What kind of knowledge is required to do a PhD? I am currently in an engineer school specialized in Computer Science, and I will be done with it in a little less than 2 years. I spent a lot of time on some advice websites and this stackexchange regarding PhD for the future, as I would like to do some research (based on a internship I did before), and later on join the academic world (or at least the teaching world).

The big issue I am facing is choosing a laboratory that I would like to join. The first thing that came to mind was of course Computer Science related, but the real question is : What kind of PhD can I join ?

What kind of knowledge is required before engaging in a PhD ? Could I join a bioinformatics laboratory and do a thesis on a specific protein because I have some general knowledge on how proteins work and use my computer science knowledge only as a tool, or does my thesis have to be computer science related as it is my main field ?



As for context, the main reason I am searching for another field than Computer Science is because I did not find yet a subject I am passionate about in this field. I have a lot of knowledge in a lot of different topics that I can apply at work, but none of them is something I would consider researching on.

On the other end, I am very passionate about different topics that I would love to contribute on and where computer science would be useful as a tool.
I think everything is about your motivation. You need a subject you're very interested in and of course you don't know everything yet you need to do the job. This is part of your work. In the end you’re getting paid to teach yourself 


  but none of them is something I would consider researching on    


This is a bit strange and dangerous. You’ll be working on a field for multiple years and you should choose an area you really really like. Imagine you're forced to do stuff every day you don't really like... You'll be demotivated and quit your research.

However, I think your future research should at least somehow be related to the stuff you do during your studies. Otherwise you have no clue what you're going to do or how your daily work will be.

Maybe you should talk to other PhD candidates from a group you might want to join after you studies - just to see how their daily work looks like and whether this might be the stuff you want to do for the next years.