Biology

What two chemical groups form the backbone of a DNA strand, and what type of bond links the nucleotides?

Alternating deoxyribose (sugar) and phosphate groups form the backbone, linked by phosphodiester bonds between the 3'-OH of one sugar and the 5'-phosphate of the next.

What is meant by the antiparallel orientation of the DNA double helix?

The two strands run in opposite directions: one is oriented 5'→3' while its complement runs 3'→5', so their phosphodiester backbones point opposite ways.

Which class of nitrogenous bases are purines and which are pyrimidines?

Purines (double-ring) are adenine and guanine; pyrimidines (single-ring) are cytosine, thymine, and uracil. A purine always pairs with a pyrimidine.

What does Chargaff's rule state about base composition in double-stranded DNA?

In dsDNA the amount of adenine equals thymine and the amount of guanine equals cytosine (%A = %T, %G = %C), because of complementary base pairing.

In which 5'→3' direction does DNA polymerase synthesize, and onto which group does it add nucleotides?

DNA polymerase synthesizes only in the 5'→3' direction, adding each incoming dNTP to the free 3'-hydroxyl group of the growing strand.