Cells are the basal unit of all living organisms (except virusses, but these are not true, independently living organisms). There are two kinds of cells on the earth: types of cells: prokaryotic cells and eukaryotic cells (eu = true; karyon = nucleus, pit). let us review both types and notice their differences:
Prokaryotic versus Eukaryotic cells
Small-sized cell; about 1 µm
A few micrometers, mostly 10 µm, up to about 100 µm
Bacteria and Archaea
Elements of protists, fungi, plants and animals
Occur in general exclusively as single cells, but can also form a kind of clusters (biofilms)
Occur as single cells or as part of multicellular tissue
Single plasma membrane (=cell membrane)
Double plasma membrane
Relatively simple architecture, no organelles, at most compartments
Complex cellular structure; contain specialized organelles, surrounded by a own bilayered lipid membrane
Nucleoid (DNA-protein complex) with large circular DNA molecule, but no distinct nucleus, nor nucleoli, and in general no membrane that separates the DNA fromthe cytoplasm
DNA linearly arranged in a number of chromosomes packed in a nucleus with a nuclear envelop and nuclear pores
Ribosomes are in general smaller than in eukaryotes
Ribosomesarein general larger tan in prokaryotes
After DNA replication, original and replicate DNA attach to a different part of the cell membrane, and binary fission occurs
After DNA replication mitotic division occurs according to the stages prophase, metaphase, anaphase, telophase and cytokinesis
Older (more rimitive in evolution) and much more represented
Occured more recently in evolution and less abundant
populate greatest diversity in environment, can also live in extreme conditions as pH, temperature, salinity, gas, pressure, and show richest biochemistry
More restrained in colonization and biochemical pathways
DNA in Prokaryotes and Eukaryotes
Summary of the location of the genome in Prokaryotes and Eukaryotes
The genome of most prokaryotes is held within an long single circular DNA that is (super)coiled in loops to form a nucleoid. Nonessential genes are commonly encoded on extrachromosomal plasmids.
More on this topic in: Griswold, A. (2008) Genome packaging in prokaryotes: the circular chromosome of E. coli. Nature Education 1(1)
Artist representation (public domain image made by Mariana Ruiz Villarreal) of the typical structure of a bacterium, the group which together with Archaea is designed as prokaryotes.
In eukaryotic cells, like in the maize cell shown here, DNA is located in the nucleus, the mitochondria and the chloroplasts (occuring only in plants and some protists).
The nucleus contains most DNA. It is present in this compartment in the form of linear chromosomes that together constitute the genome. These chromosomes are identical and equal in number in all cells of an individual (except reproduction cells and mutated cells).
Mitochondria contain a relatively small amount of DNA tha is arranged in circular molecules. This DNA carries only a few mitochondrial genes. Most genetical information regarding the mitochondrion itself are present in the nucleus.
Chloroplasts also contain a limited amount of DNA, in circular or lineare arrangement. Like in mitochondria, also this DNA carries only a small part of the genes involved in making proteins required by chloroplasts (chromosomes in the nucleus are also for these organelles the main source of genes).
Remark: Mitochondria and chloroplasts divide independently of the nucleus and follw in principle an own time cycle. However, by the end of cell division they are divided with other cell organelles as a part of the cytoplasm over the new daughter cells .
Photograph: Transmission electron microscopy view of a maize cell: nucleus  with nucleolus  and mitochondria [eng]/eng