How can I take prerequisite courses if I'm out of school?

[TheLastJedi]

I am no longer in school and am currently working. I am considering applying for graduate programs in fields outside of my major, in which I may need to take prereqs for. However, it is not clear to me if I really need academic credit for something that may be a "soft" prereq. In my case, I'm a computer science major, and I may want to apply for a program in biomedical engineering or computational biology/bioinformatics (or maybe electrical/computer engineering). However, I don't have any biology courses on my transcript. Is that going to be an issue? Some schools don't mention if any prereqs are needed at all, while others seem to want most applicants to have some biology (or chemistry or physics) background, but isn't clear on how formal that has to be. I'd think med school would want formal credits, but do programs like PhD/MS engineering care about how you obtained your "background"?

I am making a distinction between learning something, versus getting recognition for what I learned for the purposes of admission. I am comfortable with learning biology or some other subject through taking MOOCs, OpenCourseware, books, etc., and I believe I can learn as much as I would in a real life class, but will the admissions committee accept it? There are two ways to approach this:

(1) Take MOOCs (Edx, Coursera, etc.) in the subject and mention somewhere in my application that this is where I got background from. If they can accept this, then does it matter if I pay for a verified certificate vs. taking it for free? Would it even be OK if I said I went through a course on OpenCourseware which doesn't come with a certificate, or read up something on my own?

(2) Take a course for official academic credit. This will almost certainly have to be an online course. There is a local university near me, but they don't have evening/weekend courses that I need. 

If I have to do #2 then where do I take online courses then? My guess is to go to an online school like Athabasca University or Open University UK, but I know when graduate schools look at a transcript they want courses from a "top" school - so would credits from these schools be looked down upon? Maybe I could check places like Penn State World Campus or University of Maryland University College that may be more reputable, if they offer individual courses. I would prefer not having to pay a lot for this though (which is why I prefer #1 if it is possible). Does anyone know of an reputable/inexpensive online school that they can recommend?

A caveat is that I can't do lab/experimental work with an online course - if grad schools expect that, then that's going to be pretty hard to resolve. 

Thanks for any advice.