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Cellular Life
Asexual and Sexual

asexual reproduction

All independent life on this planet is cellular. The living cell is the smallest unit capable of showing all the "signs and symptoms" that define the phenomenon we call "life".

Large quantities of cells are produced by growth, DNA replication, chromosome separation and then cell division; a process termed asexual reproduction. This cycle of events occurs, using radically different means, in both major types of cells; prokaryotes and eukaryotes. It is the means whereby all the living things we can see (and cannot see without a microscope) are produced.

Starting with a single cell, asexual reproduction, therefore, produces large quantities of cells all containing the same biological information. All the cells in an individual multi-cellular animal or plant contain the same chromosomes, same DNA and the same collection of genes. Similarly, all the cells in a bacterial colony or clone are just copies of one another. They are all genetically identical.

Bacterial fission and eukaryotic mitosis are powerful mechanisms of producing copies of cells that all share the same biological information, carried as genes on their DNA molecules. However, apart from the slow process of mutation, there is no formal mechanism within this system of reproduction that can introduce variation; different combinations of genes and the products they produce.

sexual reproduction

One theory concerning how complex eukaryotic cells evolved on earth states that these large units of life came about by the fusion of small units of life; simple prokaryotic cells.

In this theory precursor prokaryotes that were very efficient in energy conversion fused with, and went into partnership with other precursor prokaryotes that were efficient in other tasks, such as trapping light, digesting food granules, storing genetic information, synthesizing proteins, etc.

Each tiny pre-prokaryote retained some of its original components (including a surrounding membrane), and gave up others in the process of becoming what we now see as organelles, such as the mitochondria, chloroplast, vacuoles, nuclei and reticulum.

Eukaryotic cells, therefore (if this theory is true) are partnerships of simpler cells all working together within the same membrane. Joining forces in this way makes the resulting partnership more efficient, faster growing and able to perform functions not possible in smaller, simpler cells.

Eventually, eukaryotic cells took this "partnership" idea to the next level and joined forces with one another to produce multi-cellular organisms with even more possibilities of form and function.

It is not difficult to see that, (if this theory is true), these first eukaryotic cells would also continue to fuse with one another long after they had accumulated all their organelles and other inclusions they needed or were using.

adding together the genes

Most importantly, after fusing with another similar cell, the resulting cell would now share all the biological information carried by the two original, different, individuals. The chromosomes, DNA and genes of two distinct cells would now be all be pooled together in a single, new, fused, cell.

Different genes mean the production of different proteins, different functions and different phenotypes. This in turn leads to much more variation, many more survival strategies, and more options for exploiting different environments or responding to environmental change. This is the raw material for evolutionary change.

Eukaryotic organisms flourished, and in the fossil record evolution took off like a rocket following the "invention" of variation produced by summing biological information. This process of "adding together biological information from two different individuals" was the basis of sexual reproduction.

Sexual reproduction in eukaryotic organisms involves mixing and then separating genetic information. In every generation the genes of two partners are mixed together to form new combinations of genes in their offspring. No two individuals produced this way are ever alike.

© 2002, Professor John Blamire