Q.44 Using the Sanger’s dideoxy chain termination method, a particular exonic region of a protein coding
gene was sequenced for two individuals referred to as Subject 1 and Subject 2. The figure below
shows a segment of the autoradiogram corresponding to a small window of the DNA sequence.
Which one of the following interpretations is correct for the sequenced DNA fragments?
(A) Subject 2 has two allelic variants.
(B) Subject 1 has the sequence 5’– TAGTCGGA –3’.
(C) Subject 2 has the sequence 5’– AGGCTAGAT –3’.
(D) Subject 1 has a single nucleotide deletion in the gene.
Concept: Reading a Sanger Sequencing Gel
- Four lanes correspond to A, T, G, and C.
- DNA sequence is read from bottom to top (5′ → 3′).
- Each band represents termination at a specific nucleotide.
- Two bands at the same position indicate heterozygosity.
Step 1: Interpretation of Subject 1
Subject 1 shows a single band at each position in the autoradiogram.
Reading the bands from bottom to top gives a continuous and unambiguous DNA sequence.
There are no extra or missing bands, indicating a single, normal allele with
no insertion or deletion.
Step 2: Interpretation of Subject 2
In Subject 2, multiple positions show two bands at the same vertical level
in different nucleotide lanes.
This indicates the presence of two different nucleotides at the same position in the DNA,
which is characteristic of allelic variation.
Critical Observation
- Subject 1: Single band per position → single allele
- Subject 2: Double bands at some positions → two alleles
- No altered band spacing → no frameshift or deletion
Final Answer
The correct interpretation is that Subject 2 has two allelic variants.
Conclusion
By carefully reading the Sanger sequencing autoradiogram from bottom to top and
analyzing the band patterns, it is clear that Subject 2 shows heterozygosity,
evidenced by double bands at specific positions. Subject 1, on the other hand,
displays a single, uninterrupted sequence. Therefore, the correct interpretation
is that Subject 2 has two allelic variants.
