9. A gene consists of two introns, a 5′ UTR and 3’UTR region, the probable number of exon will be.
(1) 2             (2) 3
(3) 4             (4) 5

Calculating the Number of Exons in a Gene with Two Introns and UTRs

Question Recap:

A gene has:

• Two introns,

• A 5′ untranslated region (5′ UTR),

• A 3′ untranslated region (3′ UTR).

What is the probable number of exons?

Options:
(1) 2
(2) 3
(3) 4
(4) 5

Explanation:

• Basic gene structure:
  In eukaryotes, genes are composed of exons (coding and non-coding sequences retained in mature mRNA) and introns (non-coding sequences spliced out).

• Relationship between introns and exons:
  The number of exons is generally one more than the number of introns, because exons flank introns.
  For example, a gene with 2 introns usually has 3 exons.

• Inclusion of UTRs:
  The 5′ UTR and 3′ UTR are parts of the exons at the beginning and end of the gene, respectively. They are included within the first and last exons (or sometimes split into multiple exons), but do not add extra exons beyond the count determined by introns.

• Therefore:

  • 2 introns → 3 exons (minimum).

  • The 5′ and 3′ UTRs are part of these exons, not separate exons themselves.

Conclusion:

The gene with 2 introns will have 3 exons.

Final Answer:

(2) 3

Additional Notes from Literature:

• Average eukaryotic genes have multiple introns and exons, with exons being one more than introns.

• UTRs are part of exons and do not increase exon count separately.

• This is a fundamental principle in gene structure across eukaryotes