Yeast cells reproduce by budding, the parent cell forms a protuberance or bud. As the bud elongates, the DNA in the cell nucleus replicates, the nucleus then divides into two and one nucleus migrates into the bud. A wall forms between the bud and the parent plant cell, and the bud breaks away. Moulds reproduce by forming spores that may be sexual or asexual. Asexual spores form in sporangia at the tips of aerial hyphae from one organism. Sexual spores form when cells of two organisms fuse, these are not produced as often as asexual spores. Sexual spores often have to survive adverse conditions before they germinate.
Protozoa are a subgroup of the kingdom Protista, which contains organisms that do not fit into the other four kingdoms. Protozoan infecting agents cause malaria, sleeping sickness and amoebic dysentery. Protozoa: grow best within a temperature range of 10- 45 degrees centigrade. Are unicellular eukaryotic cells, reproduced asexually by fission. Can reproduce sexually, two fertilised cells are produced when haploid nuclei (half the number of chromosomes) from different cells fuse. Each fertilised cell now contains DNA from another individual. When these cells divide, their daughter cells will not be genetically identical to the original parent cell. Can be parasitic and cause disease, such as amoebic dysentery, giardiasis and toxoplasmosis. The Protozoa causing these infections are transmitted via faeces or faecal contamination of food or water. Figure 17 shows a Protozoa cell.
Parasites can be spread by insects that transmit the protozoa when they bite humans or animals. The insects are the vectors but not the cause of the disease. Female anopheles mosquitoes are vectors (carriers) for Plasmodium, which causes and spreads malaria. Sandflies are vectors for Leishmania and spread leishmaniasis. Tsetse flies are vectors for trypanosomes and spread African sleeping sickness.