Video Transcript
In this video, we will learn to
describe the components that make up RNA molecules and differentiate between three
types of RNA: tRNA, mRNA, and rRNA.
In learing about genetics and
molecular biology, DNA is often in the spotlight. But DNA doesn’t act alone. Like a movie star can’t shine
without the movie crew, DNA wouldn’t be able to play its role without RNA. So let’s turn the spotlight on RNA
and give it the credit it deserves. As we’ll see, RNA and DNA share
some common similarities and differences. Let’s look at this in a bit of
detail as it relates to the composition and structure of these molecules. So here, we have a molecule of RNA
on the left and a molecule of DNA on the right. RNA is normally single-stranded,
while DNA is double-stranded. Let’s zoom in on both of these
molecules to see what the chemical structures are.
RNA and DNA are both considered
nucleic acids which are made up of repeating subunits called nucleotides. Each nucleotide is made up of three
different parts: the phosphate group as shown here and here, the pentose sugar as
shown here and here, and the nitrogenous base. You may have noticed that the only
difference between these nucleotides is the nitrogenous base. There’s different types of these
bases that we’ll talk about in a moment. In both molecules, the phosphate
group plays a role in establishing a phosphodiester bond between two consecutive
nucleotides. These phosphodiester bonds form a
backbone for both RNA and DNA that we call the sugar phosphate backbone. You can see these boxed in pink
here.
Notice that RNA has one sugar
phosphate backbone because it’s single-stranded, while DNA has two because it’s
double-stranded. You can also see the sugar
phosphate backbone indicated in black in both the RNA and the DNA diagrams, while
the different coloured lines indicate the nitrogenous bases. Now that we understand the general
structure of RNA and DNA, let’s look at some of the differences.
One of the differences has to do
with the pentose sugar. We’ll start with the pentose sugar
that’s in RNA. The carbons are numbered around the
pentose sugar with one prime, being a carbon that bonds to the nitrogenous base,
then two prime, three prime, four prime, and five prime bonds to the phosphate
group. Now let’s look at the pentose sugar
in DNA. There, the carbons are numbered one
prime to five prime, just like in RNA. Why don’t you pause the video and
see if you can see what’s different between the two? The difference is in the group
that’s attached to the second carbon of the pentose sugar.
The hydroxy group in RNA is what
makes this sugar a ribose. This is why we call it ribonucleic
acid, whereas in DNA we’re missing an oxygen and just have the hydrogen instead. And this is why we call it
deoxyribonucleic acid. This hydroxy group in RNA makes it
more chemically reactive. Because of this, RNA is less stable
or more fragile than DNA. This is why DNA is preferred by
organisms as a support for genetic information, and short-lived RNA is preferred for
transient messages in the cell.
Now let’s compare the nitrogenous
bases of RNA and DNA. You may recall that in DNA there
are four different nitrogenous bases as shown here: adenine as shown in green,
thymine as shown in pink, guanine as shown in blue, and cytosine as shown in
orange. Of these, adenine and guanine are
purines and have a double ring structure, whereas thymine and cytosine are
pyrimidines and have a single ring. You may also remember that
nucleotides can make hydrogen bonds with other nitrogenous bases. According to the rules of
complementary base pairing, adenine binds the thymine with two hydrogen bonds, and
guanine binds the cytosine by three hydrogen bonds. This is what forms these base pairs
as indicated here.
The two strands of DNA are held
together by these hydrogen bonds which are individually weak but collectively
strong. This double-stranded DNA molecule
can then twist and form the double helix shape that you are familiar with. Now let’s look at the nitrogenous
bases that we find in RNA. As you can see, the nitrogenous
bases of RNA are very similar to those of DNA with only one key difference. In RNA, we see uracil in place of
thymine. Uracil also belongs to the
pyrimidine family, and it pairs with adenine in the same way that thymine does,
using two hydrogen bonds. We can see that here. Although the nitrogenous bases of
RNA can base pair just like DNA, RNA generally doesn’t form a double-stranded
molecule like DNA. However, the ability of RNA to form
complementary base pairs with DNA is incredibly important, during the formation of
any type of RNA during transcription. Before we get into this, let’s
first review how RNA can transmit genetic information contained in DNA to the rest
of the cell.
This is a universal process in all
organisms. It has been summed up in what’s
called the central dogma of molecular biology. So here we have a stretch of DNA,
and within this DNA is a gene. A gene is a section of DNA that
codes for a functional unit such as a protein or some type of functional RNA
molecule like rRNA or tRNA that we’ll discuss later. This gene is transcribed in a
process called transcription. Transcription is the process of
converting DNA into RNA. Any type of RNA can be produced
from transcription. But in this example, we’ll be
discussing a special type of RNA called messenger or mRNA. Genes that code for proteins
produce mRNA after transcription. And this mRNA sequence can be
converted into an amino acid sequence during a process called translation. These amino acids can then fold to
form a protein with a specific function.
