## **Core Concept**
A codon is a sequence of nucleotides in DNA or RNA that corresponds to a specific amino acid. The genetic code is nearly universal and is used to translate messenger RNA (mRNA) sequences into proteins. The basic unit of the genetic code is the codon, which is crucial for protein synthesis.

## **Why the Correct Answer is Right**
The correct answer, **. 3 nucleotides**, is right because a codon is defined as a sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule. The sequence of nucleotides in a codon determines which amino acid will be added next during protein synthesis. This sequence of three nucleotides is read from the mRNA and is recognized by transfer RNA (tRNA) molecules, which bring the corresponding amino acids to the ribosome for protein assembly.

## **Why Each Wrong Option is Incorrect**
- **Option A: 1 nucleotide** is incorrect because a single nucleotide does not specify an amino acid; it's the sequence of three that does.
- **Option B: 2 nucleotides** is incorrect for similar reasons; two nucleotides are not sufficient to specify one of the 20 amino acids used in protein synthesis.
- **Option D: 4 nucleotides** is incorrect because four nucleotides would imply a different level of specificity than what is required for the genetic code; three nucleotides per codon are sufficient to encode 64 different combinations, which is more than enough to specify all 20 amino acids and the stop signals.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the genetic code is **degenerate**, meaning more than one codon can encode the same amino acid. There are 64 possible codons but only 20 standard amino acids and three stop signals. This fact is crucial for understanding genetics and molecular biology.

## **Correct Answer: C. 3 nucleotides**