Nucleic acids practice questions.

Identify the three components of a single DNA nucleotide.

• A phosphate, a deoxyribose and a nitrogenous base
• A phosphate group, a ribose sugar and an amino acid
• Two sugars and one nitrogenous base
• A fatty acid, glycerol and a nitrogenous base

Answer: A phosphate, a deoxyribose and a nitrogenous base

A DNA nucleotide is made of a phosphate group, the pentose sugar deoxyribose and one of four nitrogenous bases.

In DNA, adenine always pairs with which base?

• Guanine
• Cytosine
• Thymine
• Uracil

Answer: Thymine

Complementary base pairing in DNA joins adenine with thymine (A–T) and guanine with cytosine (G–C).

State how many hydrogen bonds form between guanine and cytosine.

• 1
• 2
• 3
• 4

Answer: 3

Guanine and cytosine are held together by three hydrogen bonds, making the G–C pair more stable than the A–T pair.

The two strands of a DNA double helix are described as antiparallel. Explain what this means.

• Both strands run in the 5'→3' direction
• One strand runs 5'→3' while its complementary strand runs 3'→5'
• The two strands are identical copies
• The strands are joined by phosphodiester bonds between the bases

Answer: One strand runs 5'→3' while its complementary strand runs 3'→5'

Antiparallel means the sugar-phosphate backbones run in opposite directions, one 5'→3' and the other 3'→5'.

Distinguish RNA from DNA by identifying a feature unique to RNA.

• It contains thymine
• It contains the sugar deoxyribose
• It contains uracil and ribose
• It is always double-stranded

Answer: It contains uracil and ribose

RNA uses ribose as its sugar and the base uracil in place of thymine, and is typically single-stranded.

Which two bases are purines?

• Cytosine and thymine
• Adenine and guanine
• Guanine and cytosine
• Thymine and uracil

Answer: Adenine and guanine

Adenine and guanine are the double-ringed purines; cytosine, thymine and uracil are single-ringed pyrimidines.