Question 41:
A group of contiguous nucleotide codons in DNA molecule that does not include a termination codon is called:
The correct answer is (A) Open Reading Frame.
An Open Reading Frame (ORF) precisely matches the description of a continuous sequence of nucleotide codons in a DNA molecule lacking a termination codon, allowing potential translation into a protein until a stop codon is encountered.
Option Breakdown
(A) Open Reading Frame
This refers to a DNA segment starting from a start codon (like ATG) and consisting of contiguous codons that translate into amino acids without a stop codon (TAA, TAG, or TGA), making it “open” for ribosomal reading.
Long ORFs often indicate protein-coding genes, especially in prokaryotes, and exist in six possible reading frames across DNA strands.
(B) Operon
An operon is a functional unit of DNA containing clustered genes under a single promoter, regulated together for coordinated expression (e.g., lac operon), but it includes both coding and non-coding regions plus stop codons.
(C) Genetic Code
The genetic code is the universal set of rules mapping 64 nucleotide triplets (codons) to 20 amino acids or stop signals, not a specific contiguous DNA sequence.
(D) Cis Trans System
This describes genetic regulation where cis-elements (on the same DNA, like operators) act with trans-factors (diffusible proteins like repressors), unrelated to codon continuity.
Introduction to Open Reading Frame DNA
In molecular biology, an open reading frame DNA sequence defines a stretch of contiguous nucleotide codons lacking a termination codon, crucial for identifying potential protein-coding genes. This concept is key for students preparing for competitive exams like GATE Life Sciences, helping distinguish it from operons or genetic codes.
What Makes an Open Reading Frame Unique?
An ORF begins at a start codon (ATG) and extends through triplets encoding amino acids until a stop codon interrupts translation—think of it as an “open road” for ribosomes. DNA’s double-stranded nature allows six reading frames (three forward, three reverse), with longer ORFs signaling true genes.
For example, prokaryotic gene prediction often scans for ORFs exceeding 100 codons.
ORF vs Other Options: Clear Comparison
| Term | Definition Key Points | Includes Termination Codon? | Relevance to Question |
|---|---|---|---|
| Open Reading Frame | Contiguous codons from start, no stop codon | No | Exact match |
| Operon | Gene cluster with promoter/control elements | Yes | No |
| Genetic Code | Codon-to-amino acid mapping rules | Includes stop codons | No |
| Cis Trans System | Regulatory elements (cis) and factors (trans) | N/A | No |
This table highlights why open reading frame fits perfectly for GATE-style questions on DNA structure.
Why ORFs Matter in Genetics and Research
ORFs aid genome annotation, especially in bacteria, by flagging translatable regions amid non-coding DNA. Tools like ORF finders analyze sequences for exam prep or biotech applications.
Understanding this boosts scores in biochemistry and molecular biology sections.
