Written Assignment 2

There are two ways in which an organism can reproduce. Many organisms, including bacteria, fungi, and even some plants and animals, undergo asexual reproduction, in which a single parent produces identical offspring. Other organisms, including most animals and plants, undergo sexual reproduction, in which offspring are produced by the fusion of two reproductive cells in the process of fertilization. And some species, particularly among plants, can use both methods. In sexual reproduction, because a combination of DNA from two separate individuals is passed on to offspring, the resulting offspring are genetically different from their parents and, for reasons explained later, from one another.

30

What would happen if sexually reproducing organisms, humans included, produced reproductive cells through mitosis? Both parents would contribute a full set of genes—that is, 23 pairs of chromosomes in humans—to create a new individual, and the new offspring would inherit 46 pairs of chromosomes in all. And when that individual reproduced, if he or she contributed 46 pairs of chromosomes and his or her mate also contributed 46 pairs, their offspring would have 92 pairs of chromosomes. Where would it end? The genome would double in size every generation. In sexually reproducing organisms, the solution to chromosome overload is meiosis. This process enables organisms to make gametes, special reproductive cells that have half as many chromosomes as the rest of the cells in the organism’s body (the somatic cells). In humans, for example, each gamete cell has only one set of 23 chromosomes, rather than two sets. In genetics, the term diploid refers to cells that have two copies of each chromosome (in humans, two sets of 23 chromosomes, for 46 chromosomes in total), and the term haploid refers to cells that have one copy of each chromosome. Thus, somatic cells are diploid, and gametes, the cells produced in meiosis, are haploid. At fertilization, two haploid cells, each with one set of 23 chromosomes, merge and create a new individual with the proper diploid human genome of 46 chromosomes. And this new individual, through meiosis, will also produce haploid gametes that have only a single set of 23 chromosomes. With sexual reproduction, then, diploid organisms produce haploid gametes that fuse at fertilization to restore the diploid state. In this way, meiosis maintains a stable genome size in a species. Although there are some variations on this pattern of alternation between the haploid and diploid states, in most cases, multicellular animals produce haploid cells for reproduction. And after two haploid gametes come together to form a diploid fertilized egg, multiple cell divisions by mitosis produce a diploid, multicellular animal.

31

Meiosis achieves more than just a reduction in the amount of genetic material in gametes. As a diploid individual, you have two copies (i.e., two alleles) of every gene in every somatic cell: one from your mother and one from your father. As a consequence of meiosis, gametes have one allele for each trait rather than two. Which of these two alleles is included in each gamete cannot be predicted. Each egg or sperm carries a unique set of alleles produced with a varied combination of maternal and paternal alleles. All offspring resemble their parents, yet none resemble them in exactly the same way. This variation among offspring can be seen among the two sisters in Figure 8-19. In all, then, meiosis has two important outcomes: 1. It reduces the amount of genetic material in gametes. 2. It produces gametes that differ from one another with respect to the combinations of alleles they carry.

32

33

Mitosis is an all-purpose process for cell division. It occurs all over the body, all the time. Meiosis, on the other hand, takes place in just a single place: the gonads (the ovaries and testes). And it takes place for just a single reason: the production of gametes (sperm and eggs). Meiosis starts with a specialized diploid cell found in the gonads that is capable of undergoing meiosis. Thus, for humans, meiosis starts with a cell that has 46 chromosomes. These include a maternal copy and a paternal copy of each of 22 chromosomes—each of these pairs is called a homologous pair, or homologues—along with two additional chromosomes (one from each parent), called sex chromosomes (Figure 8-20).

34

Before meiosis can occur (and just as we saw with mitosis), each of these 46 chromosomes is duplicated during the cell’s interphase. This means that when meiosis begins, we have 92 (2 × 46) strands of DNA, after replication. Unlike mitosis, which has only one cell division, cells undergoing meiosis divide twice. In the first division, the homologues separate. In other words, for each of the 23 chromosome pairs, the maternal sister chromatid pairs and the paternal sister chromatid pairs separate into two new cells. In the second division, each of the two new cells divides again, so that each of the four daughter cells contains a single chromosome from the homologous pair. At the end of meiosis, there are four new cells, each of which has 23 strands of DNA—that is, 23 chromosomes (Figure 8-21). Note that, in animals, none of these four cells will undergo any further cell division—they do not become parent cells for a new cycle of cell division.

