Video Transcript
A single strand of DNA has the following sequence: five prime ATTGGGCCATATA three
prime. Reading from the three prime to five prime direction, state the sequence of bases
that will form on the complementary strand. (A) TAACCCGGTATAT, (B) ATTGGGCCATATA, (C) GAACCCTTGAGAG, (D) TCCAAAGGTCTCT, (E)
CGGTTTAACGCGC.
Let’s first look at the basic structure of DNA. Remember, each DNA strand is formed from nucleotides. Each nucleotide consists of a phosphate group, deoxyribose sugar, and a nitrogenous
base. For our drawings, phosphate groups will be circles and sugars will be pentagons. Each carbon in the deoxyribose sugar is given a number, referred to as one prime to
five prime.
When looking at the DNA double helix, the backbone of the molecule is created by
alternating phosphate groups and sugars with a nitrogenous base separately bound to
each sugar. There are four types of nucleotides in DNA: adenine, guanine, cytosine, and thymine,
abbreviated as A, G, C, and T. Adenine binds to thymine, and cytosine binds to guanine.
Now we need to understand what five prime and three prime mean. DNA has a directionality to it. We have already seen that the carbons of the deoxyribose sugar are numbered one prime
to five prime. If you look at an individual nucleotide, the five prime end is the one with the
phosphate group attached. And the three prime end is the one with an OH, called a hydroxyl group, which you
will often see omitted from drawings.
Complementary DNA strands are referred to as antiparallel. This means they are oriented in opposite directions. The five prime end of one strand bonds to the three prime end of the other
strand.
Let’s look at an example where three nitrogenous bases read ACG in the five prime to
three prime direction on one strand. Recall that the other strand will have both complementary pairing and opposite
orientation. So it would read TGC in the three prime to five prime direction.
Now we can take this information and apply it to our question. The sequence of the first strand is listed in the five prime to three prime direction
as ATTGGGCCATATA. To work out the correct sequence of the complementary strand in the three prime to
five prime direction, we must start at the five prime end of the first strand and
list each complementary base of the sequence, which gives us TAACCCGGTATAT.
So now that we have worked out our complementary strand, we can select the answer
choice with this sequence. The correct answer is (A), TAACCCGGTATAT.
