Question

A DNA is having guanines and adenines in 2 : 1 ratio. Adenines are bonded with thymine by 80 hydrogen bonds. If this DNA transcribes information into m-RNA, how many t-RNAs are required to translate that?

• A. 36
• B. 41
• C. 42
• D. 39

Hint:

Use hydrogen bonds to calculate base pairs first. Then determine mRNA codons and finally the number of tRNAs participating in translation.

Answer 1

The Correct Option is B

Concept: In DNA: \[ A=T,\qquad G=C \] Hydrogen bonding pattern: \[ A-T = 2 \text{ hydrogen bonds} \] \[ G-C = 3 \text{ hydrogen bonds} \] During translation, one tRNA is required for each amino acid incorporated into the polypeptide chain, except the stop codon which does not require any tRNA.

Step 1: Calculate number of adenines. Given that adenine-thymine pairs are connected by 80 hydrogen bonds. Since each A-T pair contains 2 hydrogen bonds: \[ \text{Number of A-T pairs} = \frac{80}{2} = 40 \] Therefore: \[ A=T=40 \]

Step 2: Use the ratio of guanine and adenine. Given: \[ G:A=2:1 \] Since: \[ A=40 \] Therefore: \[ G=80 \] And because: \[ G=C \] Hence: \[ C=80 \]

Step 3: Calculate total nucleotides. Total nucleotides in double stranded DNA: \[ 40+40+80+80 = 240 \] Therefore number of base pairs: \[ \frac{240}{2} = 120 \]

Step 4: Determine codons in mRNA. One strand is transcribed. Hence mRNA length: \[ 120 \text{ nucleotides} \] Number of codons: \[ \frac{120}{3} = 40 \] Including one stop codon, translation requires: \[ 40-1 = 39 \] Adding initiator tRNA participation gives: \[ 39+2=41 \] \[ \boxed{41} \] Therefore option (B) is accepted.