Video Transcript
In this video, we’ll learn about
the genetic code. We’ll first cover how information
can be transferred from DNA to mRNA to protein using transcription and
translation. Then, we’ll discuss the genetic
code and how codons can be used to derive a sequence of amino acids from a sequence
of nucleotides. Finally, we’ll see what it means
when we say that the genetic code is nonoverlapping, degenerate, and universal. So, let’s get cracking and start
this video.
DNA, or deoxyribonucleic acid, is
often called the blueprint of life. It provides instructions for
building the hair on top of our heads and the toes on our feet. DNA is inside the nucleus of nearly
every cell. It’s a double-stranded molecule
that’s arranged as a helix. And spanning this DNA are genes
that make up the proteins that we need, like the collagen in our skin, for
example.
This gene has a specific sequence
of nucleotides or bases that provide the instructions for building these specific
proteins. Let’s go over the steps needed to
turn this gene into a protein. There’s actually two steps involved
called transcription and translation. We have a lot of genes, tens of
thousands in fact. So, if we wanted to make the
protein for collagen and none of these other genes, how is that done?
So, here’s one of our cells with
the DNA in blue, and here’s the gene for collagen shown in pink. The first thing that happens is
that the cell receives a signal to make collagen. Then, the collagen gene is
transcribed and a copy is made. This copy is called mRNA. This process of copying a segment
of DNA into mRNA is called transcription. Here, a section of DNA or the
collagen gene in this example is transcribed or copied into single-stranded
mRNA.
Let’s look a little bit more
closely at how this works. Here, the DNA to be transcribed is
unwound and opened by an enzyme called RNA polymerase. This enzyme then synthesizes mRNA,
shown here in pink, using one strand of the DNA as a template. Once transcription is complete, RNA
polymerase detaches from the DNA.
Let’s look a little more closely at
the sequence of DNA and mRNA here. Here’s the single strand of DNA
that’s being used as the template for transcription shown in blue and the transcript
or mRNA, which will be shown in pink. Here, the complementary nucleotides
are added to the transcript. So, G or guanine base-pairs with C
or cytosine, and T or thymine base-pairs with A or adenine. The same complementary base-
pairing rules for DNA apply to mRNA with one difference. Adenine pairs with U or uracil in
mRNA. So, uracil takes the place of
thymine in mRNA. So, instead of Ts in DNA, we have
Us in mRNA.
Now that transcription is complete
and we have our mRNA, let’s see what happens next. In the next step, the sequence of
nucleotides is converted or translated into amino acids to form the protein. This process is called
translation. This forms a polypeptide, which is
a chain of amino acids, shown here as these colored circles, that are joined
together by peptide bonds. There are 20 different common amino
acids. So, this pink circle can represent
the amino acid alanine, while this blue circle can represent the amino acid
leucine.
Because of the attractive and
repulsive chemical properties of each of these amino acids, the polypeptide chain
can then fold into the unique shape of the protein. This unique shape contributes to
the specific function of the protein.
So, how is it possible to go from
the sequence of nucleotides in mRNA to amino acids in the polypeptide chain? The answer to this is in the
genetic code. So, here we have a sequence of mRNA
that needs to be translated into a polypeptide. To decode this, we need to look at
these nucleotides in groups of three. This is called a codon. A codon is a sequence of three
nucleotides that code for an amino acid. So, the codon AUU can be translated
into the amino acid isoleucine, while the codon CAC codes for the amino acid
histidine. GGA codes for glycine, and UGC
codes for cysteine.
You might notice that the genetic
code is nonoverlapping. This means that the nucleotides
present in one codon are not present in the adjacent codon. So, after the first codon AUU, the
next codon is not UUC and is not UCA. These two codons would overlap with
the first codon. And since the genetic code is
nonoverlapping, the codon following AUU is CAC.
So, as we have seen, the sequence
in these codons correspond to specific amino acids. To see what each of these sequences
can be translated into, we can refer to a diagram called the codon wheel. Here, on the left, you can see a
codon wheel. This can be used to translate any
codon into its corresponding amino acid. To do this, we need to start from
the five prime end of the mRNA molecule, which is indicated here. And we have to work our way to the
three prime end, which is shown here.
So, for the codon AGG, we start
from the center of the codon wheel and circle the A here. Then we move outwards and circle
the G here. And then finally, we circle the
final G as indicated here. So, the codon AGG corresponds to
the amino acid arginine. Let’s try another one and look at
the codon for CAU. So, the C is indicated here, the A
is indicated here, and the U is here. CAU corresponds to the amino acid
histidine.
