SBA

SBA — Single Best Answer · Review

Question 583

Genetics → General

Review Question Previous Next

NextGen Workflow Status Needs Review

SQL status generated

Destination Site

Type SSBA

UUID afd85d34-2fd8-48df-ba2d-e2c2a1198d67

Classification

Speciality

Obstetrics and Gynaecology

Group

Basic

Title

Genetics

Topic

General

Subtopic 1

Deoxyribonucleic Acid

Subtopic 2

Genetic Code

Subtopic 3

Codon Redundancy

Question Type

SBA

Question

SSBA

Question Header

Question Stem

A student models the genetic code using four RNA bases, three positions in each codon, the three termination codons UAA, UGA and UAG, and 20 amino acids. Any of the four bases may occupy each position. The student proposes that every distinct amino-acid-specifying codon must identify a different amino acid.

Options

A There are 20 amino-acid-specifying codons because the number of possible triplets is limited by the number of amino acids

B There are 64 amino-acid-specifying codons because termination codons also identify amino acids

C There are 61 amino-acid-specifying codons, so the code must contain 61 different amino acids

D There are 61 amino-acid-specifying codons, so every amino acid must have exactly three codons

E There are 61 amino-acid-specifying codons for 20 amino acids, so at least some amino acids must be specified by more than one codon

Explanation

Four possible bases at each of three positions give 4 × 4 × 4 = 64 possible codons. Removing the three termination codons leaves 61 amino-acid-specifying codons in this model. Since these specify only 20 amino acids, some different codons must specify the same amino acid. This establishes redundancy without implying an equal number of codons for each amino acid, or that every pair of different codons has the same meaning.

Option Validity

A) Triplet possibilities are calculated from the four choices at each of three positions, not from the number of amino acids.

B) The 64 possibilities include three termination codons, which must be removed when counting amino-acid-specifying codons.

C) The model contains 20 amino acids. More coding triplets than amino acids require some repeated amino-acid assignments rather than additional amino acids.

D) The calculation does not establish equal allocation, and exactly three codons for each of 20 amino acids would account for only 60 coding triplets.

Source & metrics

Authored Difficulty

5.00

Evaluation Score

—

Destination

Site

Source

Saxena - MRCOG-1 Basic Sciences - Genetics

Source Section

Chapter 8 — Genetics; Deoxyribonucleic Acid

Source Locator

Printed page 254

Updated

2026-10-06 01:57:35

Difficulty Factor

Not available yet

Attended Users

No candidate-attempt data yet