Ribosomes are protein factories of cell and are composed of two subunits; one smaller and other larger unit I want to know that why ,ribosomes of eukaryotes have a 40s and 60s subunit and prokaryotic ribosomes have 30s and50s subunits ?If we combine 30s and 50s subunits shouldn't it be 80s ribosome instead of 70s ribosome .
I think I can get the point after your second comment.
Not only protein composition, but rRNA composition and sequence length of Bacterial and Eukaryotic ribosome is very different. And the difference of sequence lengths (and hence mass) in both take part in the difference.
The S units are an artifact of the historical way these RNAs were characterized, based on the rate at which they sediment in an ultracentrifuge. S stands for Svedberg, which is a non-metric unit that measures sedimentation rate, based on size and shape. Larger molecules sediment slower. A better way to refer to ribosomal subunits is by calling them large subunit (LSU), and small subunit (SSU), because ALL ribosomes consist of one SSU and one LSU. Even the 5.8S is simply a part of eukaryotic LSU. Put subunits together and the shape changes, so it they would not sediment at a rate equal to the sum of each subunit.