 5 is a RanGTP-dependent nuclear transport receptor protein.
• The cytoplasmic RNase III dicer processed the pre-miRNA and generates the mature miRNA.
• A highly conserved protein Dicer is a specific type of nuclease that cleaves the pre-miRNA exactly at 22
nucleotides and generates a mature miRNA duplex.
• After that, the mature miRNA is loaded on the Argonaute protein that forms a complex in combination with
some other proteins which helps in the gene silencing called the RISC, RNA induced silencing complex.
• Now, one of the strands of the dsRNA called the passenger strand is degraded by the RISC and the guided
strand will be loaded on the Ago protein of the RISC.

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RISC:
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• The RISC is a ribonucleoprotein complex having the miRNA guided strand, Dicer, argonaute protein and
other proteins create the primary complex often known as the miRISC. Besides these major proteins several
other proteins in the RISC are:
• TRBP, PACT, TNRC6B (RNA recognition motif-containing protein), FMRP (fragile X mental retardation
protein), tudor staphylococcal nuclease domain-containing protein and putative DNA helicase MOV10.
• The argonaute protein is one of the main functional parts of the RNA induced silencing complex having three
different domains:
• PAZ
• PIWI
• MID
• The PAZ domain binds to the 3’ end of the miRNA guided strand while the PIWI domain interacts with the 5’
end of the miRNA which structurally alike to the ribonuclease-H, although functionally not.

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• The human genome has a gene for eight different argonaute proteins.
• The structure of the argonaute protein and RISC is given into the figure below,

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Applications of miRNA:
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• Gain and loss of function assays and studies.
• Identification of specific miRNA and target-gene relationship studies.
• Gene expression and gene silencing assay.
• Cancer and viral infection studies.
• Other therapeutic techniques such as diagnosis, prognosis and
monitoring of cancer development.
• Also used in the study of cell metabolism, cell proliferation and
apoptosis etc.

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RESOURCE:
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• https://geneticeducation.co.in/microrna-mirna-and-gene-regulation/