Introduction:

Ribonucleic acid interference (RNAi) is a process of a cell which helps in controlling the expression of genes. RNAi helps in degradation of mRNA (messenger Ribonucleic acid) with the help of sequence specific double-stranded RNA. RNAi molecules are present in all eukaryotes, for example from yeast cell to mammals. RNAi is very specific in silencing the expression of any gene for which sequence is known therefore RNAi is used as tool for gene silencing in eukaryotic system.
The RNAi pathway thought to be used by cells to protect its own genome from parasites such as viruses and transposons. Researchers have found out that RNAi not only play an important role in mRNA and dsRNA expression, but also help in maintaining chromatin structure and genome integrity and also regulates translation, tran script ion. Small RNA known as microRNA are also play a very important role in cell growth and differentiation.

RNAi Pathway:

1. Enzyme dicer processes long dsRNA to ~20bp short interfering RNA (siRNA) with 3' overhangs.
2. One strand of the siRNA assembles into the complex known as RNA-induced silencing complex (RISC)
3. RISC complex cleaves the target mRNA.
4. Argonaute enzyme plays catalytic role in degradation of target mRNA.
5. Its thought that siRNA guide strand via base pairing provides the target specificity to RISC complex to degrade target mRNA.
6. Small RNAs such as short interfering RNA, repeat-associated siRNA and microRNA all play important role in providing target specificity to RISC complex.

siRNA and rasiRNA:

siRNA are 21-25bp long double-stranded RNA with dinucleotide 3'overhangs. siRNA are formed from long dsRNA molecules in the cell. Only one strand of siRNA known as guide strand is used to form RNA-induced silencing complex. siRNA guide strand thought to provide target specificity to RISC-mediated mRNA cleavage via base pairing.
siRNA produced in the cell using endogenous precursors play important role in maintenance of genome integrity and chromatin structure and also in post-tran script ional gene silencing.
The endogenous siRNA produced from the repetitive elements such as heterochromatic regions at centromeres and telomeres are known as repeat-associated siRNA (rasiRNA). rasiRNA via RNA-induced tran script ional silencing complex help in the maintainance of heterochromatic region of the chromosome.

Micro RNA (miRNA):

miRNA are 19-23 nucleotide long single-stranded RNAs, produced endogenously from single-stranded imperfectly base-paired hairpin precursor tran script s. miRNA play a critical role in developmental timing, differentiation of hematopoietic cell, cell proliferation, cell death and also in oncogenesis.

Applications:

1. RNA interference is used to study functions of particular gene in cell culture and also in model organisms.
2. RNA interference technique is also used to knock down a particular gene. That is to entirely eliminate the expression of a gene.
3. Short interfering RNA can be used to treat macular degeneration and respiratory syncytial virus.
4. Short interfering RNA can be used to treat herpes simplex virus type II infection.
5. Short interfering RNA can be used to inhibit expression of viral gene in cancerous cell.
6. Short interfering RNA can be used to treat neurodegenerative diseases.
7. Short interfering RNA can be used to knockdown host receptors and co receptors for human immunodeficiency virus.

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