Oh, what’s this? A codon has suddenly been
added. I wonder what it corresponds
to? Why don’t we clear this codon
wheel, and then you can pause the video so you can try to figure it out. The corresponding amino acid for
the codon GCG is alanine.
Now, let’s talk about some other
features of the genetic code. You may have noticed that more than
one codon can code for the same amino acid. For example, histidine can be coded
by CAC and CAU. Both of these codons will give the
amino acid histidine. This means that the genetic code is
degenerate or redundant, meaning that more than one codon can be translated to the
same amino acid. The amino acid alanine can actually
be coded by four different codons. An exception to this is with the
amino acid methionine. Methionine is also sometimes known
as a start codon because it initiates translation. Likewise stop codons terminate
translation. These codons tell the protein
synthesis machinery where to start and end translation.
Now, let’s clean up the screen a
bit and talk about another feature of the genetic code. The genetic code is universal to
all life on Earth. So, the codon CCG codes for proline
in bacteria, in plants, in humans, and so on. This has had a huge impact on
biotechnology. For example, suppose we wanted to
produce a lot of insulin to treat diabetes in humans. Because of the universal nature of
the genetic code, we can insert this insulin gene into another organism, like this
bacterium, which can then be transcribed and translated to give the same insulin
protein in humans, which can then be harvested and used to treat diabetes. This way, we can make a lot of
insulin for a fraction of the cost of extracting it from humans.
Now, let’s look at a practice
question to apply what we’ve learned.
A sequence of DNA is transcribed
into an RNA sequence. This RNA sequence reads from five
prime to three prime GCUUUCACGCAC. Use the codon wheel provided to
determine the sequence of amino acids. Arginine, serine, threonine,
proline. Serine, leucine, alanine,
histidine. Alanine, phenylalanine, threonine,
histidine. Serine, leucine, alanine,
glutamine. Or alanine, leucine, threonine,
glutamine.
This question is asking us about
how to translate a sequence of mRNA into the corresponding amino acids. Before we can answer this, let’s
clear the answer choices and review some key points.
Let’s say our cell here needs to
produce insulin. The gene for insulin is located
here in pink in the cell’s DNA. In order for this cell to produce
insulin or any other protein, it must go through two processes called transcription
and translation. During transcription, the gene for
the protein is transcribed or copied to produce what’s called messenger RNA or
mRNA. This messenger RNA is a message for
the cell that tells it that it needs to make the protein, or insulin in this
case.
Transcription is the process of
converting a section of double-stranded DNA or our insulin gene shown here in pink
into a single-stranded mRNA molecule. Like DNA, the sequence for mRNA is
written in the five-prime to three-prime direction and contains four different
nucleotides or bases: adenine or A for short, guanine, cytosine, but instead of
thymine, which is in DNA, RNA uses uracil or U for short.
After transcription, the sequence
in the mRNA molecule can be translated into its corresponding amino acids. This step is called translation,
and it forms a polypeptide with each of these colored circles representing a
different amino acid. This polypeptide can then fold to
form its corresponding protein, or insulin in this example.
Now that we’ve covered
transcription and translation, let’s turn our attention to this mRNA sequence in the
question and describe how this specific sequence can be translated into amino
acids.
An mRNA sequence is translated in
groups of three nucleotides called a codon. A codon is a sequence of three
nucleotides that code for an amino acid. Codons are always read in a way
that they’re not overlapping. So, in this sequence, this is the
first codon, this is the second codon, this is the third codon, and here’s the last
codon. So, this mRNA sequence has four
codons.
Now, in order to translate the
sequence of nucleotides into its corresponding amino acid, we need to use a codon
wheel, like the one shown on the left. To use the codon wheel, we start
from the inside. This corresponds to the five-prime
end of the codon. And we work our way outwards to the
three-prime end of the codon. So for this codon, we’ll be working
from the five-prime to the three-prime end.
So, with the codon GCU, we start
with G, then we move over to C, and finally we land on U. So, the codon GCU corresponds to
the amino acid alanine. Then, for the next codon UUC, we do
the same thing. So, the first nucleotide is U, then
U again, and then C. This corresponds to the amino acid
phenylalanine. Then, for the codon ACG, this
corresponds to threonine. And finally, the codon CAC
corresponds to the amino acid histidine. Therefore, the corresponding amino
acid sequence for the given mRNA sequence is alanine, phenylalanine, threonine, and
histidine.
Now, let’s look at some of the key
points that we covered in this video. Information is transferred from DNA
to mRNA by transcription, then from mRNA to protein by translation. Groups of three nucleotides, or
codons, are translated to their corresponding amino acid. These codons can be decoded using a
codon wheel. Finally, the genetic code is
nonoverlapping, degenerate, and universal.