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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2006.02.030 Byline: Ronald H.A. Plasterk (1) Abstract: Micro RNAs (miRNAs) are [approximately equal to]22 nucleotide single-stranded noncoding RNA molecules that bind to target messenger RNAs (mRNAs) and silence their expression. This Essay explores the importance of miRNAs in animal development and their possible roles in disease and evolution. Author Affiliation: (1) Hubrecht Laboratory (KNAW), Utrecht University, 3584CT Utrecht, the Netherlands

A model in which MUT-7, guided by transposon-derived dsRNA, represses transposition by degrading transposon-specific messengers and prevents thereby transposase production and transposition, is proposed. It appears that NRAi may work by dsRNA-directed enzymatic RNA degradation. One Tc1 silenced strain of C. elegans has been has been mutagenized. Mutants in which Tc1 was activated in the germline were isolated.

Maternal G proteins were found to play a major role in the orientation of early cell division axes possibly by coupling a membrane signal to centrosome position. The G proteins in the nematode Caenorhabditis elegans were observed to learn about the function of G proteins in vivo and to identify other active elements in the signaling cascade. G beta, which is needed in many developmental stages, is involved in the regulation of behaviors such as locomotion and egg-laying.

A study of Tc3 transposase expression from heat-shock promoters in transgenic nematodes helps understand the Tc3 transposition pathway. The linear extrachromosomal Tc3 DNA that appears during transposition is cut incompletely, with two nucleotides being absent from the 5' ends. The nucleotides at the donor site lead to a distinct footprint. A model that depicts the excision and integration of the Tc3 element and target replication and traces the roots of the somatic footprints is suggested, based on the structure of the extrachromosomal DNA element.

Transposons are used in the tagging of genes, in the generation of transgenic flies and to perform reverse genetics. The P elements of the Drosophila, when excised, cause the breakage in the DNA, which is repaired by ectopic templates. This repair introduces 8000 binding proteins of foreign DNA. This indicates that the shuttling of long sequences into the chromosomes is caused by the transposon allele. The introduction of the required fusion as a transgene into a strain results in the formation of the precise gene fusions outside a living cell.

A comprehensive analysis of Argonautes in the worm Caenorhabditis elegans, knocking out all 27 worm Argonaute genes, is presented. The results have shown that all Argonautes have shared their ability to bind to small RNAs and RNA interference (RNAi) was able to induce heritable gene silencing in C. elegans.

An estimated 122 microRNAs (miRNAs) in 10 primate species were sequenced to reveal the conservation characteristics of miRNA genes. The results suggest the presence of significantly higher numbers of miRNAs in the human genome than previously estimated.