Video Transcript
The diagram shows two adjacent
nucleotides joined together via a bond. What type of reaction occurs for
this bond to form? (A) Oxidation, (B) condensation,
(C) synthesis, (D) hydrolysis, or (E) reduction.
Let’s first remember that
nucleotides are the units that make up nucleic acids, like DNA and RNA. DNA is a double-stranded nucleic
acid. If we look at just one strand of
DNA, nucleotides are like links on the chain that makes up the entire strand. Nucleotides have three components:
a pentose sugar, which is shown in blue in the diagram; a phosphate group, shown in
yellow; and a nitrogenous base, shown in pink. When nucleotides are linked
together, the sugar and the phosphate form part of the sugar–phosphate backbone of
the nucleic acid molecule. So how do these nucleotides link to
each other?
We can see in the diagram that the
phosphate group from one nucleotide is going to join to the pentose sugar of the
next nucleotide. If we look a bit closer at the
actual chemical structures of the nucleotides, we can see that both the phosphate
and the pentose sugar have a hydroxyl, or OH, at the point of connection. For a bond to form, the sugar loses
one hydrogen atom from its hydroxyl group, while the phosphate atom loses the entire
hydroxyl group. This leaves the oxygen atom in the
sugar free to form a covalent bond with the phosphorus atom in the phosphate
group. The phosphate group now serves as a
bridge, connecting the three prime carbon atom of the upper nucleotide to the five
prime carbon atom of the lower nucleotide. This entire structure is called a
phosphodiester bond.
The hydrogen atom from the sugar
and the hydroxyl group from the phosphate combine to form a separate molecule. This molecule has two hydrogen
atoms and one oxygen atom. As you’ve probably guessed, this
molecule is water. A reaction that involves two
molecules combining to form a larger molecule with the loss of a water molecule is
called a condensation reaction.
So what type of reaction occurs for
this bond to form? The correct answer is (B). Nucleotides are joined together via
a condensation reaction.
