Video Transcript
The diagram provided shows a
simplified outline of the structure of a single DNA nucleotide. What do X and Y represent,
respectively? (A) Phosphate group, ribose; (B)
phosphate group, deoxyribose; (C) Ribose, phosphate group; or (D) glycerol,
deoxyribose.
In this question, we are being
asked about the structure of a DNA nucleotide. You might recall that a DNA
nucleotide is an individual unit in a DNA molecule. A strand of DNA is composed of
multiple nucleotides joined together in a chain. Let’s go over the structure of a
single, typical nucleotide, which is made up of three distinct components.
We can see that the first
component, the nitrogenous base, has already been labeled on this diagram. The nitrogenous base in each
nucleotide is bound to a pentose, or five-carbon, sugar. The specific type of sugar depends
on what type of nucleotide this is, and we can often deduce the name of the sugar
from the name of the nucleotide. For example, each nucleotide in a
molecule of RNA, or ribonucleic acid, contains a ribose sugar. We are looking for the sugar that
would be present in a DNA nucleotide, however. And the full name for DNA is
deoxyribonucleic acid.
With this information, we can
deduce that the pentose sugar in a DNA nucleotide, which has been labeled in the
diagram provided by the question with the letter Y, is deoxyribose. Therefore, we can eliminate options
(A) and (C), as these both state that a ribose sugar is present in the DNA
nucleotide. And we now know that this is not
the case.
Let’s take a look at the final
components of nucleotide to discern the correct answer out of the two remaining
options.
The pentose sugar in any nucleotide
is bound to one or more phosphate groups. In a single DNA nucleotide
specifically, there is just one phosphate group present, which has been labeled here
with the letter X. As option (D) says that component X
is glycerol, we know that this option must be wrong because a DNA nucleotide
contains a phosphate group, not glycerol. This means that we can correctly
identify the answer choice that describes what the components labeled as X and Y are
on the diagram of a DNA nucleotide. The correct answer is (B):
phosphate group, deoxyribose.
