A.
5'- AGTAGTATCAACTATCATGA-3'
3'- TCATCATAGTTGATAGTACT-5'

B.
5'- GACGTGCAGGTGCGGAGGTC-3'
3'- CTGCACGGTCCACCTCCAG-5'

C.
5'- TACGATGCACATGCTTGGA-3'
3'- ATGCTACGTGTACGAACCTG-5'

D.
5'- GAACGCTACGTTGCGATCCG-3'
3'- CTTGCATGCAACGCTAGGC-5'

Arrange the DNA fragments (A to D) in the order of decreasing melting temperature.

Correct option is A

Explanation-

G≡C base pairs have 3 hydrogen bonds → more stable
A=T base pairs have 2 hydrogen bonds
Higher GC content = higher melting temperature (Tm)

Step-by-Step GC Content Calculation:
Fragment A: Sequence: AGTAGTATCAACTATCATGA
Length = 20 bases
G count = 2
C count = 2
GC content = 4/20 = 20%
Fragment B: Sequence: GACGTGCCAGGTGCGAGGTC
Length = 20 bases
G count = 8
C count = 6
GC content = 14/20 = 70%
Fragment C: Sequence: TACGATGCACATGCTTGGAC
Length = 20 bases
G count = 5
C count = 5
GC content = 10/20 = 50%
Fragment D: Sequence: GAACGCTACGTTGCGATCCG
Length = 20 bases
G count = 6
C count = 6
GC content = 12/20 = 60%

Final Order Based on GC Content (Decreasing Tm):
B → 70% GC
D → 60% GC
C → 50% GC
A → 20% GC

Correct Order: B > D > C > A (Option a)