Lesson 11: Reproduce - Part 2

Transcription

1 Lesson 11: Reproduce - Part 2 Slide 1: Introduction Slide 2: Human chromosomes Fascinating Education Script Fascinating Biology Lessons Every human cell has 46 chromosomes, 23 from the father and 23 matching chromosomes from the mother. 22 of the 23 chromosomes from the mother code for the same proteins as 22 of the 23 chromosomes from the father. The 23rd chromosome, the sex chromosome off to the right, is the only chromosome that comes in two sizes, X and Y. X and Y code for different proteins. First some terminology, the 22 non-sex chromosomes are known as autologous chromosomes. All 23 chromosomes have a nearly identical matching chromosome from the other parent. Each chromosome in a pair is called a homologue, and together they are homologous chromosomes. On average, each chromosome has somewhere around 1,000 genes on it which means humans are made from about 23,000 different genes. Since each homologue chromosome makes the same proteins as its mate theoretically a cell could exist with only one set of 23 chromosomes.

2 A cell with a complete set of 46 chromosomes is called a diploid cell. A cell with only one set of 23 chromosomes is called a haploid cell. During the S-phase of interphase, each of the 46 chromosomes replicates itself with an exact copy, so that there are now 46 pairs of chromatids. The two chromatids remain attached to each other until mitosis and only then are they pulled apart. Slide 3: Chromosome replication To recap, 46 chromosomes, 23 from the father and 23 homologous chromosomes from the mother, replicate during S-phase. Each chromosome becomes two chromatids held together at their centromere. That can be a little confusing because the same term, chromosome, is used to describe a single strip of DNA and now after replication, two strips of DNA. When the two chromatids separate during mitosis, each chromatid is now called a chromosome again. Follow the numbers. We started out on the left with 23 chromosomes from the father and 23 from the mother. After replication during S-phase, we still have 23 chromosomes from the mother and 23 from the father, even though each chromosome is now made up of two chromatids. When the chromatids separate during mitosis, the 23 chromosomes from each parent now become 46 chromosomes from each parent. During telophase each parent gives 23 chromosomes to each daughter cell. Slide 4: Mitosis The separation of sister chromatids occurs during mitosis. Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase, or PMAT.

3 In the first phase, prophase, two things happen. One is that the individual strands of DNA condense and become visible and the other is the nuclear membrane dissolves. Here, for example, is a cell before it enters prophase. The two features that tell you this cell is in interphase are first, the DNA is still peppery or granular; the individual chromosomes still too long and too thin to see even under a microscope. Only when they shorten and thicken enough in prophase will they become visible under the microscope. The second feature that indicates the cell is still in interphase is that the nuclear membrane is readily visible. During the next phase, metaphase, all the chromosomes are gathered up along the midline of the cell. Notice that I didn t say the midline of the nucleus, because there is no nucleus anymore. The nuclear membrane is completely gone by now. The cell is getting ready to split the attached chromatids apart, sending one set to one daughter cell and the other set to the other daughter cell. In anaphase, half the chromatids are being pulled to one side of the cell and half to the other. Each chromatid is now called a chromosome again. Once the chromosomes have been pulled apart completely, the cell begins to pinch apart and a new nuclear membrane begins to form. This final phase of mitosis is called telophase. Slide 5: Cells during mitosis What questions occurred to you as you viewed this overview? Did you wonder how all the chromosomes knew to line up along the midline of the cell during metaphase? Did you wonder what those two fuzzy balls were that developed during metaphase? Did you wonder what was pulling the chromosomes to each side of the cell in anaphase? If you look carefully at this cell in metaphase, you can see thin strands stretching from the fuzzy balls and apparently attaching to the chromosomes. These strands are actually thin, hollow tubes, microtubules, made of protein.

