Evolution of Cells
(Cooper, 1997 p. 4-15)
In spite of the differences between prokaryotic and eukaryotic cells, the same basic molecular mechanisms govern all cells. Similarities in DNA and protein structures would indicate that all life is related. These similarities allow knowledge of one cell type to be extrapolated and generalized to other types of cells.
Evolution is the change in the genetic makeup of a population due to the selective advantage of some variation in the population.
From geological evidence the time scale of evolution for this world started with it's formation about 4.6 billion years ago (Fig. 1).
The formation of organic molecules in the reductive environment thought to be present on the early earth was demonstrated by Stanley Miller. (Cooper, Fig. 1.2)
The first genetic material was probably made up of RNA because of it's replicative and catalytic properties.
Early cells in a sea of organic molecules developed various strategies to obtain energy. "The generation and controlled utilization of metabolic energy is central to all cell activities" (Cooper, 1997).
- Initially in the anerobic atmosphere energy was derived from the breakdown of simple organic molecules such as the present-day glycolysis (Fig. 2).
- With the development of photosynthesis about 3 billion years ago(Fig. 1), sunlight was used to regenerate the organic molecule pool. This was critical to the survival of life. It also released molecular oxygen and turned the reducing environment into an oxidative one (Fig. 2).
- Oxidative metabolism was developed a little over 2 billion years ago (Fig. 1) in response to the increase in molecular oxygen. This provides a more efficient recovery of energy from the metabolism of organic molecules (Fig. 2).
Ancestral eukaryotes probably aquired oxidative phosphorylation and photosynthesis from prokaryotes and referred to as endosymbiosis (Fig. 3). Evidence supporting endosymbiosis include:
- Chloroplast and Mitochondria contain double membranes, all other organelles except the nucleus have single membranes.
- Chloroplast and Michondria contain DNA and code for their own mRNA and rRNA.
- Ribosomes in Chloroplast and Mitochondria are similar in structure and size to prokaryotic ribosomes.
Discussion Activity:
Discuss in your groups what selective advantages were gained by the development of multicellular organisms?
Discussion Activity Summary
References:
Cooper, Geoffrey M. (1997) The Cell: A Molecular Approach; ASM Press, Washington, D.C. / Sinauer Associates, Inc., Sunderland, MA.