Question

What are Twintrons?

(1). Group II introns
(2). Group III introns
(3). Composite structure made up of two or more Group II and Group III introns
(4). An archaeal introns

• A. Group II introns
• B. Group III introns
• C. Composite structure made up of two or more Group II and Group III introns
• D. An archaeal introns

Hint:

Think of twintrons as genetic Russian nesting dolls: an intron inside another intron.
They require sequential splicing (inside first, outside second) to generate the mature exon sequence.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Introns are non-coding segments within genes that are removed from precursor RNA transcripts during splicing.
In organelles like chloroplasts and mitochondria, introns are classified into Group I, Group II, and Group III based on their structural motifs and self-splicing mechanisms.

Step 2: Detailed Explanation:
A twintron is a complex splicing arrangement characterized as an “intron-within-an-intron”:
1. Structure: It is a composite structure where an internal guest intron has inserted itself into a specific region of an external host intron.
2. Splicing Mechanism: Splicing of a twintron is sequential.
The internal intron must be spliced out first.
This splicing event restores the intact sequence and functional secondary structure of the host intron.
Once reconstituted, the host intron is spliced out, joining the flanking exon segments.
3. Composition: Twintrons are commonly discovered in chloroplast genomes (e.g., in Euglena gracilis) and are typically composed of nested Group II or Group III introns.
Therefore, they represent a composite structure of multiple nested introns.

Step 3: Final Answer:
Twintrons are composite structures made up of two or more Group II and/or Group III introns, which corresponds to option (C).