4 The two fuzzy balls are made up of two small cylinders, called centrioles, surrounded by lots of short fibers that anchor the centrioles in place. The microtubules connect the centrioles to the paired chromatids, and by pulling with equal force, the two centrioles pull the attached chromatids into the center of the cell. Slide 6: Images of female chromosomes Here are chromosomes from a human female. During anaphase each chromatid inches its way toward the centriole along the microtubule stretching between the centriole and the chromatid. They come from a female because there are two X chromosomes at the lower right. At what stage in the cell cycle were these chromosomes isolated? The fact that you can see the chromosomes indicates that this is not interphase. There are 23 groups here each group containing two sets of homologous chromosomes, one set from the mother and one from the father. Each set is comprised of two chromatids joined at their centromere. Chromatids form during the S-phase of interphase but don't become visible until prophase. In mitosis, they line up along the midline of the cell during metaphase, and then separate during anaphase. So these paired chromatids must have been isolated in prophase or metaphase.

5 Slide 7: Binary fission and budding The DNA in bacteria is so short that there is no need to chop it into chromosomes. Bacterial DNA is a single loop. Each gene is represented only once on the DNA loop which makes bacterial DNA, haploid. When a bacterium reproduces, it simply replicates the single stand and gives it to the daughter cell. There s no centromere, no lining up at the midline, and no centriole pulling the chromatids apart. This type of cellular reproduction is called binary fission. These bacteria are undergoing binary fission. Yeast cells are also haploid and they duplicate their DNA without centrioles, too. Instead of dividing into equal halves however, yeast cells divide by pinching off a part of the cell. This is called budding. Slide 8: Gametes The only haploid cells in humans are the egg cells and sperm cells, what we call gametes. Egg and sperm contain only 23 chromosomes. When an egg and sperm join they form a regular diploid cell containing 23 pairs of chromosomes. The process of making a haploid gamete is called meiosis. Slide 9: Why the difference in cellular reproduction? 99.9% of the cells in a plant or animal reproduce by mitosis, but when it comes to reproducing the entire plant or animal, an entirely different method of reproduction, meiosis, was developed.

6 Why? Why should individual cells reproduce differently from the whole organism? Because reproduction serves a different purpose in individual cells than it does in the whole organism. Individual cells reproduce to replace ones that wear out and die. You want the replacement cell to look and function exactly like the parent cell. What's the purpose of reproduction of the whole organism? Is it simply to replace aging parents? If it were, then mitosis would be perfect to replace aging parents with exact replicas. But that's not the real purpose of reproduction. The real purpose of reproduction of the whole organism is to ensure survival of the species. Every species must adapt to a changing environment, either because the species has migrated into a new environment, or the environment has changed beneath their feet. In either case a species must find new ways to obtain food and water, keep warm, avoid predators, mate successfully, and care for their young. Mitosis doesn't prepare offspring for environmental changes because mitosis generates exact replicas of the previous generation. If the inevitable environmental changes make it impossible for the parents to survive, the children will be no better off than their parents because being exact replicas of their parents, they re in no better position to handle the environmental changes than their parents. Reproduction of the organism has a much more important job than simply replacing aging parents. Reproduction of the organism has to prepare the descendants for the future, a future that may change in unpredictable ways and threaten the very survival the species. What reproduction of the organism needs is a way to prepare future generations for dangerous changes in the environment when the nature of those changes is totally unpredictable. How are we going to do that?

7 Slide 10: Genetic diversity and the survival of species By making the children not look and function exactly like their parents. We want the descendants of plants and animals to differ from their parents in case the environment changes in some unfavorable way. Admittedly, if all the children differ from their parents, some of the children may not do as well in the current environment, but Mother Nature doesn t care about the individual. She only cares about the species as a whole. She s willing to let some of today s offspring be less adapted to today s environment if it means they will be better prepared for tomorrow s changed environment. The only way to make the offspring of a plant or animal differ from its parents is to change its DNA. That s the purpose of meiosis. In the process of making haploid gametes, meiosis mixes up the genetic makeup of each parent s gametes so that each sperm cell and each ovum ends up with a different set of genes from every other sperm and ovum. Meiosis creates genetic diversity. Genetic diversity is essential if plants and animals are going to adapt to a changing environment, because genetic diversity offers some of the offspring the possibility that their minor difference may provide them with a slightly better chance to adapt to a changed environment. But even in an unchanging environment, genetic diversity is still essential to the survival of the species. You can t wait for the environment to change and then hope that a plant or animal s DNA will somehow change quickly enough to survive the environmental changes. The species may be extinct by the time the necessary genetic changes are made. If a species is going to survive for the long term, genetic diversity must have a head start before the environment changes. No greater truth has ever emerged from rock n' roll than the song sung by Jerry Butler, Only the Strong Survive. Slide 11: Meiosis vs. mitosis So how does meiosis mix up the genetic pool? The easiest way to see how meiosis mixes up the genetic pool is compare it to mitosis. Here is an imaginary nucleus. In the top half are three chromosomes undergoing mitosis, and in the

