Passage of genetic information to succeeding generations

The fundamental fact about living things is that they have a limited lifetime-a fact that most people, especially younger people, have inadequately dealt with. What that means is that the most important function of a organism is to replace itself.

The thing which identifies biological entities most clearly is their ability to reproduce.

Even viruses, which show no other traits associated with life, are able to reproduce themselves

Here we will consider the mechanisms that allow eukaryotic cells to faithfully reproduce themselves.

The forms of cell division.

The simplest form of cell division occurs in bacteria. Bacteria divide by a process called binary fission, a process of asexual reproduction. Essentially, the process involves the equal (or sometimes unequal) division of the cell and all of its contents.

The DNA content of the cell is divided between the two daughter cells. The actual division involves extending the plasma membrane and external cell wall across the bacterium to create two independent cells

The process of divsion in bacteria is highly regulated, as we will see is true of eukaryotes, but it occurs without an overt change in the appearance of the cell before the actual division.

Eukaryotes, because their cells are much larger in size and contain a much greater amount of DNA, divide by processes which are much more elaborate. The form of cell division which we will consider today is mitosis.

Mitosis is a process which generates two daughter cells which are genetically identical to the parent (in a sense, they are clones of the mother cell). Mitosis is an asexual process (meaning that it does not involve the generation of new individuals by fusion of unique germ cells, or gametes). It is used as a form of asexual reproduction by single-celled eukaryotes. Bodily growth in multi-cellular eukaryotes occurs by mitosis

The second form of cell division in eukaryotes, which we will get to next time, is meiosis.

It is essential that you recognize the fundamental difference between these two processes. Meiosis importantly creates cells that differ from the mother cell. Since meiosis is the mechanism by which gametes are formed it is important that they be unique to allow for genetic variability in a species. Since gametes must fuse to form a new organism in sexual reproduction, they must have half the number of chromosomes present in a somatic cell.

("Somatic" means literally "of the body" ("soma" = "body" in Greek), but refers to all cells which are not of the germ line, cells set aside for sexual reproduction.)