Lecture 32 April 18 2003 R.Jones Chapters 10.17-22, 15.14, 16, 17.
The Domains and Kingdoms of Living Organisms. What Constitutes Life?
1. Non-living infectious agents. Prions, viroids and viruses-non-living but highly infectious agents. Prions are proteins that cause diseases such as Creutzfeldt-Jakob Disease-CJD in humans, scrapie in sheep and BSE (bovine spongiform encephalopathy BSE) in cattle. A human disease called variant CJD is caused by eating beef having BSE disease. BSE in cattle was probably caused by the feeding of animal (sheep) meat/offal to cattle as a protein supplement. To fatten cattle for beef production or to increase milk yields cattle must be given protein supplements. In the US we usually use protein supplements form plants, mostly soybean. Sheep are often infected by a prion that causes srapie and it was the scrapie prion that probably caused BSE in cattle and which brings about variant CJD in humans.
2. Viroids are small circular pieces of RNA that cause many diseases in plants and are much smaller than virsues. Unlike viruses they do not posses a protein coat.Viruses are more complex and generally have DNA or RNA and a protein coat (see section 10.17-22). They can easily infect plants and animals but fungi bacteri and protists also carry viruses. In animals and plants it is clear that viruses are disease causing agents.
3. Organisms are organized into three major groupings called domains: Bacteria, Archaea and Eukarya.
4. Living organisms and divided into three domains; the prokaryotes that include the Bacteria and Archaea and the Eukarya. The Archaea gave rise to the Eukarya about 3.5 bya (Figure 15.14B). The Eukarya are divided into 4 kingdoms of livi ng organisms the Protista, Plantae, Fungi and Animalia (Figure 15.14A). This diagram shows the evolutionary relationship of these organisms.
5. Bacteria and Archaea differ from each other in many features including the nature of the cell wall, membrane lipids, the presence of introns, RNA polymerase complexity, and senstivity to antibiotics. Many features of Archaea indicaste that they are the pprogenitors of Eukarya.
6. Protists (Chapter 16) are a bizarre mix of fungal- animal- and plant-like lifeforms. The fungal-like organisms are the slime molds, water molds and chytrids and they are common in the environment. Slime molds can grow and extend over very large areas and they may be among the largest organisms. Water molds are less conspicuous but they can cause diseases of plants and animals that are devastating. The water mold Phytophthera infestans caused the potato famine in Ireland that changed the face of the US. The fungus-like protists are characterized by being heterotrophic organisms, havinga motile part of their life cycle, and if they have cell walls, these lack chitin (found in fungi!) and often having cellulose in their walls.
7. The largest of the protists are the plant-like multicellular algae, but the algae also include unicellular organisms such as Chlorella and Euglena as well as dinoflagellates and diatoms. The brown seaweeds called kelps are extrmeely large and can reach sizes in excess of 100m. There are red algae and green algae that are also multicellular.
8. Animal-like protists include the protozoans. They are all motile and heterotrophic lacking cell walls. Amoeboid protozoans such as Amoeba, cilliate protzoans such as Paramecium and flagellated organisms such as Giraridia, Trichomonas and trypanosomes are all disease causing organisms (see Chapter 16.20). Sprozoans are also disease causing organisms, as in the case of malaria.