Video Transcript
Which of the following describes
the five prime end of DNA? (A) The five prime end of DNA is
the one with the hydroxyl group. (B) The five prime end of DNA is
the one with the phosphate group. (C) The five prime end of DNA is
the one with the covalent bond. (D) The five prime end of DNA is
the one with the ribose sugar.
DNA is a biological macromolecule
that carries the genetic information. It is made up of two strands that
wrap around each other to form the double helix shape, as shown here. Let’s zoom in so we can see the
chemical structure of DNA to answer this question.
Here we can see each of the two
strands of DNA. Each strand of DNA is made up of
repeating subunits called nucleotides. There are three components to a
nucleotide: a phosphate group, a deoxyribose sugar, and a nitrogenous base. In DNA there are four nitrogenous
bases: guanine, shown in orange; cytosine, shown in blue; adenine, shown in green;
and thymine, shown in pink. These nitrogenous bases can pair
with each other using hydrogen bonds to keep the two strands together.
DNA has directionality, and one
strand has an opposite orientation compared to the other. We can describe this orientation by
numbering the carbons in the deoxyribose sugar. Let’s look at the deoxyribose in
this nucleotide. The first carbon is called one
prime and attaches to the nitrogenous base. The subsequent carbons are numbered
clockwise. The five prime carbon is outside of
the ring structure and attaches to the phosphate group. The three prime group has a
hydroxyl group that’s reacted to form a bond with the phosphate on the adjacent
nucleotide. You can see it here when it isn’t
attached to a phosphate. If we number the carbons on the
opposing strand, we can see that everything is flipped around. This shows that the two strands
have opposite orientations.
Now to answer our question, the
five prime end of DNA is correctly described by answer choice (B). The five prime end of DNA is the
one with the phosphate group.
