Problem 2 - Campbell Biology

Question

Which of the following is not true of a codon? $$\begin{array}{l}{\text { (A) It may code for the same amino acid as another codon. }} \\ {\text { (B) It never codes for more than one amino acid. }} \\\ {\text { (C) It extends from one end of a tRNA molecule. }} \\ {\text { (D) It is the basic unit of the genetic code. }}\end{array}$$

Step-by-Step Solution

Answer

(C) is not true.

Step 1: Understand the Concept of a Codon

A codon is a sequence of three nucleotides which together form a unit of genetic code in a DNA or RNA molecule. It specifies which amino acid will be added next during protein synthesis.

Step 2: Evaluate Statement (A)

Statement (A) states that 'It may code for the same amino acid as another codon.' This is true due to the degeneracy of the genetic code, where multiple codons can code for the same amino acid.

Step 3: Evaluate Statement (B)

Statement (B) states that 'It never codes for more than one amino acid.' This is true because each codon specifies exactly one amino acid.

Step 4: Evaluate Statement (C)

Statement (C) states that 'It extends from one end of a tRNA molecule.' This is not true as codons are on the mRNA, not on the tRNA. tRNA has the anticodon that pairs with the codon.

Step 5: Evaluate Statement (D)

Statement (D) states that 'It is the basic unit of the genetic code.' This is true because codons are the fundamental units that code for amino acids in the genetic code.

Step 6: Identify the Incorrect Statement

The incorrect statement is (C).

Key Concepts

Codon Explained

A codon is a sequence of three nucleotides in DNA or RNA that codes for a specific amino acid. This triplet system forms the basis of the genetic code. Codons are fundamental in the process of translating genetic information from DNA or RNA into proteins. For example, the codon AUG codes for the amino acid methionine, which often serves as the start signal for protein synthesis. Because of the degeneracy of the genetic code, multiple codons can specify the same amino acid. This feature provides a buffer against mutations, as changes in one nucleotide might not alter the resulting amino acid.

Role of tRNA

Transfer RNA, or tRNA, plays a crucial role in translating the genetic code from mRNA into proteins. Each tRNA molecule has an anticodon at one end and an amino acid at the other end. The anticodon is a three-nucleotide sequence that is complementary to a specific codon on the mRNA. When the tRNA anticodon binds to its corresponding codon on the mRNA, the amino acid it carries is added to the growing protein chain. This pairing ensures that the protein is synthesized correctly according to the genetic instructions.

Function of mRNA

Messenger RNA, or mRNA, acts as a template for protein synthesis. It is transcribed from DNA in the cell nucleus and then travels to the ribosome, where translation occurs. The mRNA sequence is read in sets of three nucleotides, each set being a codon. The sequence of codons on the mRNA determines the sequence of amino acids in the protein, effectively translating the genetic instructions into functional molecules. Misreading the mRNA can lead to incorrect protein synthesis, which can have significant biological consequences.

Understanding Protein Synthesis

Protein synthesis is the process by which cells build proteins, and it involves two main steps: transcription and translation. During transcription, the DNA sequence of a gene is copied into mRNA. In translation, this mRNA is read by ribosomes in the cytoplasm or on the endoplasmic reticulum. tRNA molecules bring the appropriate amino acids to the ribosome according to the sequence of codons on the mRNA. This process continues until a stop codon is reached, signaling the completion of the protein. The synthesized protein then goes on to perform various functions within the cell or organism, from catalyzing metabolic reactions to providing structural support.