Question

The binding site of tRNA with mRNA and amino acids respectively are
 A. mRNA with DHU loop and amino acid with CCA end.
 B. mRNA with CCA end and amino acid with anticodon loop.
 C. mRNA with anticodon loop and amino acid with DHU loop.
 D. mRNA with anticodon loop and amino acid with CCA end.

Answer 1

Hint: There are over 100 types of tRNAs. tRNA or transfer Rna is the smallest RNA with 73-93 nucleotides and a sedimentation coefficient of 4s. tRNA is an adapter molecule that is meant for transferring amino acids.tRNA binds place-specific amino acids at particular points during polypeptide synthesis as per codons of mRNA. Codons are recognized by the anticodon of tRNAs.Specific amino acids are recognized by particular activating or aminoacyl synthetase enzymes.

Complete answer:
tRNA consist of 3 loops, variable arm, amino acid binding site
1.A 5’- terminal phosphate group.
2.Acceptor stem/ Amino acid binding site-It (acceptor stem) is the amino acid binding site. The site lies at the 3’ end opposite the anticodon and has a CCA-OH group. The 5’ end bears Guanine. Amino acid or AA- binding site and the anticodon are two recognition sites of tRNA.
3.TΨC loop- It has 7 bases out of which Ψ and rT are unusual bases. This loop is the site for attaching to the ribosome
4.DHU loop - The loop contains 8-12 bases. It is the largest loop and has dihydrouridine. It is a binding site for aminoacyl synthetase enzymes.
5.Extra arm- It is a variable side arm or loop which lies between the TΨC loop and anticodon. It is not present in all tRNAs.The exact role of the extra arm is not known.
Option-A-mRNA with DHU loop and amino acid with CCA end: CCA end or acceptor stem is the area where amino acid binds with tRNA but the DHU loop is not the area where mRNA binds.DHU loop is the binding site of the aminoacyl synthetase enzyme. So this is not the correct option.
Option-B -mRNA with CCA end and amino acid with anticodon loop: CCA end or acceptor stem binds with the amino acid and not with the mRNA.The anticodon site is a binding site for mRNA and not for an amino acid. So this is an incorrect option.
Option-C-mRNA with anticodon loop and amino acid with DHU loop: Anticodon is the binding site for mRNA. But DHU Loop is not the binding site for amino acid it is the binding site for aminoacyl synthetase enzyme. So this too does not hold good for the above statement.
Option-D-mRNA with anticodon loop and amino acid with CCA end: Anticodon loop is the binding site for mRNA and CCA end or Acceptor stem is the binding site for an amino acid.
So the correct answer to this question is an option D"mRNA with an anticodon loop and amino acid with CCA end".

Note: The 3D structure of tRNA was proposed to be inverted -L-shaped (by Kim and Klug) which is the actual structure of tRNA. The secondary structure of tRNA has been depicted as that it looks like a cloverleaf. It was Francis Crick who postulated the existence of tRNA.