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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

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.