Can an antivirus program's 'virus definitions file' be used to reverse-engineer a virus?

I failed an exam some years ago over that exact question. I said yes, every virus could be reverse engineered with a definition file and it was marked wrong. It seems that sometimes they antivirus folks cant figure out exactly how the virus works, but will figure out how to disable and remove it. Then they just incorporate this into their av and move onto the next virus. Somebody with a background in programming would have a far better idea about this and could explain in alot more detail than I can.