8 lower half three chromosomes undergoing meiosis. The three chromosomes up top represent all 46 chromosomes, as do the three chromosomes down below. The first big difference between mitosis and meiosis occurs during metaphase. In mitosis up top, chromosome replicate and line up along the midline as 46 pairs of chromatids. When the chromosomes split down the midline during anaphase, one chromatid will pull to one side of the cell and the other identical chromatid to the other side. Now that the chromatids have separated they are back to being called chromosomes. Each side of the cell now has 46 identical chromosomes, 23 from the mother and 23 homologous chromosomes from the father; when the cell divides during telophase both daughter cells will be identical to each other. Meiosis, down below, also makes a copy of each chromosome, but instead of every pair of chromatids lining up along the midline, single-file, every pair of chromatids finds the homologous pair of chromatids from the other parent and together they line up double-file. In meiosis, then, the line of chromosomes is only half as long as it in mitosis. So when the cell divides during telophase both daughter cells will have a total of 46 individual chromosomes, but the chromosomes are still attached to each other as 23 pairs of chromatids. Slide 12: Gametes These daughter cells do have 23 chromosomes, but because the chromosomes are still pairs of chromatids, the daughter cells are not gametes. A gamete, that is, a sperm or ovum, needs 23 individual chromosomes, not 23 pairs of chromatids.

9 When a gamete combines with another gamete of the opposite sex, the total DNA can add up to only 23 pairs of homologous, individual chromosomes. The two daughter cells have to somehow reduce their 23 doublechromatid chromosomes to 23 single-chromatid chromosomes. What do you suppose these two daughter cells do? Each daughter cell undergoes a second cell division, called meiosis II. Meiosis II separates the two chromatids. Slide 13: Gamete division In this second cell division the 23 chromosomes line up along the midline and the chromatids split apart, half going to one side and half to the other side. In telophase the cell divides into two gametes. To recap, in meiosis I, one particular cell in the testes or ovary divides into two daughter cells. In meiosis II, each of the meiosis I daughter cells divides into two gametes. Each gamete has 23 chromosomes and is now ready to meet another gamete of the opposite sex to form a diploid cell. Doesn't that seem like a complicated way to reduce 46 chromosomes down to 23? daughter cell? Why didn't meiosis simply take this diploid cell with 46 chromosomes, pair up the homologous chromosomes along the midline, and pull them apart 23 chromosomes to each Because the real purpose of meiosis is to mix up the genes to create genetic diversity. So let's go back and review meiosis and pick out where meiosis mixes up the genetic pool.

10 Slide 14: Genetic diversity The first place meiosis mixes up the genes is during prophase when the 23 duplicated chromosomes from mother and father first pair up as a tetrad of 4 chromatids. The mother's and father's chromatid pairs rub up against each other and stick to each other. The attached segment of chromatids is called a synapsis. During synapsis homologous genes are exchanged between the mother's and father's chromatids. Some of the mother s genes are now in the father s chromosomes, and some of the father s genes are now in the mother s chromosomes. The process is called crossing over because the genes cross over to another chromosome. Slide 15: Genetic diversity The second place meiosis mixes up the genetic pool is after a cell in the testes or ovaries has already replicated its 46 chromosomes and has 46 pairs of chromatids. As that cell enters metaphase, the 46 replicated chromosomes pair up along the midline into homologous partners, mother on one side - father on the other side. Whether a mother's chromosome lines up on the left or the right is completely random. How many different combinations can you imagine if at every one of the 23 positions, the mother and father could be on either side of midline? If there were only 1 position, there would only be two ways for the mother and father to line up: mother on the left and father on the right, or vice versa. If there were two positions, there would be four combinations. If there were 3 positions, there would be 8 possible combinations. If n is the number of positions, 2 n is the number of possible combinations. For 23 positions, there would be 2 23 combinations or 8,388,608 different combinations. For all practical purposes, no two daughter cells will have the same genetic makeup.

