Video Transcript
Which of the following is not a
base found in RNA? (A) Adenine, (B) cytosine, (C)
guanine, (D) thymine, or (E) uracil.
This question asks us about
ribonucleic acid, or RNA. In order to answer correctly, let’s
review the structure of RNA and how it differs from DNA.
RNA and DNA are very similar
molecules. They are both nucleic acids, a
family of macromolecules that play an essential role in living organisms. They are large polymers made of
monomers called nucleotides. Nucleotides are composed of three
subunits: a sugar, a phosphate group, and a base.
The sugar is called a pentose
sugar, as it has five carbons. This pentose forms a single ring
containing four carbons and an oxygen atom. It is called a ribose. In the case of RNA, the ribose has
a hydroxyl group, shown as OH, on the second carbon of the ring. In DNA, the second carbon of the
ring doesn’t bond to a hydroxyl group, but just to a hydrogen atom. This explains why in DNA the
pentose is a deoxyribose.
The last element of the nucleotide,
shown here on the right, is the nucleobase, or more simply, the base! Each base is attached to the first
carbon of the ribose and contains nitrogen atoms that form either one ring or two
fused rings. The group of bases with one ring
are called pyrimidines, whereas the group of bases with two rings are called
purines. Purine bases are adenine and
guanine. Pyrimidine bases are cytosine,
thymine, and uracil.
Now, there is an important
difference in the bases contained in RNA and DNA. DNA contains the nucleotide bases
adenine, guanine, cytosine, and thymine. RNA also contains adenine, guanine,
and cytosine. However, thymine is replaced by
uracil in RNA molecules.
Now that we have reviewed the
nucleotide bases that are specific to RNA, we can answer our question correctly. In response to which of the
following is not a base found in RNA, the correct answer is (D), thymine.
