From Seed to Seed:
Plant Science for K-8 Educators

Meiosis. Reproductive cells undergo meiosis (my-OH-sis; Gr. meioun = to lessen). As in mitosis, cell division by meiosis involves the replication of genetic material within the nucleus. However, there is an important difference. Mitosis results in two diploid (2n) daughter cells that are identical to the parent cell. Meiosis, on the other hand, results in four haploid (n) daughter cells. These daughter cells, having half the number of chromosomes, are not identical to the parent cell. Furthermore, the haploid cells are not genetically identical to one another. This introduces genetic diversity in the offspring produced by sexual reproduction.

Sexual reproduction depends on a reduction in chromosome number because it involves the union of gametes. Recall that a gamete is simply a haploid reproductive cell. When gametes fuse, the resulting zygote has double the number of chromosomes each gamete has. Without the reduction in chromosome number, the fusing of gametes would result in a continual doubling of chromosome number, generation after generation.

Similar to mitosis, meiosis is preceded by interphase, during which chromosomes are replicated.

Unlike mitosis, meiosis consists of two successive nuclear divisons, meiosis I and meiosis II. Therefore, there are two sets of phases for meiosis, each labeled with a I or II to indicate whether it is associated with meiosis I or II:.

1) In prophase I, the chromosomes first appear as long threads scattered throughout the nucleus (sound familiar?). Throughout prophase, the chromosomes become condensed, eventually appearing as sister chromatids attached at the centromere. So far, this sounds like mitosis. However, the next step is unique to meiosis. Also during prophase, the homologous chromosomes join together. Homologous chromosomes are pairs of chromosomes, each member of which has been donated by a different parent.

A helpful visualization. Here's a way to help your students visualize homologous chromosomes. Purchase two boxes of crayons. Each box should contain eight crayons of similar colors, but make sure the two boxes are made by different manufacturers so that the crayons in one box are a lighter shade than the crayons in the other box. Dump the crayons out. Together, the 16 crayons represent the chromatin of one somatic cell. Each crayon represents a chromosome and each manufacturer represents a parent. Ask students to line the crayons up according to color-the light red with the dark red, the light blue with the dark blue, etc. The resulting pairs represent homologous chromosomes. However, since they come from different manufacturers (or parents), they are similar but not identical. Homologous chromosomes are similar in their structure and in the traits they represent, but because they have been donated by two parents, the chromosomes are not identical. In the diagrams below, the solid and dashed lines indicate chromosomes contributed by the egg and sperm, respectively. These diagrams represent a nucleus with two pairs of chromosomes (2n = 4)-let's call them red and blue. At this stage in meiosis, the chromosomes have replicated (resulting in sister chromatids attached at the centromere-same as mitosis), resulting in four blue and four red crayons. The crayons-or chromosomes-lie in two pairs. These pairs represent the homologous chromosomes.

 

2) During metaphase I, the homologous chromosomes line up near the center of the cell.

3) In anaphase I, the homologous chromosomes separate from one another. As in mitosis, the chromosomes are now distributed equally to two daughter cells. If the process ended here, there would be two diploid daughter cells... but we're not through yet!

4) Each of the daughter cells undergoes another cell division. This time, there is no replication of the chromosomes. Instead, in metaphase II, the chromosomes again line up near the center of the cell...

5) ...and in anaphase II, the sister chromatids separate from one another.

6) By telophase II, the cell has divided completely to produce four daughter cells.