A student reads the following
sequence of mRNA bases: UACGAGAACCGA. They divide it up into the
following codons: UACG AGAA CCGA. What is wrong with this sequence of
codons? Option (A), the codons overlap. Option (B), codon should be three
bases long. Option (C), codon should be read as
DNA bases. Option (D), there is nothing wrong;
this sequence is correct.
Let’s start by summarizing the
structure of mRNA and defining the term codon. mRNA or messenger RNA is a message
that is transcribed from the DNA of a gene and can be translated to make the
corresponding protein. DNA is found in the nucleus of
cells. It contains the instructions for
building all of the proteins that make up our body. In the first stage of protein
synthesis, the required DNA sequence is converted into mRNA by a process called
transcription. The mRNA produced can then leave
the nucleus via a nuclear pore.
The next stage of protein
synthesis, translation, takes place at a ribosome. The mRNA attaches itself to a
ribosome, and transfer RNA or tRNA transports amino acids to the ribosome. The amino acid attached to each
tRNA corresponds to a triplet of nucleotides called the anticodon. During translation, the three
nucleotides of the anticodon on the tRNA base pair with the complementary codon on
the mRNA, which is also made up of three nucleotides. This ensures that the correct amino
acid is brought to the growing polypeptide chain.
In our example, the codon on the
mRNA CGA corresponds to the anticodon GCU on the tRNA. CGA codes for the amino acid
arginine. And the complementary codon and
anticodon ensure that the tRNA transports arginine to this section of the mRNA. And it is therefore incorporated
into the polypeptide chain correctly. These codons, or triplets of
nucleotides, are the fundamental basis of the genetic code. Each triplet of nucleotides in the
RNA sequence codes for one of 20 amino acids that make up proteins.
Now, let’s revisit our
question. The student has divided the mRNA
sequence into codons of four bases. We have just seen how codons need
to be three bases long. And so, the correct answer is (B):
codons should be three bases long.