How much depth should I have into a topic before I can claim it to be one of my research interests? I have been applying to Ph.D. programs in bioinformatics and have already gotten interview invitations from a few of them. My background is mainly on the computational side but I am interested in applying that knowledge to biological problems. 

In my application I mention that one of my research interests is modelling physiological systems (e.g. the cardiovascular system). However, I have not done substantial reading on the topic other than from a few websites and Wikipedia (mainly because of the little free time that I currently have as an undergraduate). From what I have read, I find the field to be very appealing and something that I would like to do research on. Nevertheless, my knowledge is basic at best and I am wondering if I can claim it to be one of my research interests.
You can be interested in Japanese even if you don't know more than 3 words in Japanese.

You can say it is your interest.

If they ask about your research experience - then usually a publication is a good indicator that you have some knowledge of the topic.