The Cell is the smallest thing that can definatly be called alive. There are other 'organisms', for want of a better word, such as virii, prions and virions that are smaller, but whether these are alive is a matter for philosphers. The cell is definatly alive.

The first cells, on Sol III, appeared a long time ago, 3 500 000 000 years ago, only a thousand million years after the formation of the Earth and since then they have spread everywhere. From the specialised cells in your body, and all animals, through yeasts and plants all the way down to bacteria. This abundance meant that, to study them, we needed a way to start compartmentalising them into groups and catagories. Out of this came the three main types of cells.

The Main Cell Types

Though in many ways the types of cells are similar, there are some major differences. The eukaryotic cell will be the main one explored in this project, as, in its different forms throughout the world, it contains all the major parts of cells, and many which are not found in its more primitive cousins. However, where there is a major difference between the cell types both will be explored.

Eukaryote

Eukaryotes are the most complex of the cells. These are the cells that make up most multi-cellular organisms from yeasts to frogs to man. The main difference between eukaryotes and other cells is that eukaryotes have a nuclei and other membrane bound organelles¹. No one is quite sure how they formed, but one theory is that smaller creatures, now all extinct², started banding together, loosing the abilities provided other creatures in the amalgam, and eventually becoming too atrophied to be able to survive out of its group. Some people think that all that is left of these organisms are the organells in the cell.

Prokaryote

A typical prokaryotic cell is a simple bacterial cell, say Escherichia coli. No descrete organelles in the cell, a small size³ and almost ubiquetous on Earth, cells of this type can be found almost anywhere. A simple structure, with a relativley small amount of DNA, when compared to the eukaryotes makes for an extremely fast generation time. Given optimum conditions a typical prokaryote cell can reproduce every 20 minutes, or so. They are so fast that the offspring is already starting DNA⁴ replication, for the next generation, before being fully split from the original parent. Most human infections are caused by prokaryotic cells.

Archaebacteria

The remenants of the original cells that populated the Earth. These primitive cells are now only found in enviroments hostile to the other cell types, and such can be found in the most unusual places. Boiling hot springs, in areas with extremely high salinity, deep within the earth are some placess where these primitive cells can be found. These cells are of intrest to scientists, as the extreme conditions make for unusual adaptions. The Polymerase Chain Reaction, which is one of the most powerful tools available to molecular biologists⁵ is only made possible by a DNA polymerase found in an organism which lives in hot springs, in near boiling water.