35

Interphase. Before meiosis begins, every chromosome creates an exact duplicate of itself by replication. The chromosomes that were each a single, long, linear piece of genetic material become a pair of identical long, linear pieces, held together at the centromeres. The first meiotic division takes place in four stages. 1. Prophase I: chromosomes condense and crossing over occurs. This is by far the most complex of all the phases of meiosis. As in mitosis, it begins with all of the replicated genetic material condensing. As the sister chromatids become shorter and thicker, the homologous chromosomes come together. This is where the process diverges from mitosis. The homologous chromosomes (each of which has become a pair of identical chromatids) line up, touching. Under a microscope, the two homologous pairs of sister chromatids appear as pairs of X’s lying one on top of the other. At this point, the sister chromatids that are next to each other do something that makes every sperm or egg cell genetically unique: they swap little segments of DNA. Some of the genes that you inherited from your mother may get swapped onto the strand of DNA you inherited from your father, and vice versa. This possible outcome of the crossing over of portions of the chromatids is called genetic recombination (or, more often, just recombination). It can take place at several spots (up to dozens) on each chromatid. As a result of recombination, every sister chromatid possesses a unique mixture of your genetic material. Note that crossing over takes place only during the production of gametes, in meiosis. It does not occur during mitosis. Following crossing over, the nuclear membrane disintegrates. We explore crossing over in more detail in Section 8-12. The remaining steps of meiosis are relatively straightforward. 2. Metaphase I: After crossing over, each pair of homologous chromosomes (that is, the pairs of X’s lying one on top of the other) moves to the center of the cell, pulled by the spindle fibers to form the arrangement called the metaphase plate. (Keep in mind that each pair of homologous chromosomes includes the maternal and paternal versions of the chromosome— with crossed-over segments—and the replicated copy of each, making four strands in all.) The maternal and paternal sister chromatid pairs line up at the metaphase plate in a random fashion, called random assortment, so that the pairs of sister chromatids pulled toward each pole are a mix of maternal and paternal sister chromatids. As a result of random assortment, all the products of meiosis are genetically unique. 3. Anaphase I: This phase is the beginning of the first cell division that occurs during meiosis. In anaphase, the spindle fibers pull the homologues apart toward opposite poles of the cell. One of the homologues (consisting of two sister chromatids) goes to one pole, the other to the opposite pole.

36

4. Telophase I and cytokinesis: After the pairs of chromatids arrive at the two poles of the cell, nuclear membranes re-form, then cytokinesis occurs: the cytoplasm divides, and the cell membrane pinches the cell into two daughter cells. Each daughter cell has a nucleus that contains the genetic material—two sister chromatids for each of the 23 chromosomes in humans. There is a brief interphase after the first division of meiosis. In some organisms, the DNA molecules (now in the form of chromatid pairs) briefly uncoil and fade from view. In others, the second part of meiosis begins immediately. It is important to note that in the brief interphase before prophase II, there is no replication of any of the chromosomes. 5. Prophase II: The second division of meiosis begins with prophase II. The genetic material in each of the two daughter cells once again coils tightly, making the pairs of chromatids visible under the microscope. (Unlike prophase I, no crossing over occurs during prophase II.) 6. Metaphase II: In each of the two daughter cells, the sister chromatids (each appearing as an X) move to the center of the cell, pulled by thread-like structures in the cytoskeleton attached to the centromere, where the sister chromatids are held together. The congregation of all the genetic material in the center of each daughter cell is visible as a flat metaphase plate. 7. Anaphase II: During this phase, the fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell. 8. Telophase II and cytokenesis: Finally, the sister chromatids for all 23 chromosomes have been pulled to opposite poles. The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close. In humans, the outcome of one diploid cell undergoing meiosis is the creation of four haploid daughter cells, each with a set of 23 individual chromosomes. These chromosomes contain a combination of traits from the individual’s diploid set of chromosomes.

37

38

For all sexually reproducing organisms, there is just one way to distinguish males from females. Regardless of the species, the defining feature is always the same (and if you’re thinking of anything visible to the naked eye, you’re wrong). When there are two sexes—as is the case for nearly every sexually reproducing animal and plant species—the females are the sex that produces the larger gamete, and the males produce the smaller, more motile gamete (Figure 8-23). Meiosis works differently in males and females. The female gamete is larger than the male gamete because it has more cytoplasm. During the production of sperm, meiosis takes place just as described in Section 8.9, resulting in four evenly sized cells that become sperm. During egg production, the cell divides in telophase I, and the genetic material is evenly divided, but nearly all of the cytoplasm goes to one of the cells and almost none goes to the other. The smaller cell, called a polar body, degrades almost immediately in most animals. Then, in the second meiotic division of the larger cell, there is again an unequal division of cytoplasm. As in the first division, one of the new cells gets nearly all of the cytoplasm and the

other gets almost none, forming another polar body. The net result of meiosis in the production of eggs is one large egg with lots of cytoplasm and two or three small polar bodies with very little cytoplasm that degrade and never function as gametes (Figure 8-24).

