神马文学网RNAi - What it means and how it works — Windo...
来源：百度文库 编辑：神马文学网 时间：2024/04/29 16:05:53
L. SunRNAi (RNA Interference, also called "RNA-mediated interference") is a cellular mechanism for the destruction of target RNA molecules. The Nobel Prize in Physiology or Medicine 2006 was awarded on October 2nd to two American scientists, Andrew Z. Fire and Craig C. Mello "for their discovery of RNA interference - gene silencing by double-stranded RNA". This technology is relatively young by Nobel standards; but with less than 10 years' acknowledged research it has drawn as much attention as never before due to its significance in medicine. DNA (Deoxyribonucleic acid) exists in the nucleus of every cell of our bodies. Even though it carries genetic information that gets transmitted throughout the cell, the DNA molecules never leaves the nucleus. The nuclear membrane surrounding the nucleus has openings known nuclear pore complex. Genetic information will be encoded by mRNA (messenger Ribonucleic acid) during the transcription process and exported out of the nucleus through the nuclear pores. It will guide the protein synthesis and generate final gene products like protein. Different kinds of proteins build up distinctive cells, clusters of functional cells constitute specific tissues and then organs.
Everything in our bodies is made from proteins assembled according to the genetic sequence of our own DNA. Sometimes a virus invades a cell and deposits its own double-stranded RNA carrying the viral genetic information. A virus always tries to take over its host cell and force it to produce viral proteins and thus more copies of itself. If it succeeds, hundreds of thousands of viral molecules will occupy the tissues of their host and the host will get ill. When the virus is copying itself, its gene-information carrier, RNA, has a long, double-stranded shape.
In order to fight against the viral RNA replication, cells have developed a mechanism called RNA interference. In that sense RNAi is anti-virus machinery of the cells.
The whole process is initiated by Dicer[1], an enzyme (means protein which catalyzes chemical processes) that catalyzes the cleavage of long, double-stranded RNA molecules (of a virus). They will be cleaved to many short double-stranded fragments of around 20-25 nucleotide base pairs with a few unpaired overhang bases on each end[2], as is demonstrated in part A of the following picture. These short dsRNA fragments, usually called small interfering RNA (siRNA), will be unwound by RISC complex.
The blue upper strand is called the 'sense'-strand, while the other is referred to as the 'anti-sense'-strand. Each strand has two ends, namely the 3'- and the 5'-end, indicating the encoding or decoding direction (depends on which process they attend), as the following schematic diagram demonstrates.
Because these fragments produced by the dicer are double-stranded, each strand could theoretically bind to the RISC complex (RNA-induced silencing complex) and lead to the gene silencing; however, only one of the two strands – the anti-sense strand - could actually be functional. The sense-strand will degrade beforehand.[3] Recent research has shown that the selected strand tends to be the one whose 5'-end is more stable.[4]
What happen to the other strand – the functional anti-sense strand? As shown in part B of the picture above, the anti-sense strand will bind to the RISC complex, simultaneously activating the RISC complex. Then the activated RISC complex, together with the anti-sense strand, looks around in the cell for mRNA (in that case other viral RNA that might also be loose in the cell) that has a complementary sequence with the anti-sense strand. When it finds a matching long single-stranded mRNA, the RISC complex binds to it. The binding results in partial or sometimes complete mRNA degradation, and thus blockb mRNA’s protein production (of a virus).
RNAi plays an important role in medicine. Researchers have found that although it’s difficult to introduce long dsRNA into mammalian cells due to the interferon response[5], they can artificially introduce short interfering RNA from a disease gene, triggering the RNAi system to silence that gene. It has recently be reported that a disease-causing gene was silenced in monkeys through RNAi therapy[6]. This finding affirms the promise of the new type of therapy for HIV, cancer and many other fatal diseases.
ABBREVIATIONS & RESOURCES
DNA: Deoxyribonucleic acid.
RNA: Ribonucleic acid.
mRNA: messenger RNA.
dsRNA: double-stranded RNA.
siRNA: small interfering RNA.
RISC: RNA-induced silencing complex.
RNA interference on Wikipedia: http://en.wikipedia.org/wiki/RNA_interference
NOVA ScienceNow RNAi: http://www.pbs.org/wgbh/nova/sciencenow/3210/02.html
REFERENCE
[1] Bernstein E, Caudy AA, Hammond SM, Hannon GJ. (2001). Role for a bidentate ribonuclease in the initiation step of RNA interference. Nature 409(6818):363-6. PubMed 11201747
[2] Vermeulen A, Behlen L, Reynolds A, Wolfson A, Marshall WS, Karpilow J, Khvorova A. (2005). The contributions of dsRNA structure to Dicer specificity and efficiency. RNA 11(5):674-82. PubMed 15811921
[3] Gregory RI, Chendrimada TP, Cooch N, Shiekhattar R. (2005). Human RISC couples microRNA biogenesis and posttranscriptional gene silencing. Cell 123(4):631-40.PubMed 16271387
[4] Preall JB, He Z, Gorra JM, Sontheimer EJ. (2006). Short interfering RNA strand selection is independent of dsRNA processing polarity during RNAi in Drosophila. Curr Biol 16(5):530-5. PubMed 16527750
[5] Paddison PJ, Caudy AA, Hannon GJ. (2002). Stable suppression of gene expression by RNAi in mammalian cells. Proc Natl Acad Sci USA 99(3):1443-8. PMID 11818553
[6] T.S. Zimmermann et al. RNAi-mediated gene silencing in non-human primates. Nature, published online March 26, 2006.