Video Transcript
The diagram shows a simple outline
of what happens during transcription. As RNA polymerase moves along the
DNA strand, it initiates the formation of bonds between the RNA nucleotides. What type of bonds form between RNA
nucleotides?
In order for a gene in DNA to be
converted into a protein, it needs to go through a couple of steps. The first step is transcription and
is where the gene in DNA is copied to form an mRNA transcript. The sequence of nucleotides in mRNA
can then be translated to a sequence of amino acids to form a polypeptide. This polypeptide can then fold into
a protein.
This question is asking us about
transcription, so let’s review that in some detail. The major enzyme of transcription
is called RNA polymerase, and it first binds to the promoter region of a gene in
DNA. Next, RNA polymerase unwinds the
double-stranded DNA molecule. It does this by breaking the
hydrogen bonds between base pairs. Now that the bases on DNA are
exposed, free RNA nucleotides can be added to form a strand of complementary mRNA to
one of the DNA strands.
This process continues as RNA
polymerase moves along the DNA. Along the way, the double helix is
unwound and new RNA nucleotides are added to the growing mRNA molecule. Once a stop signal is reached in
the gene in DNA, the mRNA is released. So it can exit the nucleus to be
translated into a polypeptide, which can then fold into a protein.
Now that we’ve reviewed the steps
of transcription, let’s take another look at the provided diagram. This represents the part of
transcription where new RNA nucleotides are being added to a growing mRNA strand,
while RNA polymerase is moving along the DNA molecule. This question is asking about the
type of bond that is forming between the RNA nucleotides to join them to the growing
mRNA strand. Let’s look at this in more
detail. Here we can see the strand of DNA
on the bottom and the strand of mRNA on the top. Let’s remind ourselves of some of
the key components of these molecules.
Each strand of DNA or RNA is made
up of repeating subunits called nucleotides. We can see a free nucleotide here
that hasn’t yet joined the mRNA strand. Each nucleotide has three
components: a phosphate group, a pentose sugar — in RNA the sugar is ribose, and in
DNA it’s deoxyribose — and a nitrogenous base. These include guanine, or G for
short, cytosine, adenine, and thymine in DNA. RNA has the same bases except in
place of thymine it uses uracil. These bases pair with each other to
form base pairs. We can see that this incoming free
nucleotide is going to base-pair with adenine.
When it does this, RNA polymerase
can join this nucleotide to the existing mRNA strand by reacting the phosphate group
on the free nucleotide with the hydroxy group on the terminal nucleotide of the mRNA
strand. This forms a bond, called the
phosphodiester bond, which includes the bond between the phosphate and the two
adjacent carbons on both sides. Now the free nucleotide is joined
to the mRNA strand, and the free hydroxy group can be used to join the phosphate
group from the next free nucleotide that base-pairs with the DNA strand. This process continues as mRNA is
synthesized. Therefore, the type of bond that
forms between RNA nucleotides is called a phosphodiester bond.
