Unit Three. The Continuity of Life
9.3. The Stages of Meiosis
Now, let’s look more closely at the process of meiosis. Meiosis consists of two rounds of cell division, called meiosis I and meiosis II, which produce four haploid cells. Just as in mitosis, the chromosomes have replicated before meiosis begins, during a period called interphase. The first of the two divisions of meiosis, called meiosis I (meiosis I is shown in the outer circle of the Key Biological Process illustration on the facing page), serves to separate the two versions of each chromosome (the homologous chromosomes or homologues); the second division, meiosis II (the inner circle), serves to separate the two replicas of each version, called sister chromatids. Thus when meiosis is complete, what started out as one diploid cell ends up as four haploid cells. Because there was one replication of DNA but two cell divisions, the process reduces the number of chromosomes by half.
Meiosis I is traditionally divided into four stages:
1. Prophase I. The two versions of each chromosome (the two homologues) pair up and exchange segments.
2. Metaphase I. The chromosomes align on a central plane.
3. Anaphase I. One homologue with its two sister chromatids still attached moves to a pole of the cell, and the other homologue moves to the opposite pole.
4. Telophase I. Individual chromosomes gather together at each of the two poles.
In prophase I, individual chromosomes first become visible, as viewed with a light microscope, as their DNA coils more and more tightly. Because the chromosomes (DNA) have replicated before the onset of meiosis, each of the threadlike chromosomes actually consists of two sister chromatids associated along their lengths (held together by cohesin proteins in a process called sister chromatid cohesion) and joined at their centromeres, just as in mitosis. However, now meiosis begins to differ from mitosis. During prophase I, the two homologous chromosomes line up side by side, physically touching one another, as you see in figure 9.5. It is at this point that a process called crossing over is initiated, in which DNA is exchanged between the two nonsister chromatids of homologous chromosomes. The chromosomes actually break in the same place on both nonsister chromatids and sections of chromosomes are swapped between the homologous chromosomes, producing a hybrid chromosome that is part maternal chromosome (the green sections) and part paternal chromosome (the purple sections). Two elements hold the homologous chromosomes together: (1) cohesion between sister chromatids; and (2) crossovers between nonsister chromatids (homologues). Late in prophase, the nuclear envelope disperses.
Figure 9.5. Crossing over.
In crossing over, the two homologues of each chromosome exchange portions. During the crossing over process, nonsister chromatids that are next to each other exchange chromosome arms or segments.
In metaphase I, the spindle apparatus forms, but because homologues are held close together by crossovers, spindle fibers can attach to only the outward-facing kinetochore of each centromere. For each pair of homologues, the orientation on the metaphase plate is random; which homologue is oriented toward which pole is a matter of chance. Like shuffling a deck of cards, many combinations are possible—in fact, 2 raised to a power equal to the number of chromosome pairs. For example, in a hypothetical cell that has three chromosome pairs, there are eight possible orientations (23). Each orientation results in gametes with different combinations of parental chromosomes. This process is called independent assortment. The chromosomes in figure 9.6 line up along the metaphase plate, but whether the maternal chromosome (the green chromosomes) is on the right or left of the plate is completely random.
Figure 9.6. Independent assortment.
Independent assortment occurs because the orientation of chromosomes on the metaphase plate is random. Shown here are four possible orientations of chromosomes in a hypothetical cell. Each of the many possible orientations results in gametes with different combinations of parental chromosomes.
In anaphase I, the spindle attachment is complete, and homologues are pulled apart and move toward opposite poles. Sister chromatids are not separated at this stage. Because the orientation along the spindle equator is random, the chromosome that a pole receives from each pair of homologues is also random with respect to all chromosome pairs. At the end of anaphase I, each pole has half as many chromosomes as were present in the cell when meiosis began. Remember that the chromosomes replicated and thus contained two sister chromatids before the start of meiosis, but sister chromatids are not counted as separate chromosomes. As in mitosis, count the number of centromeres to determine the number of chromosomes.
In telophase I, the chromosomes gather at their respective poles to form two chromosome clusters. After an interval of variable length, meiosis II occurs in which the sister chromatids are separated as in mitosis. Meiosis can be thought of as two consecutive cycles, as shown in the Key Biological Process illustration on the previous page. The outer cycle contains the phases of meiosis I and the inner cycle contains the phases of meiosis II, discussed next.
After a brief interphase, in which no DNA synthesis occurs, the second meiotic division begins. Meiosis II is simply a mitotic division involving the products of meiosis I, except that the sister chromatids are not genetically identical, as they are in mitosis, because of crossing over. You can see this by looking at figure 9.7, where some of the arms of the sister chromatids contain two different colors. At the end of anaphase I, each pole has a haploid complement of chromosomes, each of which is still composed of two sister chromatids attached at the centromere. Like meiosis I, meiosis II is divided into four stages:
1. Prophase II. At the two poles of the cell, the clusters of chromosomes enter a brief prophase II, where a new spindle forms.
2. Metaphase II. In metaphase II, spindle fibers bind to both sides of the centromeres and the chromosomes line up along a central plane.
3. Anaphase II. The spindle fibers shorten, splitting the centromeres and moving the sister chromatids to opposite poles.
4. Telophase II. Finally, the nuclear envelope re-forms around the four sets of daughter chromosomes.
Figure 9.7. Meiosis.
The main outcome of the four stages of meiosis II— prophase II, metaphase II, anaphase II, and telophase II—is to separate the sister chromatids. The final result of this division is four cells containing haploid sets of chromosomes. No two are alike because of the crossing over in prophase I. The nuclei are then reorganized, and nuclear envelopes form around each haploid set of chromosomes. The cells that contain these haploid nuclei may develop directly into gametes, as they do in most animals. Alternatively, they may themselves divide mitotically, as they do in plants, fungi, and many protists, eventually producing greater numbers of gametes or, as in the case of some plants and insects, adult haploid individuals.
The Important Role of Crossing Over
If you think about it, the key to meiosis is that the sister chromatids of each chromosome are not separated from each other in the first division. Why not? What prevents microtubules from attaching to them and pulling them to opposite poles of the cell, just as eventually happens later in the second meiotic division? The answer is the crossing over that occurred early in the first division. By exchanging segments, the two homologues are tied together by strands of DNA. It is because microtubules can gain access to only one side of each homologue that they cannot pull the two sister chromatids apart! Imagine two people dancing closely—you can tie a rope to the back of each person’s belt, but you cannot tie a second rope to their belt buckles because the two dancers are facing each other and are very close. In just the same way, microtubules cannot attach to the inner sides of the homologues because crossing over holds the homologous chromosomes together like dancing partners.
Key Learning Outcome 9.3. During meiosis I, homologous chromosomes move to opposite poles of the cell. At the end of meiosis II, each of the four haploid cells contains one copy of every chromosome in the set, rather than two. Because of crossing over, no two cells are the same.