Modified nucleotide is seen in ?

Ans. is 'a' i.e., tRNAA modified base is one that has been modified after nucleic acid chain has been formed. These bases are modified by methylation, reduction, acetylation or glycosylation.In t-RNA there are many modified bases, e.g. dihydrouridine, ribothymidine, pseudouridine, and 7-methylguanosine.In DNA, the most common modified base is 5-methylcytosine.Other impoant modified bases are acetylcytosine and dimethyladenine.Transfer RNA (t RNA) The function of tRNA is to transpo amino acids in an activated form from cytosol to ribosome for protein synthesis. 10-20% bases of tRNA may be modified and known as modified or unusual basese. A modified base is one that has been modified (altered) after nucleic acid chain has been formed, i.e. modified base is altered purine or pyrimidinee, e.g. ?Dihydrouridine (D)deg in which one of the double bonds of the base is reduced.Ribothymidine (T) in which methyl group is added to uracil to form thymine. Thus, tRNA is the only RNA that can contain thyminee though only some times.Pseudouridine (w) in which uracil is attached to ribose by a carbon-carbon bond rather than a nitrogen bond.t-RNA molecule get folded into a structure that appears like a clover leafe. There are four arms.1. Acceptor arm : It consists of a base paired stem that terminates in the sequence CCA at the 3' end. This is the attachment site for amino acids.2. D arm - It contains the base dihydrouridine (D).3. Anticodon arm - It contains anticodone that base pairs with the codon of coming mRNA. Anticodon has nucleotide sequence complementary to the codon of mRNA and is responsible for the specificity of the t RNA. 3) Tyr C arm : It contains both ribothymidine (T) and pseudouridine (v).Proteins are synthesized from 20 amino acids. For each of these amino acids, there is at least one specific tRNA. For some amino acids, there may be more than one species oft RNA.