Question

Select the phrases that accurately describe properties of the most common form of the DNA double helix. 

There is more than one correct answer. Select all that apply. 

• DNA contains equal amounts of adenine and thymine, and equal amounts of cytosine and guanine. 

• A helical turn consists of about 10 nucleotides. 

• Base pairs have a spacing of 20 A. 

• The phosphodiester bonds between nucleotide residues run in opposite directions in the two strands. 

• The nitrogenous bases are exposed to the solvent, whereas the sugar-phosphate backbone of each nucleotide strand is on the interior of the double helix.

Answer 1

A deoxyribo nucleic acid (DNA) strand is composed of purines (adenine, and guanine) and pyrimidines (thymine, and cytosine). Here adenine base pairs with thymine and cytosine with guanine. Hence, in a DNA the concentration of adenine is always equal to thymine, and cytosine is equal to guanine. This shows that statement (1) is correct with respect to the structure of a common double helix DNA.

The DNA has a helical structure with each helix holding about 10 nucleotides in them. This shows that statement (2) is correct with respect to the structure of a common double helix DNA.

In a helix the base pairs are spaced at about 3.4 angstroms and the total spacing of a helix would be around 34 angstroms holding 10 nucleotides. This shows that statement (3) is incorrect with respect to the structure of a common double helix DNA.

The DNA strand are complementary to each other or in other terms the nucleotides in one strand of the DNA is opposite in direction and orientation in the two strands of DNA. The phosphodiester bonds are involved in joining sugar molecule of one nucleotide with the phosphate molecule of the next nucleotide. This shows that statement (4) is correct with respect to the structure of a common double helix DNA.

In a DNA molecule the sugar phosphate backbone is found at the outside, whereas the nucleotides are harbored within the backbone. This shows that statement (5) is incorrect with respect to the structure of a common double helix DNA.