what is the RNAi and tell about the recent Noble prize winners on this RNAi mechanism?

Answer 1

I'm in Genetics at the University of Loyola Chicago so I know all about it: RNAi is a mechanism where a small portion of a piece of double stranded RNA, sometimes abbreviated (ds)RNA, whose sequence matches part of a gene's sequence, interferes with the expression of that particular gene...Now, in terms of the recent Nobel prize winners, Mello and Fire found that after they injected the dsRNA into worm, it silenced the genes which had a complementary gene sequence. Now, I don't know the rest, but I'm going to give you an excerpt that my professor gave me: "[That nailed down the mechanism for seemingly disparate and baffling observations others had made in recent years. It also laid the groundwork for RNAi findings that came later, including the machinery's natural role in mammalian cells and ways to artificially manipulate it. Today, it's thought that one type of small RNA molecule, microRNAs, control upwards of a quarter of the human genome."

Answer 2

Fire et al actualy first made the discovery back in 1997 doing some work on a nematode. RNAi basically a method of post transcriptional gene regulation tht has shown huge potential for treating various infections and diseases such as HIV. It uses short infering RNA's (siRNA) bound to an endonuclease and targets sequence specific mRNA and cuts it up.

I have written several reports and presentations on the subject, and i think it is about time that Fire and Mello recieved the Nobel Prize for Medicine.

If you want a copy of a presentation i did recently, then drop me an e-mail. I will be more than happy to explain it to you if you like.

This is quite a good site: http://www.life.uiuc.edu/shapiro/rnaiapp...

Answer 3

RNAi (RNA interference) refers to the introduction of homologous double stranded RNA (dsRNA) to specifically target a gene's product, resulting in null or hypomorphic phenotypes. The use of antisense RNA to interfere with a gene's activity in C. elegans was first utilised by Su Guo and Ken Kemphues to study par-1 ; however, it was reported that control sense RNA also produced a par-1 mutant phenotype (Cell 81: 611-20, 1995). Subsequently, it was discovered by Fire et al. '98 that it is the presence of dsRNA, formed from the annealing of sense and antisense strands present in the in vitro RNA preps, that is responsible for producing the interfering activity. Introduction of dsRNA into an adult worm results in the loss of the targeted endogenous mRNA from both the adult and its progeny. This phenomenon has been effectively harnessed to study an ever increasing number of maternal and zygotic genes in C. elegans.

The most interesting aspects of RNAi are the following:

dsRNA, rather than single-stranded antisense RNA, is the interfering agent
it is highly specific
it is remarkably potent (only a few dsRNA molecules per cell are required for effective interference)
the interfering activity (and presumably the dsRNA) can cause interference in cells and tissues far removed from the site of introduction

In recent years several studies have shed light on the underlying mechanisms of how dsRNA results in the loss of the targeted homologous mRNA. Early observations indicated that the primary interference effects are post-transcriptional. First it was observed by Craig Mello, and reported in Fire et al. ('98), that only dsRNA targeting exon sequences was effective (promoter and intron sequences could not produce an RNAi effect). Additional evidence supporting mature messages as the most likely target of RNA-mediated interference is summarised below (from Montgomery et al. ‘98, PNAS 95: 15502-07):

* primary DNA sequence of target appears unaltered
* initiation and elongation of transcription appear unaffected
* nascent transcripts can be detected but are apparently degraded before leaving the nucleus

RNAi is remarkably potent (i.e., only a few dsRNA molecules per cell are required to produce effective interference). This observation suggested that the dsRNA must be either replicated and/or function catalytically; models that have been mutually supported by further studies. Genetic and biochemical studies involving plants and flies as well as worms have uncovered similar processes in which the dsRNA is cleaved into ~23 bp short interfering RNAs (siRNAs) by an enzyme called Dicer (Bernstein et al., 2001; Hamilton & Baulcombe, 1999, Science 286: 950), thus producing multiple “trigger” molecules from the original single dsRNA. The siRNA-Dicer complex recruits additional components to form an RNA-induced Silencing Complex (RISC) in which the unwound siRNA base pairs with complementary mRNA, thus guiding the RNAi machinery to the target mRNA resulting in the effective cleavage and subsequent degradation of the mRNA (Hammond et al., 2000, Nature 404: 293-96, Zamore et al., 2000, Cell 101: 25-33; Pham et al., 2004, Cell 117: 83-94). In this way, the activated RISC could potentially target multiple mRNAs, and thus function catalytically.

In addition, a role for RNA-dependent RNA polymerases (RdRP) has been found for some species; mutations have been shown to effect the RNAi response as well as result in increased viral susceptibility, and/or developmental defects (reviewed in Hutvagner & Zamore, 2002, Curr. Opin. Genet. Dev. 12: 225-32). This ability to generate dsRNA de novo supports the replication hypothesis.