## Core Concept
The genetic code is composed of codons, which are sequences of three nucleotides (also known as bases) that correspond to specific amino acids or stop signals during protein synthesis. The structure of a codon is crucial for understanding how genetic information is translated into proteins.

## Why the Correct Answer is Right
The correct answer, ****, implies that a codon does not have an "intron." Introns are non-coding regions of a gene that are removed by RNA splicing before the mature RNA is translated into protein. Codons, being part of the coding sequence, do not contain introns; they are composed of exons, which are the coding regions.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is incorrect because a codon indeed has a start and stop signal; it specifies one of the 20 amino acids or a stop signal.
* **Option B:** - This option is incorrect because a codon is made up of nucleotides; it is the basic unit of the genetic code.
* **Option D:** - This option is incorrect because a codon does specify an amino acid (or a stop signal), which is fundamental to its role in protein synthesis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while codons are crucial for specifying amino acids, the presence of introns (non-coding regions) within genes is a critical aspect of gene structure. The process of splicing removes introns and joins exons together to form a mature mRNA molecule that can be translated into protein. Understanding the distinction between coding (exons) and non-coding (introns) regions is vital for grasping how genes are expressed.

## Correct Answer: .