Video Transcript
Which of the following best describes the structure of a nucleotide? (A) A nucleotide is a small unit made up of a glucose sugar, a phosphate group, and a base pair. (B) A nucleotide is a small unit made up of the four base pairs joined together by hydrogen bonds. (C) A nucleotide is a large subunit made up of a phosphate sugar group. (D) A nucleotide is a small unit made of a deoxyribose sugar, a phosphate group, and a base, either A, T, G, or C. (E) A nucleotide is a small unit that joins with other nucleotides to form a long chain.
Key knowledge necessary for selecting the correct option here is DNA structure. So, let’s review what DNA is made of and how it’s put together. DNA molecules are very long. The total length of DNA in a human body or somatic cell is greater than two meters. DNA is a polymer because it’s made up of similar small repeating units or monomers. The basic pattern of a DNA molecule is two spiraling chains of four types of monomer units called nucleotides. And nucleotides are the specific subject of our question.
The diagram here shows the connected strands of nucleotides without the spiraling or helical shape. And this allows us to determine that while option (E) is a true statement, it does not answer the question with the required description of a nucleotide. So, let’s review nucleotide structure.
First, nucleotides are made of three parts, and we’ll take a look at each of these one at a time. DNA stands for deoxyribonucleic acid. And the name indicates two things about DNA. DNA is a nucleic acid, and it contains the sugar deoxyribose. Deoxyribose is a five-carbon sugar, so they’re typically drawn as pentagons with an arm for the fifth carbon. Nucleotides also contain a phosphate group, and these are bonded between the deoxyribose sugars, forming the long sugar phosphate backbones of a DNA molecule. These sugar phosphate backbones run alongside each other but are oriented in opposite directions, so we can say they’re antiparallel.
The third part of a nucleotide is one of four types of bases: adenine, thymine, cytosine, or guanine. And the bases are represented by the color-coded rectangles in the diagram. As you may recall, the bases are paired across the DNA molecule according to the base-pairing rules, where A always bonds to T and C bonds to G. Two bases bonded together across a DNA molecule form a base pair. And the sequence of bases along the sugar–phosphate backbone is the genetic code. But let’s go ahead now and review the remaining four solution options and see which one best fits the information we have here.
Option (A) lists a glucose sugar, and we know that can’t be correct, since DNA nucleotides require deoxyribose sugars. Option (B) says there are four base pairs joined together and doesn’t mention a phosphate group or a deoxyribose sugar, so we can cross off this option as well. Option (C) mentions a phosphate sugar group. But the phosphate and sugar are not normally considered a group, and this option fails to mention the part of the nucleotide that is the basis of the genetic code, the base. So, the next option must be better. Option (D) is on the mark with the three parts of the nucleotide we just reviewed: deoxyribose sugar, a phosphate group, and a base, either an A, T, G, or C.
Therefore, the correct answer is (D). A nucleotide is a small unit made of a deoxyribose sugar, a phosphate group, and a base.
