Biochemistry (this level at least) is a mixture of, you guessed it, biology and chemistry. A really cool class, interesting and loads of fun (no, I'm not being facetious). I think having taken chemistry helps, but it's not necessary. I really liked chemistry (I liked it so much I tutored that too!), but I probably would not have taken it unless I had to (as a prerequisite for other nursing schools I was considering). I'm pretty confident that if I didn't take chemistry beforehand, I would have been able to make it through biochem without too much difficulty, with all the videos and lessons on the internet, it should be relatively quick and easy to learn the very basics.
Personally, I wouldn't put off applying just to take a class that might help me a little when it's not actually required. You will get resources from WGU (etextbooks and access to video lectures) that will help you through the class.
For taking the TEAS, the chemistry is very helpful. But, if you study from the TEAS study manual, hopefully everything else you have studied should carry you over and cover the questions you miss. I was taking microbiology at the time I took the TEAS, and the week after I took the TEAS, we covered the questions I missed! If you have taken micro, GOOD! It'll help. The cool thing about the TEAS is that it's not a focused test, it's pretty general, consisting of a few questions on what seemed like every class I have ever taken from high school to college! That's good because there are so many opportunities to get the answers right! I didn't know any of the answers that had to do with micro, but I knew everything for some of the others.
Just keep in mind, ATI tests are so much about understanding the questions that are being asked. Use the manual, it is not a conclusive guide though, so take what you read in it and learn more about what you are unsure of. My cohort recently got the whole set of ATI books from WGU (for free) and I hope they will start doing that for all the nursing students...those books are invaluable!