RNA that carries amino acids to the ribosome where proteins are made is called tRNA or transfer RNA. It is the smallest RNA and has a molecular weight of 25,000 daltons. About 15% of the cell is tRNA. Each tRNA molecule is made up of 90 nucleotides. It is produced in the nucleolus of the cell. tRNA molecules contain some unusual bases. Such as inosinic acid, thiamine etc. They are highly stable and make proteins for cells. There are about 100 types of tRNA in living organisms. A cell contains 31-42 types of tRNA.
According to scientist R. Holley’s clover leaf model in 1965, tRNA molecule consists of 5 arms.
(i) Subscriber arm: 3 and 5 ends of tRNA lie side by side to form the subscriber arm. It contains 7 base pairs. In addition, there are 4 additional unpaired nucleotides at the R3 end. 3 ends contain the -CCA sequence and the fourth A or G. The CCA sequence is called the amino acid attachment site. Amino acids are attached at this point. 5 ends with G or C.
(ii) D arm: The second arm of tRNA is D arm. It consists of 15-18 nucleotides. The D arm consists of 3-4 base pairs and its loop consists of 7 unpaired nucleotides. D arm loop is called loop-1 or Dihydrouridine or D loop.
(iii) Anticodon arm: The third arm of tRNA is called anticodon arm. The anticodon arm consists of 5 base pairs and its loop consists of 7 unpaired nucleotides. Its leak is called leak-2. The 3 nucleotides between the loops form the anticodon. It acts as a complementary codon to mRNA during protein synthesis.
(iv) Variable arm: Variable arm is of two types. Stemless and stemmed arms. Stemless arms have 4-5 bases and a loop. Stem arms consist of 13-21 bases and a loop.
(v) T arm: T arm has both stem and loop. Its stem is made up of 5 base pairs and the loop is made up of 7 nucleotides. The T arm contains a ribosome site. Its loop contains ribothymine, pseudouracil and cytosine bases.
tRNA carries amino acids to mRNA during protein synthesis.