Question Video: Calculating the Number of a Given Base in a Section of DNA | Nagwa Question Video: Calculating the Number of a Given Base in a Section of DNA | Nagwa

# Question Video: Calculating the Number of a Given Base in a Section of DNA Biology • Third Year of Secondary School

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In a section of DNA that contains 100 bases in total, 20 of these bases are thymine. Which of the following is the correct number of adenine and guanine bases? [A] 30 bases are adenine and 20 bases are guanine. [B] 20 bases are adenine and 30 bases are guanine. [C] 20 bases are adenine and 60 bases are guanine. [D] 80 bases are adenine and 20 bases are guanine.

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### Video Transcript

In a section of DNA that contains 100 bases in total, 20 of these bases are thymine. Which of the following is the correct number of adenine and guanine bases? (A) 30 bases are adenine and 20 bases are guanine. (B) 20 bases are adenine and 30 bases are guanine. (C) 20 bases are adenine and 60 bases are guanine. (D) 80 bases are adenine and 20 bases are guanine.

DNA is a biological macromolecule that carries genetic information. It is made up of two strands that wrap around each other to form the double helix shape as we can see here. Let’s zoom in on DNA’s structure so we can get a better look in order to answer this question.

Here, you can see a segment of DNA with its two strands indicated. Each strand is made up of repeating subunits called nucleotides. There are three components in a nucleotide: a phosphate group, a deoxyribose sugar, and a nitrogenous base. In DNA, there are four nitrogenous bases: guanine, represented in orange; cytosine, represented in blue; adenine, represented in green; and thymine, represented in pink.

One base on one strand can pair with the base on the opposite strand through hydrogen bonding. This is what holds the two strands together. Guanine pairs with cytosine and adenine pairs with thymine. This means that for every guanine, there is also a cytosine that pair with it in DNA. And for every adenine, there is also a thymine.

In this question, we are told there’s 100 bases in total. And 20 of them are thymine. If 20 bases are thymine, then 20 bases must be adenine. So that makes 40 bases between adenine and thymine. 100 minus 40 is 60. So there must be 60 bases between guanine and cytosine. Since there needs to be an equal number of guanine and cytosine bases, we simply divide 60 by two to get 30 bases for each.

So now we can see that the correct option is given by answer choice (B). 20 bases are adenine and 30 bases are guanine.

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