Materials
Introduction
Meiosis takes place in all organisms that reproduce sexually. In animals, meiosis occurs in special cells of the gonads; in plants, in special cells of the sporangia. Meiosis consists of two nuclear divisions, meiosis I and II, with an atypical interphase between the divisions during which cells do not grow and synthesis of DNA does not take place. This means that meiosis I and II result in four cells from each parent cell, each containing half the number of chromosomes, one from each homologous pair. Recall that cells with only one of each homologous pair of chromosomes are haploid (n) cells. The parent cells, with pairs of homologous chromosomes, are diploid (2n). The haploid cells become sperm (in males), eggs (in females), or spores (in plants). One advantage of meiosis in sexually reproducing organisms is that it prevents the chromosome number from doubling with every generation when fertilization occurs.
What would be the consequences in successive generations of offspring if the chromosome number were not reduced during meiosis?
Working with another student, you will build a model of the nucleus of a cell in interphase before meiosis. Nuclear and chromosome activities are similar to those in mitosis. You and your partner should discuss activities in the nucleus and chromosomes in each stage. Go through the exercise once together and then demonstrate the model to each other to reinforce your understanding. Compare activities in meiosis with those in mitosis as you build your model.
Procedure
Cell activities in G1 are similar to those activities in G1 of the interphase before mitosis.
In G1, are chromosomes single-stranded or double-stranded?
As in mitosis, in G2 the cell prepares for meiosis by synthesizing proteins and enzymes necessary for nuclear division.
Meiosis consists of two consecutive nuclear divisions, called meiosis I and meiosis II. As the first division begins, the chromosomes coil and condense as in mitosis. Meiosis I is radically different from mitosis, however, and the differences immediately become apparent. In your modeling, as you detect the differences, make notes in the margin of your lab manual.
Procedure
As chromosomes begin to coil and condense, prophase I begins. Each chromosome is double-stranded, made up of two sister chromatids. Two pairs of centrioles are located outside the nucleus.
The nuclear envelope breaks down and the spindle begins to form as in mitosis.
Early in prophase I, each chromosome finds its homologue and pairs in a tight association called the synaptonemal complex. The process of pairing is called synapsis. Because the chromosomes are double-stranded, this means that each paired doubled chromosome complex is made of four strands. This complex is called a tetrad.
How many tetrad complexes do you have in your cell which is 2n = 4?
Crossing over takes place between nonsister chromatids in the tetrad. In this process a segment from one chromatid will break and exchange with the exact same segment on a nonsister chromatid in the tetrad. The crossover site forms a chiasma (plural, chiasmata).
Late in prophase I, tetrads move to the equator.
During this phase, tetrads lie on the equatorial plane. Centromeres do not split as they do in mitosis.
How does the structure of chromosomes in anaphase I differ from anaphase in mitosis?
Two nuclei now form, followed by cytokinesis. How many chromosomes are in each nucleus?
Would you describe the new nuclei as being diploid (2n) or haploid (n)?
The interphase between meiosis I and meiosis II is usually short. There is little cell growth and no synthesis of DNA. All the machinery for a second nuclear division is synthesized, however.
The events that take place in meiosis II are similar to the events of mitosis. Meiosis I results in two nuclei with half the number of chromosomes as the parent cell, but the chromosomes are double-stranded (made of two chromatids), just as they are at the beginning of mitosis. The events in meiosis II must change double-stranded chromosomes into single-stranded chromosomes. As meiosis II begins, two new spindles begin to form, establishing the axes for the dispersal of chromosomes to each new nucleus.
Procedure
The events that take place in each of the nuclei in prophase II are similar to those of a mitosis prophase. In each new cell the centrioles move to the poles, nucleoli break down, the nuclear envelope breaks down, and a new spindle forms. The new spindle forms at a right angle to the axis of the spindle in meiosis I.
As the chromosomes reach the equator, prophase II ends and metaphase II begins.
As metaphase II ends, the centromeres finally split and anaphase II begins.
In anaphase II, single-stranded chromosomes move to the poles.
As telophase II begins, chromosomes arrive at the poles. Spindles break down. Nucleoli reappear. Nuclear envelopes form around each bunch of chromosomes as the chromosomes uncoil. Cytokinesis follows meiosis II.
Results
Summarize the major differences between mitosis and meiosis.
William R. Morgan wmorgan@acs.wooster.edu