Question

why m-RNA is unstable

Answer 1

mRNA is unstable because of the environment in which it is present. When it is synthesized by RNA polymerase inside the cell’s nucleus (in eukaryotes), it is protected from nucleases by the nucleus membrane, but once it is outside to be read and translated by ribosomes, the clock starts ticking until nucleases chop down the RNA chain.

It is protected on one side by the 5′ guanine cap, because none of the nucleases are able to degrade RNA through this side, and any organism with an enzyme capable of doing this is already dead.

And on the other side (3´) it is protected by a long chain of adenines (known as a polyadenine chain) that is used as a sort of expiring date, because nucleases start by degrading mRNA through this site, and it is still readable until nucleases degrade the coding sequence.