Question

1a. There are no fossils of ancient prokaryotes, nor of the early eukaryotes. No one was present to observe events first-hand, yet we know that eukaryotes arose from an endosymbiotic event between two prokaryotic organisms... Describe TWO lines of evidence that strongly supports the endosymbiotic origin of eukaryotic life; and, state what is compelling (convincing) about this evidence

Answer 1

Endosymbiosis is the hypothesised process by which prokaryotes gave rise to the first eukaryotic cells. It attempts to explain the origin of eukaryotic cell organelles such as mitochondria in animals and fungi and chloroplasts in plants. Mitochondria are one of the many different types of organelles in the cells of all eukaryotes. In general, they are considered to have originated from proteobacteria (likely Rickettsiales) through endosymbiosis. Chloroplasts are one of the many different types of organelles in the plant cell. In general, they are considered to have originated from cyanobacteria through endosymbiosis. There are many compelling evidences that support the endosymbiosis theory for origin of eukaryotes from prokaryotes as listed below :

• Size and Function- Both mitochondria and chloroplast range in size from 1-10 microns, which is the standard size for prokaryotes. Also, Mitochondria have very similar characteristics to purple-aerobic bacteria. They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle and oxidative phosphorylation. Similarly, chloroplasts are very similar to photosynthetic bacteria in that they both have similar chlorophyll that harnesses light energy that is converted into chemical energy.
• Semi-autonomous Nature of Mitochondria and Chloroplasts- Mitochondria and chloroplasts DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis is similar to that for bacteria. Each mitochondrion has its own circular DNA genome, like a bacteria's genome, but much smaller. This DNA is passed from a mitochondrion to its offspring and is separate from the "host" cell's genome in the nucleus. Mitochondria multiply by pinching in half, the same process used by bacteria. Every new mitochondrion must be produced from a parent mitochondrion in this way; if a cell's mitochondria are removed, it can't build new ones from scratch.