Table 3.
Comparison between different small RNA species.
| siRNAs | miRNAs | piRNAs | 21U-RNAs | |
|---|---|---|---|---|
| Length (in nts) | 21-24 | ~22 | 25-31 | 21 |
| Requirement of dsRNA precursor | Yes | Yes | No | No |
| Genomic location | Dispersed throughout | Dispersed throughout | Discrete loci | Chromosome IV |
| Frequency (in %) of 5′ U Monophophate | ~80% | ~76% | ~94% | 100% |
| Location of Biogenesis | Cytoplasm/Nucleus | Nucleus and Cytoplasm | Nucleus? | Nucleus |
| Nature of gene | Autonomous /clustered | Autonomous | Tightly clustered | Autonomous |
| Proteins strictly associated with biogenesis (animals) | Dcr 2, AGO2 | Dcr 1, AGO1, Drosha/ Pasha, Exportin-5 | Piwi/ Aubergine, AGO3 | ? |
| Detected in | All eukaryotes studied | All eukaryotes but S. cerevisiae | Worms, Zebrafish mammals | C. elegans, C. briggsae |
| Expression | All tissues | Every tissue but few shows tissue specificity | Male germ line cells | All tissues |
| Downstream effects | Target cleavage, Chromatin remodeling, Translation repression, Genome reorganization | Translation repression, Target cleavage, Chromatin remodeling? | Genome organization, Enhances translation and mRNA stability | Nucleosome phasing |
| 3´ end modification | Yes | Yes | Yes | Yes |
| Mode of transcription | Divergent but partial overlapping Convergent | Autonomous | Divergent | Autonomous |
| Strand biasedness | Yes | Yes | High | Yes |
| Selection pressure | No | High | No | No |
| Nature of transcript | Polycistronic | Polycistronic/ Monocistronic | Polycistronic | Monocistronic |
| Potential tool without adverse effects | Yes | In plants | No | No |