# DBT JRF 2020 — Question 59

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Medium

## Question

In the DNA helix, the GC and AT base pairs

## Options

- **A.** stack on top of each other, perpendicular to the helix axis
- **B.** stack sideways, parallel to the helix axis
- **C.** stack on top of each other, parallel to the helix axis
- **D.** stack sideways, perpendicular to the helix axis

## Answer

Option A

## Explanation

In the tRNA cloverleaf secondary structure: The 3' end has the universal sequence 5'-CCA-3' (the acceptor stem, also called the amino acid acceptor stem or 3' acceptor arm). The cognate amino acid is covalently attached (aminoacylation) via an ester bond between its carboxyl group and the 2'- or 3'-OH of the terminal adenosine (A76) of the CCA sequence. The reaction is catalysed by aminoacyl-tRNA synthetase (aaRS) using ATP (forming aminoacyl-AMP then transferring to tRNA, releasing AMP + pyrophosphate). Each of the 20 amino acids has its own cognate aaRS. The anticodon loop (at the opposite end of the tRNA) base-pairs with the complementary codon on mRNA at the ribosomal A site during translation.

Source: https://mypyqbuddy.com/questions/in-the-dna-helix-the-gc-and-at-6a05a8a02adafec470293253