[BioC] GO analysis for time-series data

[Srinivas Iyyer]

dear group, 

What are the GO analysis methods for time series data
analysis. 

experiment:

50 samples (cell line) treated with drug
4 time points (2hr, 4hr, 8hr, 24hr)

Once the clusters of genes identified for 2,4,8 and 24
hrs, what are the steps to identify over represented
GO groups. 

could any expert help me how to proceed further. 
thank you
srini