Reproduction: Protists and Fungi

Objectives:

Before you leave at the end of the laboratory period you should be able to tell a plant-like protist (alga) from a fungus, and specify the kingdom of each. You should know how to recognize the haploid and diploid (and dikaryotic) phases of each organism studied, and what cells were formed by mitosis and by meiosis.


Introduction:

Most plant and animal cells are diploid (2n), meaning that they contain two complete sets of their genetic materials, located on two complete sets of chromosomes. Alternately, some plant and animal cells are haploid (1n), containing only one complete set. [You may want to review the discussion of these terms in Lab 2.]

Dependent upon the organism, both diploid and haploid cells can divide by mitosis. In both cases, each "daughter cell" has the same amount of genetic material (chromosome number) as the "mother cell".


During a second type of cell division called meiosis, a diploid cell undergoes a "reduction division" to form four haploid cells. (The reason that four cells are formed will be covered later in the semester.)


The diploid number is restored when two haploid cells fuse during fertilization.

The actual location of meiosis and fertilization in a sexual cycle depends upon the organism. For example, the only haploid cells produced by humans (and other animals as well) are the sex cells (gametes). The gametes, egg and sperm, are formed by meiosis; they immediately combine to restore the diploid condition by fertilization, forming the zygote. The mature adult is then formed by a large number of mitotic divisions. Therefore, the relative number of haploid cells is small, as indicated in the sexual cycle to the right.


On the other hand, many protists have a sexual cycle where the only diploid cell is the zygote. This zygote immediately undergoes meiosis upon "germination"; all of the other cells are formed by mitosis - including the cells that are capable of fusion again.

Compare these sexual cycles.

These two cycles can be abbreviated by simply showing meiosis and fertilization as gray arrows. On the right, the only diploid cell is the zygote; on the left, the only haploid cells are the gametes. To help you keep track of which cycle goes with each organism you will study in this lab, a simplified cycle will be found at the upper right corner of each illustrated cycle.

In some instances the "many haploid cells" remain separate (e.g., Chlamydomonas), in other cases the cells are attached to form a filament (e.g., Oedogoniium and Spyrogyra), and in still other cases the cells actually form a multicellular organism (not seen in lab, but described in your textbook).

Plants (and some larger plant-like protists) have a third sexual cycle in which both diploid and haploid phases are multicellular, resulting in an "alternation of generations". We will study this cycle in the next two laboratories.

Finally, the alternation of meiosis and fertilization, and the resultant recombination of genes, is associated with the sexual cycle. Many organisms also have an asexual cycle where the offspring are simply produced by mitosis and therefore are genetically identical to the parent (in other words, they are "clones"). This can be an asexual reproduction of diploid organisms, as in the propagation of some commercial plants. It can be an asexual reproduction of the haploid organisms, as the production of spores by many fungi. And some organisms, such as yeast, can produce both diploid and haploid cells asexually.

Most biologists today subdivide all of life on earth into six kingdoms. The prokaryotic organisms, such as the bacteria you observed in Laboratory 1, are in Kingdom Eubacteria and Kingdom Archaebacteria. Many if not all of the simpler plant-like organisms ("algae") are placed with simpler animal-like organisms into a Kingdom Protista; they are therefore called protists. The fungi that you will observe today are given their own kingdom.

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