Question Video: Understanding the Rules of the Genetic Code for Reading Codons Biology

Video Transcript

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.