Last Second Scoring

[agentorange]

Would be interested to know what the rule on the following situation in the NFL and College is:

Team A is down by 5 points there is 0:01 on the clock. Team A scores a TD to take a 1 point lead as time expires. Does Team A have to attempt the try?

I've seen highlights where a team scores to go up by 1 and rushes off the field but it seems to me like they should have to attempt the try which would give the defense a chance to score (at least in NCAA I believe the play is dead in the NFL if the D gets the ball).

I know that if a team is down by 6 and they score they can attempt the try to tie the game. But are they forced to do so?

Any input would be appreciated.