Model Answer
0 min readIntroduction
The genetic code is a set of rules used by living cells to translate information encoded within genetic material (DNA or RNA) into proteins. It’s essentially a dictionary that relates codons – three-nucleotide sequences – to specific amino acids. This code is nearly universal across all organisms, demonstrating a common ancestry. Understanding the genetic code is fundamental to comprehending gene expression and the molecular basis of heredity. The question asks for a visual representation of this code, specifically highlighting the crucial roles of initiation, termination, and nonsense codons in the protein synthesis process.
The Genetic Code: A Chequerboard Representation
The genetic code consists of 64 possible codons, formed by combinations of four nucleotide bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T) in DNA (or Uracil (U) in RNA). Each codon specifies either an amino acid or a signal to start or stop protein synthesis. The following table represents the standard genetic code, often visualized as a chequerboard:
| U | C | A | G | |
|---|---|---|---|---|
| UUU | Phe | Ser | Tyr | Cys |
| UUC | Phe | Ser | Tyr | Cys |
| UUA | Leu | Pro | His | Arg |
| UUG | Leu | Pro | His | Arg |
| UCU | Ser | Thr | Cys | Arg |
| UCC | Ser | Thr | Cys | Arg |
| UCA | Ser | Thr | Leu | Arg |
| UCG | Ser | Thr | Leu | Arg |
| UAU | Tyr | His | Phe | Gln |
| UAC | Tyr | His | Phe | Gln |
| UAA | STOP | STOP | STOP | STOP |
| UAG | STOP | STOP | STOP | STOP |
| UGU | Cys | Ser | Trp | Arg |
| UGC | Cys | Ser | Trp | Arg |
| UGA | STOP | STOP | STOP | STOP |
| UGG | Trp |
(Note: This table represents only the first page of the complete codon table. The remaining codons follow a similar pattern with A, C, and G in the first position.)
Initiation Codon
The initiation codon is AUG, which codes for the amino acid Methionine (Met). However, in bacteria, a modified form of methionine, formylmethionine (fMet), is often used as the initiating amino acid. The initiation codon signals the ribosome where to begin translating the mRNA sequence. It is crucial for the accurate start of protein synthesis. The initiator tRNA carrying methionine binds to the small ribosomal subunit and then scans the mRNA for the AUG start codon.
Termination Codons
Termination codons, also known as stop codons, signal the end of protein synthesis. There are three termination codons: UAA, UAG, and UGA. These codons do not code for any amino acid. Instead, they are recognized by release factors, which bind to the ribosome and cause the polypeptide chain to be released. Without termination codons, the ribosome would continue translating the mRNA indefinitely, resulting in a non-functional protein.
Nonsense Codons
Nonsense codons are synonymous with termination codons (UAA, UAG, and UGA). The term "nonsense codon" emphasizes their role in creating a premature stop signal during translation. Mutations that introduce a nonsense codon into an mRNA sequence can lead to truncated, non-functional proteins. These mutations are often referred to as "nonsense mutations."
The degeneracy of the genetic code (multiple codons coding for the same amino acid) provides some buffering against the effects of point mutations, but nonsense mutations are generally more detrimental than missense mutations (which change one amino acid for another).
Conclusion
The genetic code is a remarkably conserved system that underpins all life. The accurate identification and function of initiation, termination, and nonsense codons are essential for the faithful translation of genetic information into functional proteins. Understanding these components is crucial for comprehending the molecular mechanisms of inheritance, gene expression, and the consequences of genetic mutations. Further research continues to refine our understanding of the nuances of the genetic code and its role in biological processes.
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.