Dallmer el al., 1966, reported that the endoplasmic reticulum is originated from the plasma membrane by invagination giving rise to a canalicular system.
But De Robertis et al., 1970, suggested that the endoplasmic reticulum originates from the nuclear envelope which it resembles physically and chemically.
Both eukaryotic and prokaryotic cells contain ribosomes. These are the smallest structures of the cell which can be seen only under the electron microscope, first reported, by Palade (1953).
These are roughly spherical bodies of about 150A diameters and are not bounded by any membrane.
They are found usually attached to outer surfaces of the ER and its vesicles (Palade, 1953). Each ribosome consists of two structural sub-units, one slightly larger (60 Svedberge or 60S sub-unit), than the other (4QS sub- rait).
The ribosomes remain attached with the membranes of ER by 60S sub- units. The 40S sub-units occur on the larger sub-units and form caplike structure.
Ribosomes are originated in nucleolus and are composed of relatively small molecules of RNA and of protein.
Petermann, 1964, reported several kinds of RNA from ribosomes on the basis of analysis of purine and pyrimidine bases.
These are ribosomal RNA’s or rRNA. 5S, 18S and 28S rRNA’s. The 28S and 5S rRNA’s occur in large (60S) subunits, while 18S rRNA occurs in the smaller ribosomal subunits.
Ribosomes are the sites where all the proteins of the cell, including all enzymes, are synthesized.
These literally carry out the instructions contained within the DNA code of the nucleus. In cells with rough endoplasmic reticulum, the membrane ret-work forms a pathway by which substances are transported in and out of the cell.