Practicing Success

Target Exam

CUET

Subject

Biology

Chapter

Molecular Basis of Inheritance

Question:

Read the given passage and answer the following question :

In 1953 that James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of DNA. One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one. The base pairing confers a very unique property to the polynucleotide chains. They are said to be complementary to each other, and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.

In DNA molecule, two polynucleotide chains are :

Options:

Parallel

Discontinuous

Semiconservative

Antiparallel

Correct Answer:

Antiparallel

Explanation:

The correct answer is Option (4) - Antiparallel

The DNA molecule consists of two polynucleotide chains that form a double helix structure. The DNA double helix is a right-handed helix, meaning the strands twist around each other in a right-handed manner along a common axis. 

The Double-helix structure of DNA  is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside. The two chains have anti-parallel polarity. It means, if one chain has the polarity 5'-3', the other has 3'-5'. The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.