Q.26 Heterologous expression of green fluorescent protein is possible because the genetic code is
(A) universal
(B) triplet
(C) degenerate
(D) non-overlapping

The correct answer is (A) universal. The green fluorescent protein (GFP) from Aequorea victoria can be expressed in bacteria, yeast, plants or mammalian cells because almost all organisms share the same universal genetic code, so the same codons specify the same amino acids across species.​

Correct Answer Explained: Genetic Code is Universal

Heterologous expression means expressing a gene in a different host organism from the one in which it naturally occurs, for example expressing the jellyfish GFP gene in E. coli or human cells. This works because the universal genetic code uses the same codon–amino acid assignments in almost all known organisms (e.g., AUG codes for methionine, UUU for phenylalanine, etc.), so the host translation machinery reads the foreign mRNA correctly to synthesize the same protein.​

In GFP heterologous expression, once the GFP gene is cloned under a suitable promoter in a host expression vector, the host ribosomes and tRNAs decode its codons exactly as in the original jellyfish, producing correctly folded GFP that fluoresces green under blue/UV light. If different organisms used radically different codon meanings, the same nucleotide sequence would specify different amino acids, and functional GFP would not be produced in a foreign host.​

Therefore, heterologous expression of green fluorescent protein is possible because the genetic code is universal (Option A).​

Why Option A (Universal) is Correct

The genetic code is described as universal because:

• The same 64 codons code for the same 20 amino acids and stop signals in almost all species, from bacteria to humans.​

• Only rare exceptions occur, such as in mitochondria or some protozoa, but even there most codons still retain their standard meaning.​

This near-universality is what allows a gene from one organism to be transplanted and expressed in another without changing the protein’s amino acid sequence. For GFP, once codon usage is optimized for the host (to improve expression level), the amino acid sequence remains identical because the codon assignments are conserved.​

Why Option B (Triplet) is Not the Main Reason

The genetic code is a triplet code, meaning each codon consists of three nucleotides (e.g., AUG, UGC), and each triplet specifies one amino acid or a stop signal. While this triplet nature is fundamental for encoding enough combinations (64 codons) to cover 20 amino acids, it does not by itself explain heterologous expression of GFP.​

• Triplet coding explains how information is packaged (3 bases per codon), not whether different organisms interpret those triplets the same way.​

• Even if the code were duplet or quadruplet, heterologous expression would still be possible provided codon meanings were the same across organisms.​

Thus, being triplet is a structural property of the code, but it is not the reason GFP can be expressed across species; universality is the key.

Why Option C (Degenerate) is Not the Main Reason

The genetic code is degenerate, meaning most amino acids are encoded by more than one codon (for example, leucine has six codons, serine has six, etc.). This degeneracy leads to synonymous codons, where different triplets specify the same amino acid, providing robustness against some mutations and flexibility in codon usage.​

In heterologous expression:

• Degeneracy allows codon optimization: the DNA sequence of the GFP gene can be redesigned to use codons preferred by the host without changing the amino acid sequence.​

• However, degeneracy alone does not guarantee that a host will interpret a foreign gene correctly; that still depends on which amino acid each codon stands for, i.e., the universality of assignments.​

So, while degeneracy is very important for optimizing expression levels and translation efficiency, it is not the fundamental reason heterologous GFP expression is possible; that role is played by universality.

Why Option D (Non-overlapping) is Not the Main Reason

The genetic code is non-overlapping, meaning each nucleotide is part of only one codon during translation, and codons are read in a continuous, non-overlapping fashion (e.g., ABC DEF GHI, not ABC BCD CDE). This simplifies decoding and reduces errors compared with an overlapping code.​

For GFP expression:

• Non-overlapping reading ensures a single, correct reading frame and a well-defined amino acid sequence from start to stop codon.​

• However, even an overlapping code could, in principle, be used heterologously if all organisms shared the same overlapping rules and codon meanings.​

Therefore, the non-overlapping nature of the code is a general feature of translation, but it does not specifically explain why a GFP gene from one species can be accurately translated in another; that again depends on the universal mapping of codons to amino acids.

Key Takeaways for Exam Preparation

• Correct option: (A) universal – this directly enables heterologous expression of GFP across different species because codon meanings are conserved.​

• Triplet: describes codon length, necessary for coding capacity but not the reason for cross-species expression.​

• Degenerate: multiple codons per amino acid; important for codon optimization and robustness, but not the core reason for heterologous expression.​

• Non-overlapping: each base belongs to one codon; important for clear reading frames, but again not what allows a foreign gene to be expressed in a different host.​