"Protist" is an umbrella term that refers to a varied group of living organisms that have little in common other than they have nuclei within their cells and are of a relatively simple structural organization. The Kingdom Protista is no longer in use among modern biologists because of the broad diversity of the group. Advances in the study of genetics have led to systems of classification based on relatedness to other organisms. In general, protists are thought to recommend the very earliest stages of biological evolution, but scientists now recognize some protists as being derived from more complex organisms. Reproduction in protists can be either sexual or asexual.
Amoebas are an example of a single-celled protist that reproduce asexually, which simply means a single parent produces an offspring without fertilization by another. First, the amoeba creates an exact replica of its nucleus, which is called mitosis. Then, in a process called fission, the amoeba splits itself into two cells with one nucleus in each. As a result , the pair are genetically identical. Fission is the predominant method of asexual reproduction in single-celled protists, though some fungus-like protists produce spores, another method of asexual reproduction.
Asexual reproduction produces faster population growth than sexual reproduction because it can occur more frequently and because it only requires a single parent. Many protists, however, are capable of true sexual reproduction under certain stressful circumstances. It is believed that environmental stress triggers the exchange of genetic material in an attempt to create offspring better suited to the environment. Sexual reproduction requires two parents to undergo the process of meiosis, which creates a special reproductive cell with exactly half a set of DNA. This is called a gamete. When the gametes of the two parents meet, a zygote is formed with a full set of DNA partially reflecting each parent.
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Another method of reproduction that has aspects of both sexual and asexual reproduction is the process by which bacteria exchange genetic information, called conjugation. After joining with a another bacterium through a special straw-like tube, a single DNA strand from a donor is injected into a recipient. The process is not properly sexual, however, because there are no haploid gametes formed.