**5. Characteristic features of non-coding RNA**

Non-coding RNAs are RNA molecules that are transcribed from DNA but cannot code for a protein. With the advances in transcriptomics and sequencing techniques, thousands of small and long non-coding RNAs were identified. They generally play a major role in gene regulation at the transcriptional or post-transcription level and some regulatory ncRNAs possess high target specificity and some ncRNAs are involved in epigenetic mechanisms too. A few major ones are discussed below.

#### **5.1 tRNA (transfer RNA)**

Next to mRNA which is the coding RNA, tRNA, and rRNA are considered more prominent. tRNA typically contains less than 100 nucleotides and, as their name indicates, their job is to carry an amino acid to the protein-synthesizing machinery. tRNA usually takes a clover leaf secondary structure, which forms a 3D L- shaped structure by stacking the helices. The structure of tRNA was found to be almost similar among different tRNA species. The secondary structure includes the acceptor arm, anticodon arm, T-arm and D-arm, and a variable arm. It was found that the size of the acceptor arm, anticodon arm, and T-arm were conserved whereas the D-arm and variable arm differ in their sizes [28]. This difference in the sizes of the variable arm led to the grouping of tRNAs into two classes. The major proportion of tRNA belongs to class I with less than 10 nucleotides in the variable loop; the class II tRNAs included tRNASer, tRNALeu, and tRNATyr with more than 10 nucleotides [8].