39

40

Genetically speaking, there are two ways to create unique individuals. The obvious way is for an organism to carry an allele that is not present in any other individuals. Alternatively—and equally successful in creating uniqueness—an individual can carry a collection of alleles, no single one of which is unique, that has never before occurred in another individual. Both types of novelty introduce important variation into a population of organisms. The process of crossing over, or genetic recombination (Figure 8-25), which occurs during prophase I in meiosis, creates a significant amount of the second type of variation. Let’s look at crossing over more carefully. Take, for example, the homologous pair of human chromosome 15. It includes two copies of chromosome 15: one copy from your mother (which you inherited from the egg that was fertilized to create you) and one copy from your father (which you inherited from the sperm that fertilized the egg). Each chromosome in the pair carries the same genes, but because they came from different people, they don’t necessarily have the same alleles. Once the sister chromatids of the homologous chromosome pairs line up in prophase I (so that there are now four chromatids in two pairs lying very close together), regions that are close together can swap segments. A piece of one of the maternal chromatids—perhaps including the first 100 genes on the strand of DNA—may swap places with the same segment in a paternal chromatid. Elsewhere, a stretch of 20 genes in the middle may be swapped from the other maternal chromatid with one of the paternal chromatids. The points at which chromatids exchange genetic material during recombination are called chiasmata (sing. chiasma). Every time a swap of DNA segments takes place, an identical amount of genetic material is exchanged, so all four chromatids still contain the complete set of genes that make up the chromosome. The combination of alleles on each chromatid, though, is now different. Suppose there are genes relating to eye color and height on a particular chromosome. After crossing over, a chromatid that carried instructions for brown eyes and short height may now carry instructions for brown eyes and tall height. All of the alleles from your parents are present on one DNA molecule or another. But the combination of alleles (and the traits they determine) that are linked together on a single chromatid is new. And when a gamete, let’s say it’s an egg, carrying a new combination of alleles is fertilized by a sperm, the developing individual will carry a completely novel set of alleles. Without creating new versions of any traits (such as yellow eyes or purple hair), crossing over creates gametes with collections of alleles that may never have existed together before. In Chapter 9, we’ll see that this variation is tremendously important for evolution.

41

42

Sexual reproduction leads to offspring that are genetically different from one another and from either parent, through three different processes (Figure 8-26). 1. Combining alleles from two parents at fertilization. First and foremost, with sexual reproduction, a new individual comes from the fusion of gametes from two different individuals. Each of these parents comes with his or her own unique set of genetic material. 2. Crossing over during the production of gametes. Crossing over during prophase I of meiosis causes every chromosome in a gamete to carry a mixture of an individual’s maternal and paternal genetic material. 3. Shuffling and reassortment of homologues during meiosis. When homologues for each chromosome are pulled to opposite poles of the cell during the first division of meiosis (anaphase I), the maternal and paternal homologues are randomly separated. Many different combinations of maternal and paternal homologues could end up in each gamete.

43

The variability among the offspring produced by sexual reproduction enables populations of organisms to cope better with changes in their environment. After all, if the environment is gradually changing from one generation to the next, the production of many genetically different offspring increases the likelihood that some of the offspring will carry sets of genes particularly suited to the new environment. Over time, populations of sexually reproducing organisms can quickly adapt to changing environments. It’s like buying different lottery tickets—the more tickets you buy, the more likely it is that one of them will be a winner. In the yellow dung fly (Scathophaga stercoraria), males sometimes wrestle each other for mating access to a female. The female awaits the outcome of the battle in a pile of dung. Occasionally, females drown in the dung pile as they wait. Males, too, can be at risk during reproduction. This is seen most dramatically in species such as praying mantises and black widow spiders, in which the female will attempt to eat the male (and often succeed in doing so) during or after mating, gaining not just gametes but a nutritious meal as well! Dangers associated with mating are one of the downsides to sexual reproduction. As we see with the dung flies, sex can be a risky proposition because organisms make themselves vulnerable to predation, disease, and other calamities during mating and reproduction. There are other risks, too. First, when an individual reproduces, only half of its offspring’s alleles will come from that organism. The other half will come from the other parent. The complex cellular division required for sexual reproduction offers opportunities for mistakes, sometimes leading to chromosomal disorders. With asexual reproduction, an individual produces nearly identical offspring, so there is a very efficient transfer of genetic information from one generation to the next. Second, with sexual reproduction it takes time and energy to find a partner. This is energy that asexual organisms can devote to additional reproduction. Because asexual reproduction involves only a single individual, it can be fast and easy. Some bacteria can divide, forming a new generation, every 20 minutes (Figure 8-28). And for organisms in isolated habitats or those establishing new populations, asexual reproduction can be advantageous as well. Asexual reproduction is efficient, too. Offspring carry all of the genes that their parent carried—they are genetically identical. If the environment is stable, it is beneficial for organisms to produce offspring as similar to themselves as possible. The downside to asexual reproduction is that the more closely an offspring’s genome resembles its parent’s, the less likely it is that the offspring will be suited to the environment when it changes. In the end, we still see large numbers of species that repro- duce asexually and large numbers that reproduce sexually. It seems that conditions favoring one or the other occur in the great diversity of habitats in our world. That we see both sexual and asexual reproduction also highlights the recurring theme in biology that there often is more than one way to solve a problem.

44

45