Birthed by gamma radiation, The Incredible Hulk grew to his immense size out of the scrawny Bruce Banner and warped the scientist’s DNA to get that prodigious strength. But how does the big green meanie get so shredded and gain an avocado hue by twisting up Banner’s DNA?
Stan Lee and Jack Kirby wanted a Dr. Jekyll and Mr. Hyde character and mucked around with fictional science to have Banner transform when under emotional duress into the rampaging Hulk. His transformation is not elaborated on in any comprehensive way in comics other than a large dose of gamma radiation created the Hulk and can even separate the green guy from Banner. But blowing up to 7 or 8 feet tall and possessing nearly limitless strength, Hulk’s DNA has to change for that to be even remotely feasible. A real-life dose of gamma radiation would likely mutate or destroy a normal person’s DNA; similar to what happened to chemist Marie Curie, who died of leukemia related to gamma radiation poisoning.
A Stanford biologist posited that there is some advanced genome editing involved with creating the Hulk. Sebastian Alvarado, a postdoctoral research fellow in biology at Stanford, picked up where Lee and Kirby gave up on a scientific explanation and suggests, while it’s a stretch of the imagination, Banner’s double-helix DNA could have been utterly broken by gamma radiation. In a 2014 Stanford news article Alvarado postulated that Banner’s DNA repaired itself, but miscoded some of the links and the DNA was subsequently triggered by anger, making him into the Hulk. Hulk has his sickly green coloration due to comic publishers being unable to accommodate Lee’s initial preferred design of a grey Hulk. But Alvarado suggests the green is due to the traumatic reaction of Banner’s body stretching to a great degree and forming a whole-body greenish bruise.
What do comics have to say about this? It’s mostly left to jargony science terms that loosely explain how gamma radiation changed Banner’s DNA. But the gist is that, where a normal human’s DNA would link in the common recipe for a genome, Banner’s DNA was broken and repaired in such an incorrect configuration that it allows for the Hulk to be born. But rather than the DNA mutation that normally occurs over many generations, Banner’s DNA changes quickly and reacts to emotion.
Now, all this science isn’t the full explanation. Hulk lives in Banner’s consciousness as a split personality and loathes being trapped behind Banner’s emotional repressions. But he has been split from Banner in a number of ways. Doctor Doom removed parts of Hulk’s brain (yeah Doom really does have a PhD) and moved them into a cloned body of Banner in Incredible Hulk #5. These Banner brain parts formed a consciousness of their own, but the clone later goes insane due to the separation from Hulk before being ultimately killed. Then some kind of magic or whatever reinserts Banner’s consciousness into the Hulk, it’s not well explained.
And leaving things unexplained is a favorite tool of comic book writers. Hulk’s transformation isn’t clearly delineated to the degree that would show where his DNA is altered. Most have left it up to interpretation as to how Hulk transforms. It’s clear the twin personalities exist in one body that was mutated at the genetic level. But Hulk and Banner share a deeper spiritual and emotional bond that reflect the feelings Banner has repressed.