Now we’re going to look at this
process of transcription in more detail using mRNA as an example. Then, we’ll see how this mRNA can
be translated. During translation, there’s two
other types of RNA that we’ll be looking at. These are called rRNA and tRNA. We’ll first look at how
transcription works to produce mRNA as an example. But remember, transcription
produces any kind of RNA including the rRNA and tRNA that we’ll be discussing in a
moment. First, an enzyme called RNA
polymerase binds to the DNA. Then, the double helix is unwound
by breaking the hydrogen bonds between the nitrogenous bases. This exposes the different
nucleotides which then act as a template for mRNA synthesis. So here we can see the mRNA
molecule that’s being synthesized by RNA polymerase.
Different RNA nucleotides are
matched to the complementary DNA nucleotide, which are then added to the growing
mRNA molecule by RNA polymerase. RNA polymerase moves forward as
this mRNA molecule elongates. You can see the mRNA emerging from
RNA polymerase here. Transcription continues until RNA
polymerase reaches the end of the gene. Now that we have our mRNA molecule,
the nucleotides within it need to be translated into a sequence of amino acids to
form a protein. This process is called translation,
and it involves two more types of RNA that we’ll be discussing called rRNA and
tRNA. Translation is performed by an
organelle called the ribosome. The ribosome is made of two
separate structures called the large and small subunits. These two subunits combine together
to form the ribosome. The ribosome is made up of a
combination of proteins and ribosomal RNA or rRNA.
Ribosomal RNA does not code for
proteins like mRNA. Instead, it’s used as a structural
and functional component for the ribosome. The ribosome combine to mRNA and
start translation. Now that the ribosome is bound to
mRNA, specific amino acids need to be brought in to be matched to the corresponding
nucleotides in mRNA. This is done by another type of RNA
called transfer or tRNA. Like rRNA, tRNA is not converted
into a protein like mRNA. Instead, the RNA nucleotides fold
upon themselves to form hydrogen bonds as indicated by these black dots. This gives tRNA its characteristic
cloverleaf shape.
At one end of the tRNA molecule is
an amino acid. The amino acid that’s attached to
the tRNA is not a random choice. In fact, there is a sequence of
three nucleotides at the other end of the tRNA molecule that determines which amino
acid is attached. This sequence is called the
anticodon site. The three nucleotides in the
anticodon site can base pair with three complementary nucleotides in mRNA that we
call a codon. Each codon in mRNA codes for a
specific amino acid. So when these tRNA molecules bind
to mRNA, they bring the corresponding amino acid with them which can then be linked
together as we can see here. The ribosome can move along the
mRNA sequence to bring in the next tRNA molecule to join the next amino acid, while
the earlier tRNA exits.
This process continues, codon after
codon, until a special codon is reached called the stop codon. This causes the ribosome to
separate and the amino acid chain or polypeptide can be released. It’s important to point out that,
in reality, polypeptides can be much longer than what’s indicated here. If we look at a diagram of the
different parts of an mRNA molecule, we can see the stop codon indicated here. The ribosome actually binds toward
the beginning of the molecule, but it doesn’t start translation until it reaches a
start codon. The region in between is a coding
sequence, and this is what’s actually translated to the amino acid sequence.
The poly-A tail is another
structure that’s made up of around 200 adenine nucleotides. This helps to stabilize mRNA to
prevent its degradation so it can be translated. Now that we understand more about
the different types of RNA, let’s try out a practice question.
Ribosomes are formed from ribosomal
RNA and polypeptides. What role do ribosomes have in a
cell? (A) Controlling what enters and
leaves the cell, (B) providing the site for aerobic respiration, (C) transporting
enzymes around the cell, (D) intracellular signalling, or (E) providing the site for
protein synthesis.
In every living cell, DNA carries
the genetic information that gives the instructions the cell needs to properly
function. These instructions come in the form
of genes which are sections of DNA that can be converted into functional units, such
as proteins. When a gene needs to be converted
into protein, the sequence of DNA must first be converted into a molecule of
messenger RNA or mRNA by transcription. Then, the sequence of mRNA is
converted into a sequence of amino acids or a polypeptide in a process called
translation. This polypeptide can then go on to
fold into a protein with a specific function.
Translation is performed by an
organelle called the ribosome, which is made up of a special kind of RNA called
ribosomal RNA and polypeptides, as indicated in the question. This ribosome can attach to mRNA
and convert its sequence into a sequence of amino acids. This forms a polypeptide, which can
then fold to form a protein. Therefore, the role of the ribosome
in the cell is to provide the site for protein synthesis.
Now let’s take a moment to go over
the key points that we covered in this video. RNA, like DNA, is made up of
repeating subunits called nucleotides. RNA contains the nitrogenous bases
adenine, uracil, guanine, and cytosine. Unlike DNA, RNA is normally
single-stranded instead of double-stranded, contains a ribose sugar instead of a
deoxyribose, and contains uracil instead of thymine. In order to form proteins, genetic
information flows from DNA to mRNA by transcription which is then translated into a
protein. Transcription can produce any type
of RNA including rRNA and tRNA. Ribosomes, the organelle that
performs translation, are made up of rRNA and polypeptides. During translation, tRNA delivers
amino acids to the ribosome to be matched to the sequence in mRNA.
