Organization and diversity of cells

All organisms more complex than viruses consist of cells. All cells are derived by cell division from other cells. Ultimately, there must be an unbroken chain of cells leading back to the first successful primordial cell that lived maybe 3.5 billion years ago. How that cell formed is an interesting question.

Prokaryotes lack a defined nucleus and internal organisation is simple. Under the electron microscope they appear featureless. They comprise two kingdoms of life: eubacteria which include most of the bacteria; and the archaea, resemble bacteria and often grow in unusual environments, such as in acid hot springs, saturated brines, etc. The genome of a prokaryote typically consists of a single small circular chromosome in which the DNA is not packaged in any obviously organized way. Prokaryotes may be simple, but they are not primitive .

Fig: Prokaryotic and eukaryotic cell anatomy.

Eukaryotes are thought to have first appeared about 1.5 billion years ago.The organization of the Eukaryotes is complex. Membrane bound- organelles including nucleus are present. Eukaryotic cells have several linear chromosomes in their cell nuclei, in each of which a single very long DNA molecule is elaborately packaged by histone and other proteins. The number and DNA content vary greatly between species .In general the genome size tends to parallel the complexity of the organism, but there are many exceptions. Humans do not have especially large genomes, while the cells of an onion and a lily contain respectively about five and 30 times as much DNA as a typical human cell.