Video Transcript
An mRNA sequence is provided below:
five prime AUG GGG CCC UAG GAC UAG three prime. What is the sequence of the
template DNA strand that was used to create the mRNA strand? (A) Three prime TAC CCC GGG ATC CTG
ATC five prime. (B) Five prime TAC CCC GGG ATC CTG
ATC three prime. (C) Five prime AUG GGG CCC UAG GAC
UAG three prime. Or (D) three prime AUG GGG CCC UAG
GAC UAG five prime.
DNA is a double-stranded helix,
which is made up of a combination of four nucleotides: adenine, or A for short;
cytosine, C; guanine, G; and thymine, T. Nucleotides on one strand can
base-pair with nucleotides on the opposing strand of DNA in a very specific way. Adenine can base-pair or form
hydrogen bonds with its complementary base thymine. And guanine can base-pair with
cytosine. You can remember which nucleotide
can bind with which by looking at the shape of the beginning letter. The nucleotides with the rounded
letters G and C bind together. And the nucleotides with the
letters with edges A and T can bind together.
Before getting into translation,
let′s unwind a short piece of DNA to have a closer look at it. Note that DNA has
directionality. And its upper sequence is written
in the five prime to three prime direction, while the lower sequence is written in
the three prime to five prime direction. During translation, RNA polymerase
will attach to the DNA strand and unwind the DNA helix. Once the two strands are separated,
RNA polymerase can use one of the strands as a template to start forming the mRNA
transcript. The three prime to five prime
strand is used as a template. And bases are added following
complementary base-pairing rules.
It is important to know that RNA is
made up of nucleotides but does not contain thymine but uses the nucleotide uracil
instead. So, while in DNA normally thymine
would base-pair with adenine, in RNA adenine pairs with uracil. So G pairs with C, T pairs with A,
and A pairs with U. To make things clearer, let′s
create an RNA sequence pairing with our example. RNA polymerase would aid to build
the following RNA strand: GUC AAG. This is because G binds with C, U
binds with A, C binds with G, and A binds with T. To distinguish DNA from RNA, the
RNA backbone in this image is represented in blue.
Note that the produced mRNA strand
is again directional. And as the three prime to five
prime DNA strand was its template, its direction is five prime to three prime. With this information, we can now
find out what the sequence of the template DNA strand was that was used to create
the given mRNA strand.
The given mRNA strand starts at
five prime, so the DNA strand has to start at the three prime end. This information allows us to
exclude the two answer options (B) and (C). Because we know that thymine is
found in DNA strands and uracil is found in RNA strands, we can exclude answer
option (D) as well. This would indicate that the
correct answer option is option (A). However, with what we learned in
this video, we can check, so let′s do this.
Remember, we backtrack an RNA
strand to a DNA strand. So the adenine in the RNA strand
binds to a thymine on a DNA strand, not to uracil. The uracil on a RNA strand couples
with an adenine on a DNA strand. Guanine and cytosine couple with
each other no matter if we deal with RNA or DNA strands.
By filling in the remaining
nucleotides, we can confirm that the correct answer is given by answer option (A):
three prime TAC CCC GGG ATC CTG ATC five prime.
