Question Video: Identifying Complementary Bases in DNA Biology

The two strands of DNA are held together by complementary base-pairs. In DNA, which other base does C bond to? [A] A [B] T [C] C [D] G [E] U. In DNA, which other base does T bond to? [A] U [B] C [C] G [D] A [E] T

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Video Transcript

The two strands of DNA are held together by complementary base pairs. In DNA, which other base does C bond to? A, T, C, G, or U. In DNA, which other base does T bond to? U, C, G, A, T.

The key knowledge required to select the correct solution options here is DNA structure, specifically how DNA bases pair together. So let’s review DNA structure, the molecule that stores the genetic information of life.

The question starts out by mentioning the two strands of DNA. So let’s start by taking a look at what the strands of DNA are made of. The long sides of a DNA molecule are chains of alternating deoxyribose sugar and phosphate groups that act sort of like backbones. So these long chains are called the sugar phosphate backbones of DNA. These backbones are parallel while running in opposite directions, as you can see. And this orientation is called antiparallel.

Between the sugar phosphate backbones are the complementary base pairs from the question, with each base bonded to one sugar, along the inward-facing sides of each sugar phosphate backbone. DNA has four types of bases, often called nitrogenous bases: adenine or A, thymine or T, cytosine or C, and guanine G.

You can see that A and G are larger than T and C. And each of the larger A and G bases pairs with one of the smaller bases. Specifically, A pairs with T and C pairs with G. This is known as Chargaff’s base pairing rules, after Erwin Chargaff who discovered them in the mid-1900s.

One helpful way to remember these rules is “Apples grow in Trees and Cars go in the Garage.” And we can use this information to answer our question. We know that C bonds to G and T bonds with A. Therefore, the answer to our first question, which other base does C bond to, is G. And the answer to the second question, which other base does T bond to, is A.

You may be wondering why the letter U shows up in the solution options at all. U stands for the base uracil, which replaces the base thymine in the single-stranded nucleic acid RNA. But the answers to our questions here are G for guanine and A for adenine.

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