**4. Biogenesis of non-coding RNA**

Basically, ncRNAs are consequence of various processes, such as the process of duplication, modification of transposons during the evolutionary process, pseudogenization of actual coding sequences, doubling of RNA viruses, part of some hairpin structures, double-stranded RNAs from heterochromatin regions and DNA repeats [19]. Despite having a similar structural build, the sRNA varies greatly in their method of biogenesis.


*Non-Coding RNA and Its Prospective Utilization in Plant Breeding DOI: http://dx.doi.org/10.5772/intechopen.106429*

> **Table 3.** *Brief details about the ncRNA classification.*

#### **4.1 miRNA**

MicroRNAs are generated out of primary microRNAs (pri-miRNAs) by the action of RNA polymerase II. These precursor molecules have a double-stranded secondary structure which later on gets cleaved by DICER-LIKE 1 (DCL1) molecules to form single-stranded siRNA. These raw miRNAs is further processed in the nucleus before being exported to the cytoplasm [1]. Mature single miRNA that incorporates the RISC complex binds with other complementary mRNA sequences [20].

#### **4.2 IsomiRNAs**

IsomiRNAs (IsomiRNAs) are one of the variants of miRNAs that arise because of the inaccurate cleavage action performed through the RNase III enzyme. These may also be developed via the process of post-transcriptional RNA editing [21, 22].

### **4.3 siRNA**

siRNAs are derived from long dsRNAs produced during varied mechanisms, such as folding in an inverted sequence, from a long non-coding RNA, hybridization of two fully or partially complementary sequences *.* This further gets processed by the action of DCL2, DCL3, and DCL 4 proteins, leading to the formation of 22 nt, 24 nt, and 21 nt siRNA, respectively [3]. RNA-dependent RNA polymerases 2 and 6 (RDR2, RDR6), and DNA-dependent RNA polymerases IV and V in plants also take part in the production of siRNAs [23].

### **4.4 phasiRNA and tasiRNA**

In plants, phasiRNAs are the partially degraded product of mRNA as being passed through the RISC complex. DCL proteins are involved in this slicing process of dsRNAs to form a group of 21- or 24-nt siRNAs, termed phasiRNAs, *Trans*-acting siRNAs (tasiRNAs) are a class of DCL4-dependent 21-nt phasiRNAs generated from non-coding *TAS* transcripts [24, 25].

#### **4.5 lncRNA**

lncRNAs have appeared from intergenic, particularly regions giving rise to long intergenic ncRNAs (lincRNAs) and those developed from intronic regions giving intronic ncRNAs (incRNAs) [26]. lincRNAs and IncRNAs belong to conventional linear lncRNAs. Circular RNAs (circRNAs) generally develop from coding regions or intronic regions.

#### *4.5.1 Linear-long non-coding RNAs*

The lncRNAs, such as lincRNAs and IncRNAs are linear lncRNAs transcribed by Pol II. Because of having similar features as that of mRNA with a 5\_ m7G cap and a 3\_ poly (A) tail, they undergo similar modifications later on called mRNA mimics [27].
