Video Transcript
In a sequence of RNA, three bases
code for an amino acid. There are 64 possible
combinations. However, there are only 20 standard
amino acids in humans. What does this suggest? (A) We have not discovered 44 amino
acids yet. (B) The same amino acid can be
coded for by more than one codon. (C) The same codon can be
determined by more than one amino acid. Or (D) we have not discovered 44
codons yet.
This question is testing our
knowledge of the genetic code. Do you remember what that is? Let’s recall this together.
As you know already, the DNA of our
somatic cells contains our genome, which is the entirety of our genetic material
that encodes all of the information necessary to make us. Genes are regions of our DNA which
code for specific RNA or proteins. In the process of transcription, a
gene is transcribed by RNA polymerase into messenger RNA, or mRNA. As you can see in the image, the
mRNA transcript of a gene is a single strand shown in purple, as opposed to the
double-stranded DNA in blue above it. The RNA sequence is made of
nucleotides that bear four types of bases: adenine, guanine, cytosine, and
uracil.
Then, in the process of
translation, this mRNA is translated into proteins by a complex of ribosomes and
tRNA. The genetic code is the set of
instructions or the set of rules used by all living cells to translate a particular
sequence of RNA into a specific sequence of amino acids. Let’s have a look at how this
genetic code functions in more detail.
To create the protein from the mRNA
transcript, the ribosome screens the mRNA in the five prime to three prime
direction. The mRNA is read in groups of three
nucleotides, moving from one sequence of three nucleotides to the next sequence of
three nucleotides, and so on. Each group of three RNA nucleotides
is called a codon, and it will code for a specific amino acid. Each amino acid is brought to the
ribosome by a tRNA, a specialized RNA that bears one amino acid. This tRNA presents a sequence of
three nucleotides, called an anticodon, that is complementary to the mRNA codon. The ribosome reads each codon, and
the tRNA with the complementary anticodon will bring the corresponding amino acid to
the ribosome.
The amino acids brought by tRNA
molecules are bonded together by the ribosome to form a polypeptide chain. Once the polypeptide chain is
complete, it will fold and may join with other polypeptide chains to form the final
protein product.
Now that we have an overview of the
translation process, you may ask yourself how it was determined that codons are made
of sequences of three nucleotides. Why not two or four? We can use a little math to help us
answer this. If a codon were made of only one
nucleotide, then only four possible amino acids could be coded for because there are
only four different nucleotides. There are 20 amino acids, so this
doesn’t allow for as much diversity as we need. If codons were two nucleotides,
then by squaring the number four, we can see that this would allow for 16 amino
acids, which still isn’t enough. Finally, if codons were made up of
three nucleotides, we can see by cubing the number four that this allows for 64
potential amino acids to be coded for.
However, life on Earth doesn’t use
64 amino acids. There are hundreds of amino acids
available in nature, but all life on Earth uses only 20 amino acids. Therefore, because we have more
codons than amino acids, it means that some codons code for the same amino acids as
other codons. We say that the genetic code is
degenerate or redundant.
Here is a codon wheel, a useful
tool that we use to find out which amino acid corresponds to each possible codon
found in mRNA. To use it, we start in the center
and read outwards to find out which amino acid is coded for by our codon. If we observe this codon wheel
closely, we can see, for example, that the codons GUU and GUC both code for the
amino acid valine. Notice also that some codons code
for nothing. These are called stop codons as
they terminate the translation process.
Let’s look back at our
question. We now know that the answer is
(B). The same amino acid can be coded
for by more than one codon.
