SBA — Single Best Answer · Review
Question 576
Genetics → General
Classification
Question
SSBAOptions
Explanation
Complementary transcription converts the original DNA template CCA–ATT–CCA into mRNA GGU–UAA–GGU. The first GGU specifies glycine, but the following UAA is a stop codon, so the final displayed GGU is not translated in that passage. Replacing the template's ATT with CCA produces GGU–GGU–GGU without changing transcript length. Translation can therefore proceed through the former stop position and reach the final glycine codon. A tRNA anticodon written directly beneath GGU in complementary orientation is CCA. The result concerns termination of polypeptide synthesis, not truncation of the RNA transcript.
Option Validity
A) The transcription and termination interpretation is correct, but GGU is the mRNA codon. The opposed complementary tRNA anticodon is CCA.
C) This copies the DNA template's base sequence rather than making its complementary RNA sequence. The given template produces GGU–UAA–GGU before alteration, not CCA–AUU–CCA.
D) UAA is a stop codon, whereas GGU is the glycine codon used in this experiment. Treating UAA as glycine cannot account for the original failure to translate the final triplet.
E) A stop codon terminates the translation step. Both equal-length RNA transcripts already contain the final displayed triplet, so failure to transcribe it cannot explain the observation.
Further Reading
Saxena R. Textbook for MRCOG-1: Basic Sciences in Obstetrics and Gynaecology. Jaypee Brothers Medical Publishers; 2016. Chapter 8, Genetics: Deoxyribonucleic Acid and Process of Protein Synthesis. Printed pages 254–256, including Figures 8.1A and 8.1B.