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Molecular Biology: DNA structure and the central dogma · Nucleotides and the Double Helix

How DNA's building blocks, base pairing and antiparallel double helix give the molecule its stability and its ability to be copied.

Molecular Biology · 31 questions

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1. Which three components make up a single DNA nucleotide?

  1. Phosphate, ribose sugar and an amino acid
  2. Phosphate, glucose and a nitrogenous base
  3. Phosphate, deoxyribose sugar and a nitrogenous base
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Answer: Phosphate, deoxyribose sugar and a nitrogenous base

Each DNA nucleotide joins a phosphate group to the five-carbon sugar deoxyribose, which carries one of four nitrogenous bases.

2. What chemically distinguishes the sugar in DNA from the sugar in RNA?

  1. It carries an extra phosphate group
  2. It lacks an oxygen on the 2' carbon
  3. It has six carbons instead of five
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Answer: It lacks an oxygen on the 2' carbon

Deoxyribose means 'ribose minus an oxygen': it has a hydrogen where ribose has a hydroxyl group on the 2' carbon, which makes DNA chemically more stable.

3. Which DNA base is replaced by uracil in RNA?

  1. Cytosine
  2. Thymine
  3. Guanine
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Answer: Thymine

RNA uses uracil where DNA uses thymine; both pair with adenine, and thymine differs from uracil only by a methyl group.

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