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.