Which of the following pairs of base sequences could form a short stretch of a normal double helix of dna?
a) 5'-purine-pyrimidine-purine-pyrimid ine-3' with 3'-purine-pyrimidine-purine-pyrimid ine-5'
b) 5'-agct-3' with 5'-tcga-3'
c) 5'-gcgc-3' with 5'-tata-3'
d) 5'-atgc-3' with 5'-gcat-3'
e) all of these pairs are correct.

D) 5'-ATGC-3' with 5'-GCAT-3'

Explanation:

In a double helix DNA, adenine always pairs with thymine and cytosine pairs with guanine. Also, the two strands of a double helix DNA are antiparallel, that is, the 5' end of one strand lies opposite to the 3' end of another strand.

Therefore, a DNA strand with base sequence 5'-ATGC-3' would have a complementary strand 5'-GCAT-3'.

Double stranded DNA: 5'-ATGC-3'                                          

                                      3' TACG 5'

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