Question

What is an open reading frame (ORF)?

• A. All of the nucleotides of a gene that are transcribed into mRNA
• B. The nucleotides of a gene that make up the codons specifying amino acids
• C. The nucleotides of an mRNA molecule before the introns have been removed
• D. The amino acid sequence of a polypeptide

Hint:

An Open Reading Frame (ORF) is a continuous run of codons starting with ATG and ending with a stop codon.
It represents the actual protein-coding region of a gene.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
An Open Reading Frame (ORF) is a portion of a genetic sequence that has the potential to be translated into a protein.
It is defined by a continuous sequence of triplets (codons) that begins with a start codon and ends with a stop codon.

Step 2: Detailed Explanation:
An ORF is a stretch of DNA or RNA sequence that contains:
1. A start codon (typically $\text{ATG}$ in DNA or $\text{AUG}$ in mRNA), which signals the initiation of translation.
2. A series of consecutive, non-overlapping codons that specify the sequence of amino acids in a polypeptide chain.
3. A terminal stop codon (one of $\text{TAA}$, $\text{TAG}$, or $\text{TGA}$ in DNA), which signals the termination of translation.
Because an ORF does not contain any internal stop codons that would prematurely terminate translation, it represents the portion of the gene that directly codes for the amino acid sequence of the protein.
Let us review the other options:
- Option (A) describes the entire transcribed region, which includes untranslated regions ($5'$-UTR and $3'$-UTR) flanking the coding sequence.
- Option (C) describes pre-mRNA containing both exons and introns.
- Option (D) describes the primary structure of the protein, which is the product of translation, not the nucleotide sequence itself.

Step 3: Final Answer:
An open reading frame (ORF) consists of the nucleotides of a gene that make up the codons specifying amino acids, which corresponds to Option (B).