A single strand of DNA undergoing
transcription reads three prime to five prime AATCCGATCG. Reading five prime to three prime,
what will the sequence on the complementary strand of mRNA be? (A) TTCGGATCGA, (B) GGAUUCGAUC, (C)
UUAGGCUAGC, (D) AATCCGATCG, or (E) TTAGGCTAGC.
This question is asking us to
transcribe a sequence of DNA into mRNA. You’ll recall that when a gene
needs to be expressed as a protein, it first needs to be transcribed or copied into
mRNA. This process is called
transcription. This mRNA transcript can then be
converted into a sequence of amino acids in the polypeptide. This is called translation, and
once the polypeptide is formed, it can go on to fold into a protein with a specific
function. The enzyme that converts DNA into
mRNA is called RNA polymerase, which attaches to the DNA double helix as shown
here. Once attached, RNA polymerase can
unwind the helix and begin copying one of the DNA strands to form an mRNA transcript
of the gene.
RNA polymerase moves along the DNA
until it reaches the end of the gene and the mRNA transcript is released. Let’s look at this process of
transcription in a bit more detail to see how this looks in the DNA sequence. The sequence we’ll use is the
sequence in the question. Here you can see the two strands of
DNA. You’ll recall that DNA has
directionality. So, one strand is in the five prime
to three prime direction, while the opposing strand is in the three prime to five
prime direction. The sequence in this question is on
the three prime to five prime strand. The three prime to five prime
strand is actually what’s used as a template during transcription. So, once RNA polymerase binds and
unwinds the helix, which is now represented here, RNA polymerase can start adding
nucleotides to build the mRNA molecule.
Since the three prime to five prime
strand is used as a template, the corresponding mRNA, shown here as this green
arrow, will be assembled in the five prime to three prime direction. mRNA is
synthesized using the same complementary base-pairing rules as in DNA. In DNA, guanine or G pairs with
cytosine by forming hydrogen bonds indicated here as these black dots, and adenine
pairs with thymine. There is one exception. In RNA, there is no thymine, and
thymine is actually replaced by another nucleotide called uracil or U for short.
Now, let’s start filling in the
mRNA sequence by adding the complementary bases. Adenine normally base-pairs with
thymine, but since we’re forming mRNA and there is no thymine, uracil is used
instead. Thymine in DNA pairs with adenine
in mRNA, cytosine in DNA pairs with guanine in mRNA, and guanine pairs with
cytosine. Why don’t you pause the video and
see if you can work out the rest of the sequence?
Alright, now let’s fill it in. Therefore, the sequence of mRNA
read in the five prime to three prime direction is UUAGGCUAGC.