Yes, it contains all the DNA, but that doesn't mean it's the 'God of all Pokémon'. If so, there would be more information about Mew, story-wise, but in the beginning of the anime, manga and games, nobody knew about Mew. Of course, Arceus is fairly new, so nobody from the first games knew about Arceus, but in Sinnoh, and even in Johto from Generation IV, people know about Arceus, the dragon-trio and the lake-trio, which have been created by Arceus, as well as all the other Pokémon in the Pokémon World.

Mew is the ancestor of all Pokémon, because it carries the DNA of each Pokémon and can transform in every Pokémon, because probably in some unknown Pokémon history, Arceus also created Mew, and made it possible to transform into any Pokémon.

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