why do RNA show smear instead of discrete bands?

[tracyliu]

if the isolated DNA is not treated RNase, then RNA are present, but how come RNA molecules don't get separated like the digested DNA fragments? aren't they also in different sizes?

[sorin]

They get separated like DNA, but remember how many different RNA molecules are in a cell,its not like chromosomes or fragments of restriction digests having a discrete size. Apart from this if you do not carefully prepare your samples RNAse contamination can occur quite often...

[JackBean]

as sorin wrote, there are plenty of RNAs in the cell each with different length (even the polyA can only differ)