Question

which of these is a difference between a dna and an rna molecule?
dna is a polymer composed of nucleotides, whereas rna is a polymer composed of nucleic acids.
dna contains nitrogenous bases, whereas rna contains phosphate groups.
dna contains five - carbon sugars, whereas rna contains six - carbon sugars.
dna contains uracil, whereas rna contains thymine.
dna is usually double - stranded, whereas rna is usually single - stranded.

Explanation:

• Option 1: Both DNA and RNA are polymers composed of nucleotides. Nucleic acids are made up of nucleotides, so this statement is incorrect.
• Option 2: Both DNA and RNA contain nitrogenous bases and phosphate groups. This is a common feature, not a difference.
• Option 3: Both DNA (deoxyribose - a five - carbon sugar) and RNA (ribose - a five - carbon sugar) have five - carbon sugars. So this is wrong.
• Option 4: DNA contains thymine, and RNA contains uracil. The reverse of what is stated here.
• Option 5: DNA is usually double - stranded (forming a double helix), while RNA is usually single - stranded. This is a key structural difference.

Answer:

DNA is usually double - stranded, whereas RNA is usually single - stranded.