Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following statements is true for protein synthesis (translation)? (a) amino acids are directly recognized by m-RNA (b) the third base of the codon is less specific (c) only one codon codes for an amino acid (d) every t-RNA molecule has more than one amino acid attachment.

Step-by-Step Solution

Answer

Option (b) is true; the third base of the codon is less specific.

Step 1: Understanding the Question

The question asks us to determine which statement correctly describes a feature of protein synthesis, specifically translation. We need to evaluate each given option to identify the true one.

Step 2: Analyzing Option A

Option (a) suggests that amino acids are directly recognized by mRNA. In translation, mRNA codons are not recognized directly by amino acids. Instead, tRNA molecules carry amino acids and have anticodons that pair with mRNA codons. Therefore, this option is false.

Step 3: Analyzing Option B

Option (b) states that the third base of the codon is less specific. This is true because of the 'wobble' hypothesis, where the third nucleotide in a codon can often vary without affecting the amino acid it codes for, allowing for flexibility in the genetic code.

Step 4: Analyzing Option C

Option (c) claims that only one codon codes for an amino acid. This is false because most amino acids are coded by more than one codon, a feature known as the redundancy of the genetic code.

Step 5: Analyzing Option D

Option (d) indicates that every tRNA molecule has more than one amino acid attachment. This is false. Each tRNA molecule is specific to only one amino acid, ensuring that it transfers the correct amino acid during protein synthesis.

Step 6: Conclusion

After evaluating all the options, only Option (b) is true as it correctly describes the 'wobble' position in codons, which is less specific.

Key Concepts

Translation

Translation is the process of converting genetic information from mRNA into functional proteins. It occurs in the ribosome, the cellular machinery that synthesizes proteins. The mRNA serves as a template for assembling amino acids in the correct sequence. This process occurs in three stages:

• Initiation: The small ribosomal subunit binds to the mRNA, followed by the attachment of the first tRNA molecule and the large ribosomal subunit.
• Elongation: The ribosome traverses along the mRNA. As this occurs, tRNA molecules deliver specific amino acids, forming a growing polypeptide chain.
• Termination: The process concludes when a stop codon is reached, prompting the release of the newly formed protein.

This careful orchestration ensures that the genetic code is translated accurately, turning mRNA sequences into the proteins essential for life.

Genetic Code

The genetic code is a set of rules that dictates how the sequence of bases in DNA and RNA is translated into proteins. It is universal among almost all organisms, making it a fundamental aspect of genetics. The code is composed of codons, which are sequences of three nucleotides. Each codon specifies a particular amino acid.

• The genetic code consists of 64 codons, with 61 coding for amino acids and 3 acting as stop codons, which signal the end of protein synthesis.
• One of the key features of the genetic code is its redundancy, meaning multiple codons can code for the same amino acid.
• This redundancy adds an element of stability and reliability to protein synthesis, as it minimizes the impact of mutations.

Despite its complexity, the genetic code is an elegant example of nature's efficiency.

tRNA

Transfer RNA (tRNA) is crucial in translation as it acts as the link between the mRNA codon sequence and the amino acid sequence of proteins. Here's how it functions:

• tRNA molecules have two main roles: they carry a specific amino acid and have an anticodon that pairs with the mRNA codon.
• The structure of tRNA resembles a cloverleaf, with the acceptor stem binding to an amino acid, and the anticodon loop pairing with mRNA codons.
• Each tRNA is "charged" with its corresponding amino acid by an enzyme called aminoacyl-tRNA synthetase, ensuring the accuracy of protein synthesis.

By bringing the correct amino acids to the ribosome, tRNA ensures that proteins are synthesized as dictated by the genetic code.

Codon Specificity

Codon specificity refers to the precise pairing of mRNA codons with the appropriate tRNA anticodons during translation. This ensures that proteins are constructed with the correct amino acids in the proper order. Several important facets highlight codon specificity:

• Despite the redundancy of the genetic code, specific codon-anticodon pairings are crucial for accurate protein synthesis.
• The third base of the codon is often less critical, allowing "wobble" in pairing, which is why some codons can code for the same amino acid.
• This wobble flexibility allows cells to adapt to mutations and variability without compromising protein function.

While codon specificity is highly regulated, the molecular dance allows the genetic code to be robust yet flexible, ensuring proteins are correctly formed in diverse organisms.