Meiosis, Sexual Reproduction, & Genetic Variability

Transcription

1 Meiosis, Sexual Reproduction, & Genetic Variability Teachers Guide NARRATION FOR MEIOSIS, SEXUAL REPRODUCTION, AND GENETIC VARIABILITY Since the members of no species, even California redwoods or giant Galapagos tortoises, live forever, all species have to have members that successfully reproduce in order for the species to continue to survive. The process of reproduction requires that offspring obtain the genetic instructions or the blueprints of life contained in strands of DNA from their parent or parents. The two basic types of reproduction - asexual reproduction and sexual reproduction differ in how they provide offspring with the genetic instructions that will define their traits and characteristics. During asexual reproduction, a means of reproduction often used by single-celled organisms such as paramecium and amoebas and certain multi-cellular organisms such as hydra, a single parent passes exact copies of their genetic material on to the next generation through the process of mitosis. The result is offspring genetically identical to the parent. During sexual reproduction the genetic material provided from two parents in the form of sexual gametes join. The cell created by the union of the two gametes is called a zygote. The newly formed zygote has a genetic makeup that varies somewhat from that of either parent. Chromosomes: Chromosomes are Strands of DNA The individual strands of DNA that carry the blueprints of life are called chromosomes. In eukaryotic organisms, or organisms made up of cells that have nuclei, chromosomes are found within the nucleus of each cell. Every living species has a characteristic number of chromosomes. For example, we humans have 46 chromosomes in the nuclei of all our cells, except our sex cells which have only 23. Chromosomes: Genes and Homologous Pairs Chromosomes are made up of sections of DNA called genes. Each chromosome may have hundreds or thousands of genes. Most genes program cells to synthesize specific enzymes or other proteins. It is the cumulative action of these proteins that produces an organisms inherited traits. If we were to look at the chromosomes in the nucleus of a eukaryotic cell organism, we would notice that the chromosomes could be

2 distinguished from one another by their length, banding pattern, and the location of their centromere. In the cells of most organisms we would also note that, except in their sex cells, two matching copies of each type of chromosome exist. These matching pairs of chromosomes are called homologues. The two chromosomes in each homologue carry genes for the same inherited characteristics and the gene or genes for each trait such eye color are situated at identical locations on each chromosome in a homologous pair. Chromosomes: Diploid and Haploid Cells Cells that have two matching sets of chromosomes are referred as diploid cells. The bodies of most eukaryotic organisms such as humans are made up of diploid cells. When a single-celled diploid organism reproduces asexually via mitosis and cellular division, the offspring is assured of having diploid cells because the process of mitosis and cellular division produces cells genetically identical to the parent and each other. Thus a diploid cell reproducing through mitosis will always produce two diploid cells identical to the parent. But what if multi-cellular diploid organisms produced sex gametes such as eggs and sperm through mitosis, just as they do the rest of their cells? If two such gametes joined, the zygote formed would have four sets of chromosomes. The next generation of zygotes would have eights sets, the next sixteen and so on, each succeeding generation having double the number of chromosomes of the preceding one, until finally the number of chromosomes in the cells of each organism would become virtually infinite. Thus, in order for diploid organisms to reproduce sexually they must produce sex gametes that have just a single set of chromosomes, so that the zygotes they produce are merely diploid like the parents. Cells like sex gametes, which have a single set of chromosomes in their nucleus are referred to as haploid. Meiosis: Meiosis Produces Haploid Sex Gametes The special form of cellular division by which organisms create haploid sex gametes is called meiosis. Meiosis not only forms sex gametes with one set of chromosomes it also, as we ll see later, moves genetic material around in ways that increase the genetic diversity within a species population. GENETIC DIVERSITY and SEXUAL REPRODUCTION The competitive advantages arising from genetic diversity are probably the driving force behind the evolution of sexual reproduction in living organisms. While asexual reproduction has a number of advantages over sexual reproduction, not the least of which are that individuals don t have to spend energy producing sex gametes or competing with sexual rivals, asexual reproduction has the major disadvantage that it doesn t create genetic diversity within the species as whole. Lack of genetic diversity may not be that negative, in fact it may be an advantage, if the parent organism is well adapted to the environment. But what if the environment changes? For example, the climate becomes drier or a new predator enters the