11 And remember, some of the mother s and father s chromosomes had already undergone crossing-over with each other over during prophase. This further enhances the variety of genes being separated as the chromosomes from each parent line up along the midline during metaphase I before being pulled apart in anaphase I. Slide 16: Metaphase II The third place meiosis increases genetic diversity is during metaphase II when the 23 chromosomes, each consisting of a pair of sister chromatids, line up along the midline in preparation to be separated. Because of crossing over during prophase I, the two sister chromatids are not identical, so when these two nonidentical sister chromatid line up along the midline in metaphase II, every crossed-over chromatid randomly lines up on either side of the midline. The result is that the crossed-over chromatids are randomly pulled into the final gamete during anaphase. Slide 17: Random combination of gametes The fourth place meiosis increases genetic diversity is at fertilization when an egg and sperm unite. Every sperm and every ovum has a one of a kind genetic makeup. What better way to mix up the genes of an individual than to combine one unique genetic makeup with another. Slide 18: Catching our breath Before we go on, I think we should stop here and catch our breath. Meiosis only occurs in cells that are to become gametes. The purpose of meiosis is to mix up the gene pool to increase genetic diversity in the offspring. That way some of the offspring may be better able to adapt to any changes in the environment that have already taken place, or may take place in their lifetime.

12 Meiosis mixes up the gene pool at four separate places. The first place is during metaphase, when the 46 chromosomes line up along the midline. Here, for purposes of demonstration, are 18 chromosomes, 9 from the mother and 9 homologous chromosomes from the father, lining up along the midline in metaphase of mitosis. There are two columns for each chromosome because by metaphase, each chromosome has already duplicated itself into two chromatids, and each column represents a chromatid. During telophase, the chromatids separate, and each daughter cell in mitosis gets exactly the same set of 46 chromosomes. In meiosis, instead of lining up single-file as they do in mitosis, the 46 pairs of chromatids line up double-file with the mother s pair of chromatids on one side of midline and the homologous pair of chromatids from the father on the other side of midline. Whether the mother s is to left and the father s is to right, or viceversa, is completely random. The second opportunity for mixing up the gene pool occurs during this period when the homologous pairs of chromatids are lined up along the midline during metaphase. Some of the mother s and father s chromosomes undergo synapsis and cross over, offering a second opportunity to mix up the gene pool. The third opportunity occurs after the two daughter cells have split and separated. The 23 pairs of chromatids line up along the midline randomly, meaning that each crossed-over chromatid is randomly assigned to one or the other gamete. A fourth opportunity for genetic diversity, of course, is the unpredictability of which sperm is going to fertilize which egg. Slide 19: Advantages of Mitosis Meiosis is great at increasing genetic diversity, but meiosis is used by only a few cells. In plants meiosis is used by the ovary and pollen located on the anther. In animals, meiosis is used by a few cells in the ovary and testes. 99% of plant and animal cells use mitosis.

13 Why is mitosis favored over meiosis for the vast majority of plant and animal cells? What does mitosis have to offer? Mitosis can be depended on to deliver the exact same cell as the parent cell. This is exactly what you want if you damage say, your liver. You want new liver cells that function just like the original liver cells. The second thing mitosis has to offer is that it's fast, much faster than meiosis, because it doesn t involve crossing over or a second cell division, and it doesn t depend on two gametes finding each other to make a diploid cell. So mitosis is perfect for rapidly forming a scar when you cut yourself. What would you use to grow the tips of this root, mitosis or meiosis? Mitosis, because you want the root to grow quickly into the soil. Slide 20: Asexual Reproduction Most plants and animals reproduce through sexual reproduction, but some plants reproduce asexually, that is, by mitosis. Strawberry plants, for example, send shoots, called stolons, above ground and every so often, a strawberry plant will grow from a node along the stolon. Stolons grow asexually by mitosis, and the new plants that sprout up are clones of the original strawberry plant. This type of reproduction is called vegetative reproduction. Here s a question, in mitosis the DNA of the daughter cell is the same as the DNA in the parent cell, if an adult strawberry plant sends off a shoot and a new plant sprouts from that stolon, the new plant begins life with the same DNA as its parent. If that were true, the daughter plant should only live as long as the parent, but the daughter plant lives longer than the parent.

