This is a monster movie in the style of Jaws - nature runs amok and all that, except that in this case the animal is an alligator that was was flushed down the toilet as a baby and then grew to giant size due to chemical pollution. (I discuss the alligator parts of the movie here.) There's also a romance between two people the likes of which have rarely been seen in a movie - David, an honest cop and a beautiful woman herpetologist named Marisa. Unknown to Marisa, the giant sewer alligator is her pet alligator named Ramon that her father flushed down the toilet 12 years earlier. Marisa has grown up to be an alligator expert (Robin Riker) who wrote a book about crocodilians, and works as a herpetologist for the city. (Because Chicago needs a full-time herpetologist?) Men like to call her the Snake Lady or the Lizard Lady.
In the rattlesnake scene, David has seen a giant alligator alive in the sewer so he goes to talk to Marisa, the herpetologist and alligator expert. When we first see her, she is in her lab milking a rattlesnake of its venom (because that's what all city herpetologists do every day, right?) She doesn't believe David either, telling him that it's impossible for an alligator to grow so large. But after Ramon eats a sleazy journalist who managed to get some pictures of the gator before he became lunch, everybody sees that the monster alligator is for real. Marisa accompanies David as he searches for Ramon and eventually we see them in bed together. (They probably don't live happily after, though, because movie cops are incapable of any kind of fidelity except to their job and their neighborhood bartender.)
We see a rattlesnake that looks like a Red-diamond Rattlesnake in a hand with manicured fingernails so we will think that it's a woman's hand, but it could also be a man's hand. Then we see Marisa holding a snake that is not a rattlesnake, but a gophersnake, as she walks by a glass terrarium that contains a second rattlesnake. In a nice touch of continuity, her fingernails are the same color as those holding the rattlesnake. Then she puts the gophersnake into a glass terrarium. I don't blame them for switching snakes. You can't ask an actor to handle a venomous snake. The difference between the snakes is very obvious, but I suspect most of the audience didn't notice.