In the genetic code, how many nucleotides (bases) together form one codon that specifies a particular amino acid or a start or stop signal?

Introduction / Context:
The flow of genetic information from DNA to protein follows a precise code. In this code, sequences of nucleotides are read in groups to specify amino acids during protein synthesis. These groups are called codons. Understanding how many nucleotides make up a single codon is a fundamental concept in molecular biology and genetics. This question checks your knowledge of the triplet nature of the genetic code.

Given Data / Assumptions:

• The question asks how many nucleotides together form one codon.
• Options range from 2 to 6 nucleotides.
• We assume the standard genetic code used in almost all organisms.
• We also assume that codons are read on mRNA during translation.

Concept / Approach:
The genetic code is described as a triplet code. Each codon consists of three nucleotides in sequence on the messenger RNA (mRNA). Each such triplet either specifies one amino acid or acts as a start or stop signal during translation. With four possible nucleotides (A, U, G, C in RNA), a triplet code allows 4^3 = 64 possible codons, enough to encode 20 amino acids plus start and stop signals. A doublet code (two nucleotides) would give only 16 codons, which is insufficient to encode all amino acids. Thus, the genetic code is based on triplets of nucleotides, not pairs or larger fixed groups such as 4 or 5 for each codon.

Step-by-Step Solution:
Step 1: Recall that a codon is a unit of the genetic code on mRNA that specifies one amino acid or a start or stop signal. Step 2: Remember that the genetic code is triplet in nature, meaning codons consist of three nucleotides. Step 3: Use the combination idea: with four nucleotides available, a 3-base code gives 4^3 = 64 possible codons. Step 4: Recognise that 64 codons are sufficient to code for 20 amino acids plus start and stop signals, which matches known biology. Step 5: See that a 2-base code (4^2 = 16) would not provide enough combinations and larger codes like 4 or 5 bases would be unnecessarily complex. Step 6: Conclude that three nucleotides make up one codon.

Verification / Alternative check:
Standard genetic code tables list codons as three-letter sequences such as AUG, UUU, GCU, and UGA. Each entry has exactly three nucleotides. The start codon AUG codes for methionine, while stop codons such as UAA, UAG, and UGA are also triplets. No official codon consists of only two or more than three nucleotides. These widely used tables in textbooks, genetic databases, and laboratory work confirm that codons are always triplets, never doublets or quadruplets, in the standard genetic code.

Why Other Options Are Wrong:
Option B: Two nucleotides per codon would allow only 16 combinations, not enough to code for 20 amino acids plus start and stop signals. Option C: Four nucleotides per codon (4^4 = 256 combinations) would be more than necessary and does not match the actual observed genetic code. Option D: Five nucleotides per codon is not used in the standard genetic code and would also produce far more codons than needed. Option E: Six nucleotides per codon has no basis in real biological coding systems and would be highly inefficient.

Common Pitfalls:
Learners sometimes misinterpret diagrams of the genetic code and think that codons can vary in length or include more than three bases, especially when they see longer DNA sequences. Another common mistake is confusing codons (three-base units on mRNA) with anticodons (three-base units on tRNA) or thinking that one nucleotide corresponds to one amino acid. To avoid confusion, remember the phrase "triplet code" and associate codon directly with three nucleotides read together during translation.

Final Answer:
One codon in the genetic code is made up of 3 nucleotides, which together specify a particular amino acid or a start or stop signal.