3 environment. If all the parents and offspring are genetically identical, the entire species may be destroyed as a result of one or two changes in the environment. However, if the members of a species vary somewhat genetically and thus in some of their physical characteristics, it is much easier to imagine that at least some of the members of the species will have characteristics that will enable them to escape succumbing to a change in environment such as less rain or the arrival of a new predator. Meiosis and sexual reproduction are a means by which species create genetic diversity within their population. Meiosis: Meiosis Takes Place in Reproductive Organs The process of producing gametes via meiosis takes place in the reproductive organs of an organism. In humans and other mammals, male gametes or sperm cells are produced in the testes of males, while female gametes or eggs are produced in the ovaries of females. To illustrate the process of meiosis let s look at an imaginary male diploid animal with three homologous pairs of chromosomes. Meiosis: Meiosis Begins with Diploid Cells The process of meiosis begins in the testes of our imaginary organism with diploid cells that are genetically identical to those in the rest of the body. Of the two complete sets of chromosomes in the nucleus of these diploid cells, one set came from the organism s male or paternal parent, and the other set from their female or maternal parent. Before meiosis begins both the paternal and maternal chromosomes that make up each homologous pair duplicate themselves. These copies remain attached to each other at their centromeres. Each copy of the original chromosome is called a chromatid. Even though each chromatid will eventually function as an individual chromosome, as long as the two chromatids are attached to one another we will refer to them as a single chromosome. It's not until they separate later that we will refer to each as an independent chromosome. Meiosis: Meiosis is Divided into Meiosis I and II. Meiosis is divided into two parts meiosis I and meiosis II. Both Meiosis I and II are divided into four phases; prophase, metaphase, anaphase, and telophase. Meiosis: Meiosis I: Prophase I- Chromosomes Line Up-Cross Over Occurs During the first phase of meiosis I-prophase I- chromosomes begin to condense and the maternal and paternal chromosomes that form each homologous pair line up next to one another, in a process called synapsis. Each complex of four chromatids produced by a homologous pair of chromosomes lining up next to one another is called a tetrad. Networks of protein strands join the homologues in each tetrad together, shifting their

4 positions relative to one another until the corresponding genes on the homologues lineup exactly with one another. After the genes of the homologues are lined up, special enzymes snip out corresponding sections of genes from each of the homologues and the protein strands that brought the homologues together then exchange the sections between the two homologues. In the course of exchanging sections the paternal and maternal chromosomes come to cross over one another at points along their length. These points at which the homologues cross over are called chiasmata. Besides reflecting where chromosomes have exchanged sections of genes, chiasmata also hold homologous pairs together until they are later separated. The exchanging of sections of genes between homologues is appropriately enough called crossing over. Crossing over, by creating chromosomes with unique combinations of genes, is one of the events that occur during meiosis that help increase the diversity of individuals within a species. After crossing over is compete the network of proteins that lined up the homologues and exchanged sections of chromosomes between them disintegrates and the chromosomes coil further and finish condensing. Meiosis: Meiosis I: Prophase I: Spindle Forms and Captures Chromosomes While homologues are lining up and crossing over in the nucleus, outside in the cell cytoplasm, other processes critical to meiosis are also taking place. All eukaryotic cells have a structure called a microtubule organizing center or centrosome near the nucleus that synthesizes the protein tubulin. Tubulin is used to create long thin filaments called microtubules. A network of these microtubules radiates out from the centrosome, helping to form and maintain the cell's shape. During meiosis microtubules also become involved in separating homologues and sister chromatids from one another. Animal centrosomes, differ somewhat from plant centrosomes in that they have a pair of structures called centrioles within them. Relatively early in prophase I the centrioles of animal cells separate and a new centriole develops near the base of each "parent" centriole. Once the animal centrosome has two pairs of centrioles, the network of microtubules that radiate out from the centrosome disintegrate and the centrosome divides into two daughter centrosomes. After division the two new centrosomes move to opposite poles of the cell each forming a network of microtubules that interconnect with the microtubules radiating from the other to form a structure called the spindle. The spindle consists of polar microtubules that radiate from either centrosome and overlap at the equator and kinetochore microtubules each of which will eventually attach to one of the chromosomes at their kinetochore which is located near the centromere where the two sister chromatids are tied together. Once the centrosomes are at opposite ends of the cell and the spindle is completely formed the nuclear envelope disintegrates allowing the kinetochore microtubules to break in and attach to the chromosomes at their kinetochores.

