â¢ Medium-grained (1/16 mm to 2 mm) rocks are usually called sandstone. Medium-grained rocks can be composed of quartz grains, feldspar grains (arkose), or rock fragments and clay (graywacke). Sandstone (Photograph by Parvinder Sethi). Medium-sized grains in clastic sedimentary rocks form sandstone. This sample is a quartz sandstone.
Eukaryotes are those organisms in which the cell body contains a nucleus whereas prokaryotes are those organisms in which the cell body does not contain a nucleus. In amoeba, the reproduction is carried out under the instruction of the nucleus thus making it a eukaryote organism.
There are two primary types of cells: eukaryotic cells and prokaryotic cells. Eukaryotic cells are called so because they have a true nucleus. The nucleus, which houses DNA, is contained within a membrane and separated from other cellular structures. Prokaryotic cells however have no true nucleus. DNA in a prokaryotic cell is not separated from the rest of the cell but coiled up in a region called the nucleoid. As organized in the Three Domain System, prokaryotes include archaeans and bacteria. Eukaryotes include animals, plants, fungi and protists.