When you fill out your transcript, you will enter in the grades you've already received. You can mark in classes that you will be taking or are presently taking (that is, you do not have grades for them) as "current/future." They will be recorded in your app, but will not be included in the verification process or in the calculation of your GPA.
So in your case, you'd mark the microbio class as "current/future" and not enter a grade in for it, then complete and submit the rest of the application as you would normally. The time to verify your transcripts is not affected.
Note that when you do finish the class, you would not be updating your primary app with the new grade. The med school at which you matriculate will request a copy of all transcripts to be received before you start classes; it is at this time they would note the microbio class and apply it against your prerequisites.