Your DNA is like a library. The cells of your body access the information it needs from DNA so it can build some of its critical components. In DNA there are long lines of letters that are precisely arranged in order to create protein structure. To build one protein you typically need 1200-2000 pieces of information. These pieces of information can be mixed and matched and rearranged as necessary...just like you can pull different books from a library shelf and read them, your body can pull different pieces of information from your unique DNA.
Cats have cat DNA. Dogs have dog DNA. Humans have human DNA. We're all created with our own librarys. We can pull books off at will to read or adapt to a situation....this is referred to as "Microevolution" But we can't reinvent ourselves into something that is not represented in our DNA's makeup. (Macroevolution)
You say scientists have found fossils that are supposedly transitional (going from one species to another). Please point me to those.