### 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.