Alternation Of Generations

As we have noted, meiosis occurs at some point in the life cycle of all organisms that reproduce sexually. The chromosomes that result from the process constitute a complete set in each cell, since one member of every original pair ends up in each cell. The original chromosomal complement, consisting of two complete sets of chromosomes, is restored when gametes unite, forming a zygote.

Any cell having one set of chromosomes is said to be haploid, and any cell with two sets of chromosomes is said to be diploid. By the time meiosis is complete, four haploid cells have been produced from one diploid cell. The gametes of any organism are haploid, while a zygote of the same organism is diploid. This holds true regardless of the number of chromosomes of any given organism. We can state that an organism having n (a specified quantity) chromosomes in its haploid cells will have twice as many, or 2n, chromosomes in its diploid cells.

F. Prophase II. The chromosomes coil and contract again; because of crossing-over, the chromatids are no longer identical with each other.

G. Metaphase II. The chromosomes of each cell become aligned along their respective equators.

H. Anaphase II. The chromatids separate completely and migrate to the poles.

I. Telophase II. The four groups of chro-matids (now called chromosomes again) are at the poles; new cell walls begin to form.

J. Formation of cell walls. Cell walls form; the four cells will become pollen grains.

Anaphase Pollen GrainsAlternaion Generations PlantsAlternaion Generations Plants

Figure 12.4

Continued

Alternaion Generations PlantsFlowering Plants Egg Formation

Figure 12.4

Continued

Occasionally, spindles may not form properly during meiosis or something else goes wrong, ultimately resulting in cells with more than one or two sets of chromosomes. Cells having three sets of chromosomes are said to be triploid (3n), and those with four sets are said to be tetraploid (4n). Since the homologous chromosomes of a triploid cell undergoing meiosis cannot pair properly, any resulting gametes, if they survive, invariably produce a sterile individual.

In most animals, the only haploid cells (i.e., cells with n or a single set of chromosomes) are the gametes (egg and sperm) and the cells that become the gametes. In plants and other green organisms, however, this is generally not so. In a complete life cycle involving sexual reproduction, there is an alternation between a diploid (2n) sporophyte phase and a haploid (n) gametophyte phase. This is commonly referred to as Alternation of Generations.

The diploid body itself is called a sporophyte. It develops from a zygote and eventually produces sporo-cytes (meiocytes), each of which undergoes meiosis, producing four spores. The haploid bodies that develop from these spores are called gametophytes. These eventually form sex structures, or cells, in which gametes are produced by mitosis.

pole bivalent of homologous chromosomes pole bivalent of homologous chromosomes

Bivalentof Metaphase

spindle fibers centromere

Figure 12.5 A diagram of homologous chromosomes at metaphase I of meiosis.

spindle fibers centromere

Figure 12.5 A diagram of homologous chromosomes at metaphase I of meiosis.

Chapter 12

Plant Thaty Undergo Sexual Reproduction
Figure 12.6 A typical life cycle of plants or related organisms that undergo sexual reproduction.

As becomes evident in chapters to follow, the gameto-phytes of many primitive forms constitute a large part of the visible organism, but as we progress up through the Plant Kingdom to more complex plants, they become proportionately reduced in size until they may be only microscopic. The switch from one generation to the other takes place as spores are produced when sporocytes undergo meiosis and again when a zygote is produced through fusion of gametes, or fertilization (also called syngamy).

Although the sporophyte generation of some primitive organisms may consist of a single cell (the zygote), the basic plan of Alternation of Generations can be seen in the Protistan, Fungal, and Plant Kingdoms. It becomes most conspicuous, however, in the Plant Kingdom, and it differs from one organism to the next in the forms of the various bodies and cells.

If you will note, along with the accompanying diagram (Fig. 12.6), the following six rules pertaining to Alternation of Generations in the majority of plants and other green organisms, you should have little trouble following the life cycle of any sexually reproducing organism discussed in this book.

1. The first cell of any gametophyte generation is normally a spore (sexual spore or meiospore), and the last cell is normally a gamete.

2. Any cell of a gametophyte generation is usually haploid (n).

3. The first cell of any sporophyte generation is normally a zygote, and the last cell is normally a sporocyte (meiocyte).

4. Any cell of a sporophyte generation is usually diploid (2n).

5. The change from a sporophyte to a gametophyte generation usually occurs as a result of meiosis.

6. The change from a gametophyte to a sporophyte generation usually occurs as a result of fertilization (fusion of gametes), which is also called syngamy.

The word generation as used in Alternation of Generations simply means phase of a life cycle and should not be confused with the more widespread use of the word pertaining to time or offspring.

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Responses

  • cynthia
    What process accomplished the change from sporophyte to gametophyte?
    7 years ago

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