The type of mutation you are describing is called a point mutation. Chemicals can cause this or just an error of DNA replication. In it, a single nucleotide is exchanged for another, resulting in a new amino acid being substituted in the polypeptide chain. The new amino acid inserted in the chain, in this case, is UAG, rather then the amino acid CAG. The codon UAG is a STOP codon. It causes protein synthesis to end here and this will result in a shorter polypeptide chain in the protein being constructed. In reality, glutamine was supposed to be added here and it won't be and this would have been followed by a stop codon. A shorter protein can be partially functional or non-functional. This is called a nonsense mutation, a point mutation with a premature stop codon.
Well that kind of mutation could be harmful and could not be it depends on weather or not that mutation is in the "junk" or useless DNA and also weather or not the mutation changes the sequence as a "roll on effect" (meaning one changes and so do many down the line). but because you mentioned mRNA this means that this segment will be used as the coding to produce a protein, it could produce a completely different protein or it could have 0 effect. this mutation is a point mutation (where only one base is changed)