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Question Video: Describing the Structure of Nitrogenous Bases Biology

Which of the following nitrogenous bases has a double-ring structure? [A] Guanine [B] Thymine [C] Cytosine [D] Uracil


Video Transcript

Which of the following nitrogenous bases has a double-ring structure? (A) Guanine, (B) thymine, (C) cytosine, (D) uracil.

DNA is made up of two strands that wrap around each other to form a double helix shape as shown here. Let’s zoom in so we can see more details of DNA’s structure and answer this question.

Here, we can see the two strands of DNA. Each strand is made of repeating subunits called nucleotides. Each nucleotide contains a phosphate group, a deoxyribose sugar, and a nitrogenous base. In DNA, there are four nitrogenous bases: guanine, shown in orange; cytosine, shown in blue; adenine, shown in green; and thymine, shown in pink. In contrast to DNA, the related nucleic acid RNA contains the same nitrogenous bases, except it contains uracil in place of thymine. Uracil is shown in red.

These nitrogenous bases form hydrogen bonds with each other, and this is what holds the two strands together. Let’s look at the chemical structure of these nitrogenous bases to answer our question.

Adenine and guanine are known as purines. And cytosine, thymine, and uracil are known as pyrimidines. As we can see here, adenine and guanine contain a double-ring structure that is not seen in cytosine, thymine, or uracil. Therefore, the nitrogenous base that has a double-ring structure is given by answer choice (A), guanine.

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