Small RNA (sRNA) are polymeric RNA molecules that are less than 200

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...

s in length, and are usually non-coding.

RNA silencing RNA silencing or RNA interference refers to a family of gene silencing effects by which gene expression is negatively regulated by non-coding RNAs such as microRNAs. RNA silencing may also be defined as sequence-specific regulation of gene express ...

is often a function of these molecules, with the most common and well-studied example being

RNA interference RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...

(RNAi), in which endogenously expressed

microRNA MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. mi ...

(miRNA) or exogenously derived

small interfering RNA Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA at first non-coding RNA molecules, typically 20-24 (normally 21) base pairs in length, similar to MicroRNA, miRNA, and op ...

(siRNA) induces the degradation of complementary messenger RNA. Other classes of small RNA have been identified, including

piwi-interacting RNA Piwi-interacting RNA (piRNA) is the largest class of small non-coding RNA, non-coding RNA molecules expressed in animal cells. piRNAs form RNA-protein complexes through interactions with piwi-subfamily Argonaute proteins. These piRNA complexes are ...

(piRNA) and its subspecies repeat associated small interfering RNA (rasiRNA). Small RNA "is unable to induce RNAi alone, and to accomplish the task it must form the core of the RNA–protein complex termed the RNA-induced silencing complex (RISC), specifically with Argonaute protein". Small RNA have been detected or sequenced using a range of techniques, including directly by MicroRNA sequencing on several sequencing platforms, or indirectly through genome sequencing and analysis. Identification of miRNAs has been evaluated in detecting human disease, such as breast cancer. Peripheral blood mononuclear cell (PBMC) miRNA expression has been studied as potential biomarker for different neurological disorders such as

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

, Multiple sclerosis. Evaluating small RNA is useful for certain kinds of study because its molecules "do not need to be fragmented prior to library preparation". Types of small RNA include: *

(miRNA) *

Piwi-interacting RNA Piwi-interacting RNA (piRNA) is the largest class of small non-coding RNA, non-coding RNA molecules expressed in animal cells. piRNAs form RNA-protein complexes through interactions with piwi-subfamily Argonaute proteins. These piRNA complexes are ...

(piRNA) *

(siRNA) *

small nuclear RNA Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribe ...

(snRNA), also commonly referred to as U-RNA * small nucleolar RNA (snoRNA) * small rDNA-derived RNA (srRNA) * tRNA fragment (tRF) * Y RNA-derived small RNA (ysRNA)

In plants

The first known function in plants was discovered in mutants of '' Arabidopsis''. Specifically with decline in function

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...

s for RNA-dependent RNA polymerase and DICER-like production. This impairment actually enhanced ''Arabidopsis'' resistance against ''

Heterodera schachtii ''Heterodera schachtii'', the beet cyst eelworm or sugarbeet nematode, is a plant pathogenic nematode. It infects more than 200 different plants including economically important crops such as sugar beets, cabbage, broccoli, and radish. ''H. sch ...

'' and ''

Meloidogyne javanica ''Meloidogyne javanica'' is a species of plant-pathogenic nematodes. It is one of the tropical root-knot nematodes and a major agricultural pest in many countries. It has many hosts. ''Meloidogyne javanica'' reproduces by obligatory mitotic part ...

''. Similarly, mutants with reduced Argonaute function - '' ago1-25'', '' ago1-27'', '' ago2-1'', and combined mutants with ''ago1-27'' and ''ago2-1'' - had greater resistance to ''

Meloidogyne incognita ''Meloidogyne incognita'' (root-knot nematode - RKN), also known as the "southern root-nematode" or "cotton root-knot nematode" is a plant-parasitic roundworm in the family Heteroderidae. This nematode is one of the four most common species world ...

''. Altogether this demonstrates great dependence of nematode parasitism on plants' own small RNAs.

References

Non-coding RNA {{microbiology-stub de:Kleine RNA hr:Mala RNK