Model Answer
0 min readIntroduction
The genetic code is the set of rules by which information encoded within genetic material (DNA or RNA sequences) is translated into proteins by living cells. It’s essentially a dictionary relating nucleotide sequences to the amino acid sequences of proteins. Discovered in the early 1960s through the work of Marshall Nirenberg, Har Gobind Khorana, and Severo Ochoa, the genetic code exhibits remarkable characteristics that ensure both fidelity and flexibility in protein synthesis. This answer will characterize the features of degeneracy and universality of the genetic code, and further explain the Wobble hypothesis.
(i) Degeneracy of the Genetic Code
Degeneracy, also known as redundancy, refers to the fact that multiple codons can code for the same amino acid. There are 64 possible codons (43) formed by combinations of the four nucleotide bases (Adenine, Guanine, Cytosine, and Thymine/Uracil). However, there are only 20 standard amino acids. This means that most amino acids are specified by more than one codon.
- Reasons for Degeneracy: Degeneracy primarily arises due to the fact that the third base in a codon often exhibits less strict base-pairing rules. This allows for some ‘wobble’ in the third position, as explained by the Wobble hypothesis (discussed later).
- Examples:
- Leucine (Leu) is coded by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG.
- Serine (Ser) is coded by six different codons: UCU, UCC, UCA, UCG, AGU, and AGC.
- Arginine (Arg) has the most codons – six.
- Methionine (Met) and Tryptophan (Trp) are exceptions, being coded by only one codon each (AUG and UGG, respectively).
- Biological Significance: Degeneracy minimizes the impact of mutations. A change in the third base of a codon often results in the same amino acid being incorporated into the protein, thus preventing a change in the protein’s structure and function. This is a form of buffering against deleterious mutations.
(ii) Universality of the Genetic Code
The universality of the genetic code means that the same codons specify the same amino acids in almost all organisms, from bacteria to humans. This remarkable consistency suggests a common evolutionary origin of all life on Earth.
- Evidence for Universality: Experiments involving the introduction of foreign mRNA into different organisms have consistently shown that the mRNA is translated correctly according to the universal code. For example, introducing human mRNA into a bacterial system results in the production of the corresponding human protein.
- Exceptions to Universality: While largely universal, some minor variations exist.
- Mitochondrial Genetic Code: Mitochondria have their own genetic code, which differs slightly from the standard nuclear code. For example, AUA codes for methionine in mitochondria, whereas it codes for isoleucine in the standard code.
- Certain Protozoa and Ciliates: Some protozoa and ciliates exhibit minor variations in codon usage, where certain codons are re-assigned to different amino acids.
- Biological Significance: The universality of the genetic code facilitates genetic engineering and biotechnology. Genes can be transferred between different species, and they will be expressed correctly in the recipient organism. This is the basis for the production of human insulin in bacteria, for instance.
Wobble Hypothesis
Proposed by Francis Crick in 1966, the Wobble hypothesis explains the degeneracy of the genetic code, particularly the reduced importance of the third base in codon-anticodon pairing. It states that the first two bases of a codon pair in a standard Watson-Crick manner (A-U, G-C), but the pairing at the third position is more flexible – hence the ‘wobble’.
- Wobble Base Pairing: The Wobble hypothesis proposes that a single tRNA molecule can recognize multiple codons that differ only in their third base. This is possible because the third base in the codon can pair with multiple bases in the anticodon of the tRNA. For example, the anticodon base G can pair with U or C in the codon.
- Implications: The Wobble hypothesis reduces the number of tRNA molecules required for translation. Instead of needing a tRNA for every possible codon, organisms can use fewer tRNAs to recognize multiple codons.
Conclusion
In conclusion, the degeneracy and universality of the genetic code are fundamental properties that underpin the process of protein synthesis and the continuity of life. Degeneracy provides robustness against mutations, while universality allows for the transfer of genetic information across species. The Wobble hypothesis elegantly explains the mechanism behind degeneracy, highlighting the flexibility inherent in the codon-anticodon interaction. These features collectively demonstrate the remarkable efficiency and elegance of the genetic code.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.