5 Due to the lining up of chromosomes, chromosomes crossing over, and formation of the spindle, Prophase I, is by far the longest stage of meiosis often accounting for over 90% of the time spent by cells in meiosis. Meiosis: Meiosis I: Metaphase I- Chromosomes Align, Independent Assortment The next phase of meiosis I, is metaphase I, during which the kinetochore microtubules move the homologous pairs of chromosomes to the equator of the cell, or what biologists refer to as the metaphase plate, where the two chromosomes in each tetrad lie directly across from one another. The chromosomes in each homologous pairs are now positioned to be drawn to opposite poles. Which chromosome in a homologous pair is positioned to be drawn to a given pole of the spindle is completely random and totally independent of the positioning of any other homologous pair. This has a number of important ramifications not the least of which is, that if the genes for any two traits are on different chromosomes those two traits will be inherited totally independently of one another. This results in what geneticists refer to as an independent assortment of genes. The independent assortment of homologous pairs means that species with a substantial number of chromosomes can create gametes with huge numbers of genetic combinations. For example, each human with their 23 homologous pairs of chromosomes can, just based on the random assortment of homologous pairs, can produce 2 to the 23 rd power or approximately 8 million genetically different gametes. When we consider that human sexual reproduction occurs between two individuals each capable of producing 8 million different gametes based on random assortment alone, we realize that the chances of any two children produced by a couple (other than identical twins who develop from the same sperm and egg) being genetically identical are over one in 64 trillion. When we add variations due to cross over which occur at about the rate of three per homologous pair in humans the odds of any two children produced by a couple being identical become astronomical. Thus any individual is extremely unique genetically even within their own family, let alone the entire world. Meiosis: Meiosis I: Anaphase I- Chromosomes Separate To Opposite Poles During anaphase I the randomly assorted chromosomes are separated and segregated to different ends of the cell, as the chiasmata holding the homologous pairs of chromosomes together let go and the homologues start their journey down the kinetochore microtubules to opposite poles of the spindle, where they eventually form two randomly assorted clusters of chromosomes at each end of the cell. Meiosis: Meiosis I: Telophase I- Nuclei Re-Forms, Cell Division Meiosis I concludes with telophase I, during which the chromosomes conclude their journey to one end of the spindle or the other and the spindle breaks down. What happens

6 next depends on the species of organism. In many species nuclear envelopes form around the chromosomes clusters while the chromosomes relax and de-condense and the cell divides. In other species none of these events occur and cell enters immediately into the first stage of meiosis II- prophase II, with chromosomes still condensed, no nucleus having formed around them, and without cell division having taken place. Meiosis: Meiosis II: Prophase II- Spindle Re-Forms Attaches To Chromosomes Prophase II begins with a new spindle starting to form in the cytoplasm at a right angle to the old one. If the chromosomes extended and a nuclear envelope formed around them during telophase I the chromosomes how condense again and the nuclear envelope dissolves. Once the chromosomes are condensed and free of a nuclear envelope, the kinetochores of each of the two sister chromatids are grabbed by kinetochore microtubules that extend to opposite poles of the spindle. Meiosis: Meiosis II: Metaphase II- Chromosomes Align at Equator Metaphase II is marked by chromosomes being lined up at the cell s equator or metaphase plate. Note that unlike metaphase I where homologous pairs of chromosomes line up along the metaphase plate, in metaphase II it is individual chromosomes made up of two sister chromatids that line up at the metaphase plate. Meiosis: Meiosis II: Anaphase II- Sister Chromatids Separate As anaphase II begins, the centromere that holds each of the sister chromatids together splits in two and the two sister chromatids separate into two individual chromosomes. Each of these new chromosomes works it s way down the kinetochore microtubules to opposite poles of the spindle. Meiosis: Meiosis II: Telophase II- Cell Divides and Gametes Form Telophase II completes meiosis. The spindle is disassembled and the cytoplasm divides. Chromosomes go back into their extended state while a nuclear envelope forms around each cluster of chromosomes producing a new nucleus for each cell. In these new nuclei a dark nucleolus forms indicating that the cells are once again in interphase and beginning to build the proteins critical to the cells growth and development into a mature sex gamete, in the case of our imaginary male animal, sperm. The Importance of Meiosis and Mitosis The result of meiosis is four haploid cells that have only one set of chromosomes, unlike their diploid parent cell that was formed by mitosis and therefore had two sets of chromosomes. Also note that as a result of crossing over and the independent assortment of chromosomes that occurs in meiosis I all of these cells are different genetically from one another. As our imaginary animal is a male, the cells formed by meiosis will develop

7 into sperm, if one of these sperm happens to unite with a egg produced in a similar manner by a female of the species a diploid zygote will be formed that is genetically different from either of the parent organisms. The zygote will develop into an adult organism as a result of cellular growth and division. In the process of cellular division as it occurs in the new developing organism, exact copies of the chromosomes that paired initially to form the zygote, are passed on to each new cell as a result of mitosis. Thus both mitosis and meiosis are critical biological processes for our imaginary species as they are for all species of living organisms. Mitosis enables organisms to grow and develop by insuring that each new cell resulting from cellular division receives exact copies of the genes it needs to function. Meiosis is critical to living organisms in that it provides the means by which diploid organisms form the haploid, genetically diverse, sex cells necessary for sexual reproduction and the production of the genetically diverse offspring critical to increasing a species odds of survival. Genetic variation not only increases a species odds of survival but, along with the mutations that arise in the genes of sex gametes, it is the driving force behind adaptive change and the evolution of species from paramecium in a pond to our own human population.