RNA is basic, DNA is sophisticated
It took a long time for life on earth to switch from a purely RNA based genome to the more stable DNA. RNA is still essential for life processes.
RNA is chemically simple and its ingredients are sythesied in Urey-style exeriments. So it is pretty likely that an alien lifeform will use RNA as an information encoding molecule. It may later develop DNA or something different.
While RNA is basic, the code is arbitrary
An exobiological life form will for sure have a different genetic code being incompatible with the Earth's life forms. Even if it uses the same bases (they look quite optimal for their purpose, but so-called exotic bases exist in nature and can be synthesised by chemists), the mapping to aminic acids is completely arbitrary and will be different for sure (for statistical reasons). The set of aminic acids will be also different.
There may be mirror life
Sugars, aminic acids, and bases are asymetrical molecules, they have mirror images with the same physical features (melting point, acidity, etc.) but that are still different. Biological life has selected for a long time one of the mirror forms to build their more complex molecules from and cannot use the mirror images in general. On another planet, the mirror images of the Earth's molecules may be chosen by life.
