Question

Which of the following statement(s) are correct in describing the gene in DNA?
Statement I: The start/initiator codon for the genes can be found on the \(5^{\prime} \rightarrow 3^{\prime}\) strand of DNA
Statement II: The start/initiator codon for the genes can be found on the \(3^{\prime} \rightarrow 5^{\prime}\) strand of DNA
Choose the correct answer from the options given below:

• A. Statement I is incorrect and statement II is correct
• B. Statement I is correct and statement II is incorrect
• C. Both the statements are correct
• D. Both the statements are incorrect

Hint:

Always remember: mRNA sequence = DNA Coding Strand sequence (with U instead of T). Both are written and read in the \(5^{\prime}\) to \(3^{\prime}\) direction.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A gene consists of two strands of DNA: the template strand (antisense) and the coding strand (sense). By convention, a DNA sequence is written in the \(5^{\prime}\) to \(3^{\prime}\) direction, which corresponds to the coding strand.

Step 2: Detailed Explanation:
RNA polymerase reads the template strand in the \(3^{\prime} \rightarrow 5^{\prime}\) direction to synthesize mRNA in the \(5^{\prime} \rightarrow 3^{\prime}\) direction. The sequence of the resulting mRNA is identical to the coding strand of the DNA (except that T is replaced by U).
The start codon (AUG in mRNA) corresponds to the sequence ATG on the coding strand. This coding strand runs in the \(5^{\prime} \rightarrow 3^{\prime}\) direction. When we look at a genome sequence or a gene map, we always refer to the \(5^{\prime} \rightarrow 3^{\prime}\) (sense) strand because the codons are read in this direction during translation. Therefore, the start/initiator codon is located on the \(5^{\prime} \rightarrow 3^{\prime}\) strand. It is not found as a codon on the \(3^{\prime} \rightarrow 5^{\prime}\) strand; that strand contains the complement of the codon (CAT), which is used only as a template for transcription.

Step 3: Final Answer:
Codons are read \(5^{\prime} \rightarrow 3^{\prime}\) and are mapped to the sense strand of DNA. Thus, Statement I is correct. Option 2 is the answer.