Amino acids are the building blocks of proteins. Like lego! If you build a castle, that's the protein and the single blocks are the amino acids.

When we alter DNA, we usually switch one base for another (for example A for C) or remove one. This leads to a different mRNA and thus different protein.

Nitrogen has different jobs, it's a very important part of amino acids (see the structure I drew)

Yes and now I'm too lazy to correct it. It's phosphor in German.