Question

In a double stranded DNA, the sequence of base pairs in one strand is AGCTAAGC. What is the complementary sequence on the other strand?
A) TCGATTCG
B) UCGAUUCG
C) AGCTAAGC
D) CAUTAUCG

Answer 1

Hint: Base pair describes the association between the building blocks on the strands of DNA. Therefore each DNA molecule is made up of two strands, and there are four nucleotides present in DNA that is A, C, T, and G. And every nucleotide on one side of the strand pairs with a definite nucleotide on the other side of the strand, and this makes up the double helix.

Complete answer:
According to Watson and Crick model, DNA has four nitrogenous bases, which includes two purines and two pyrimidines, within their nucleotide chains. The purines are adenine and guanine and pyrimidines are named cytosine, thymine. In a double stranded DNA, a purine always pairs with a pyrimidine and a pyrimidine base pairs with purine base only. Accordingly, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). The two strands are held jointly by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.

 A DNA strand with sequence of "AGCTAAGC" will have a complementary strand with the sequence "TCGATTCG". Uracil (U) is a pyrimidine base which is there in RNA in place of T. It is not found in DNA. The sequence given in both option B and D have U, and it makes them wrong. A DNA strand does not have the same base sequence in its complementary strand, thus option C is also wrong.

Hence the correct answer is option ‘C’.

Note: A base pair is a two chemical bases bonded to one another which forms a ring of the DNA ladder. The DNA molecule is composed of two strands that twist around each other like a twisted ladder. Each strand has a backbone made up of alternating sugar that is deoxyribose and phosphate groups. Attached to every sugar there is one of four bases including the adenine (A), cytosine (C), guanine (G), or thymine (T).