The infinite variety: the beginning of life
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Oxygenating the World The arrival of blue-greens dictated the rest of the development of life. The oxygen they produced accumulated and created the atmosphere as we know it today. Atmospheric oxygen and ozone forms the screen which filters ultra-violet rays which provided the original energy to synthesize the first amino-acids and sugars. From primitive blue-greens the first single-celled organisms evolved (Eukaryots). Such organisms are called protista. Each celled organism is more complex than any bacteria and includes a DNA filled nucleus and elongated bodies called mitochondria which provide energy from burning oxygen. Some of these unicellular organisms have tail or flagellum which resemble the filamentous bacterium called a spirochaetae. These unicellular organisms may also contain chloroplasts (packets of chlorophyll which like blue-greens use energy from sunlight to assemble complex molecules as food for the cell). Consequently each of these tiny unicellular organisms appear to be a committee of simpler organisms. It is even possible that the first cells engulfed and incorporated bacteria and blue-greens to form a communal life (Endosymbiosis). Cells of this complexity first appeared about 1200 million years ago. 
One of the best examples of a protista is Parmecium. Paramecium Paramecium
Paramecium aurelia Scientific classification Kingdom: Protista Phylum: Ciliophora Class: Oligohymenophorea Order: Peniculida Family: Parameciidae Genus: Paramecium Müller, 1773 Protista – basic unicellular organisms These protistans and bacteria can reproduce by binary fission, but since their internal organization is more complex, the division process is more complex and includes the division of the separate structures within the cell. The division of mitochondria and chloroplast (each with their own DNA) may be independent of division of the main cell. 
Binary fission begins when the DNA of the cell is replicated. Each circular strand of DNA then attaches to the plasma membrane. The cell elongates, causing the two chromosomes to separate. The plasma membrane then invaginates (grows inwards) and splits the cell into two daughter cells through a process called cytokinesis. There are, however, other means of reproduction which involves the exchange of genetic material when two individual cells conjugate. Some protistans contain two complete sets of genes which after exchange of genetic material divide to make new cells with only one set of genes. These cells are of two types, a large and comparatively immobile one and a smaller active one that possesses a flagellum and are referred to as egg and sperm cells. When the two types unite in a new amalgamated cell the genes are once again in two sets but with new combinations of genes that occur from two parent sources. This sexual reproduction increases the possibilities for genetic variation and an accelerated rate of evolution. Download 2.01 Mb. Share with your friends:
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