14 How was the daughter plant s biological clock reset if its DNA is identical to the parent's? Aspen trees can rapidly repopulate after a forest fire because, like strawberries, they reproduce by vegetative reproduction. Their shoots travel underground as rhizomes. Because all the aspen trees are connected, aspen trees turn color at the same. Redwood trees can also reproduce by vegetative reproduction, so they often grow in clusters called fairy rings. Slide 21: Fungi That fuzzy stuff on strawberries is a fungus. So is this. Some of you may have this fungus growing in your basement. Fungi use both sexual and asexual means to reproduce. Slide 22: Frogs When plants and animals first evolved in the ocean many of them reproduced by meiosis. The male gamete would swim through the water to fertilize the female gamete. Ferns and frogs were some of the first plants and animals to move onto land from the ocean. Neither of them abandoned this way to reproduce.

15 Frogs to this day mate by the male frog clasping a female loaded with eggs. The female releases her eggs into a puddle of water, and the male releases his sperm over the eggs. Slide 23: Ferns Ferns likewise require the male sperm to swim through water to reach the female ovum. This takes place in tiny droplets of water collecting along the undersurface of a tiny fern plant called a prothallus which is a tiny leaf maybe a quarter of an inch across growing in a warm shaded area less than an inch off the ground. On the undersurface of prothallus are haploid eggs and haploid sperm. When a prothallus sperm fertilizes a prothallus ovum the resulting diploid zygote grows into a typical fern plant. When the fern matures, spores develop along the undersurface of its leaves. These haploid spores are released into the air to find a shady spot of moist soil. There, they grow into the tiny prothallus plants, and the reproductive cycle starts again. Why should a plant use both mitosis and meiosis to reproduce itself? What advantage do ferns gain by doing this? To understand that, we need to understand the meaning of dominant and recessive alleles, which we'll cover in the next lesson.

16 Slide 24: What you know so far 1. Humans have 46 chromosomes, 23 from the mother and 23 from the father. 22 of the 23 pairs of chromosomes are the autologous chromosomes, and the remaining pair are the X and Y sex chromosomes. 2. Almost all cells reproduce by mitosis where each chromosome makes a chromatid copy of itself and holds onto the chromatid at the centromere. During mitosis, the two chromatids separate and move into each daughter cell. Slide 25: What you know so far 3. Mitosis ensures that the daughter cell functions exactly like the parent cell. Mitosis is also quick, making it useful when time is of the essence. 4. The sex cells use meiosis to reproduce. Meiosis serves two purposes. One purpose is to reduce the number of chromosomes from diploid to haploid, and the other is to ensure that each gamete is genetically different from every other gamete. 5. To reduce the genetic material from diploid to haploid, meiosis performs two cell divisions. In the first cell division, each daughter cell receives a complete set of 23 chromatid pairs. Slide 26: What you know so far 6. The composition of the 23 chromatid pairs is the result of a random distribution of the mother s and father s chromatid pairs. 7. Moreover, before the two sets of 23 chromatid pairs separated, many of autologous chromatid pairs underwent synapsis and crossing over, so that not only are the 23 chromatid pairs in each daughter cell a random distribution of their parent s chromosomes, in each chromatid pair, one of the chromatids likely contains random alleles from the autologous chromosome of the other parent. Slide 27: What you know so far 8. So that during the second cell division in meiosis, each chromatid that separates into a final gamete is very likely unique. 9. Further mixing of the gene pool occurs in the random fertilization of a sperm and an ovum. 10. Mitotic, or vegetative, reproduction of entire plants occurs with strawberries, aspen trees, and redwood trees.

17 Slide 28: What you know so far 11. Ferns undergo a life cycle that requires haploid sperm cells to swim through a drop of water on the undersurface of a prothallus leaf to fertilize a haploid egg. The fertilized egg grows into a fern when produces spores that grow into a small prothallus plant. Thus, ferns use both meiosis and mitosis to reproduce.