Day 10 september 30th chapter 6
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Meiosis, sexual reproduction, and karyotyping
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Day 10 september 30th chapter 6
- 1. Day 10 September 30th Chapter 6 Dr. Amy B Hollingsworth The University of Akron
- 2. 6.10–6.14 Meiosis generates sperm and eggs and a great deal of variation.
- 3. 6-10 Sexual reproduction requires special cells made by meiosis.
- 4. Meiosis Gametes Diploid Haploid Maintains a stable genome size in a species
- 5. Meiosis achieves more than just a reduction in the amount of genetic material in gametes. You have two copies of every gene!
- 6. Meiosis has two important features: 1. It reduces the amount of genetic material in gametes. 2. It produces gametes that all differ from each other with respect to the combinations of alleles they carry.
- 7. 6.11 Sperm and egg are produced by meiosis: the details, step-by-step. Mitosis occurs almost everywhere in an animal’s body. Meiosis only occurs in one place. Where?
- 8. Meiosis starts with a diploid cell. One of the specialized diploid cells in the gonads
- 9. Meiosis starts with a diploid cell. A homologous pair, or homologues • The maternal and paternal copies of a chromosome
- 10. Chromosomes are duplicated. Sister chromatids • Each strand and its identical duplicate, held together at the centromere
- 11. Cells undergoing meiosis divide twice. There are two major parts to meiosis: 1. The homologues are separated. 2. Each of the two new cells divides again, separating the sister chromatids into two even newer cells.
- 13. 1. Prophase I The most complex of all of the phases of meiosis Crossing over
- 14. 2. Metaphase I Each pair of homologous chromosomes moves to the equator of the cell.
- 15. 3. Anaphase I Beginning of the first cell division that occurs during meiosis The homologues are pulled apart toward opposite sides of the cell. The maternal and paternal sister chromatids are pulled to the ends of the cell in a random fashion.
- 16. 3. Anaphase I
- 17. 4. Telophase I and Cytokinesis This phase is marked by the chromosomes arriving at the two poles of the cell. The cytoplasm then divides and the cell membrane pinches the cell into two daughter cells.
- 18. 4. Telophase I and Cytokinesis
- 19. Meiosis Division 2 Separating the sister chromatids
- 20. 5. Prophase II The genetic material once again coils tightly making the chromatids visible under the microscope. It is important to note that in the brief interphase prior to prophase II, there is no replication of any of the chromosomes.
- 21. 6. Metaphase II The sister chromatids (each appearing as an X) move to the center of the cell.
- 22. 7. Anaphase II The fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell.
- 23. 8. Telophase II The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close.
- 24. Outcome of Meiosis The creation of four haploid daughter cells, each with just one set of chromosomes which contains a completely unique combination of traits
- 26. 6.12 Male and female gametes are produced in slightly different ways. How do you distinguish a male from a female?
- 29. 6.13 Crossing over and meiosis are important sources of variation.
- 32. 6.14 What are the costs and benefits of sexual reproduction?
- 33. Sexual reproduction advantages? Sexual reproduction leads to offspring that are all genetically different from each other and from either parent in three different ways: 1. Combining alleles from two parents at fertilization 2. Crossing over during the production of gametes 3. Shuffling and reassortment of homologues during meiosis
- 34. Bacteria reproduce asexually, while most plants and animals reproduce sexually. Which is a better method?
- 35. Sexual reproduction disadvantages? Half one parent’s alleles, half the other (and you need the other!)
- 36. With asexual reproduction, the advantages and disadvantages are more or less reversed.
- 37. 6.15–6.16 There are sex differences in the chromosomes.
- 38. 6-15. How is sex determined in humans? Which parent determines the sex of their baby? Why?
- 39. Sex Chromosomes
- 41. 6.16 The sex of the offspring is determined in a variety of ways in other species.
- 42. The sex of the offspring is determined in a variety of ways in other species.
- 43. The sex of the offspring is determined in a variety of ways in other species.
- 44. 6.17–6.18 Deviations from the normal chromosome number lead to problems.
- 45. 6.17 Down syndrome can be detected before birth: Karyotypes reveal an individual’s entire chromosome set. Karyotype • A display of an individual’s complete set of chromosomes
- 47. 1. Amniocentesis
- 48. 2. Chorionic Villus Sampling (CVS) Tissue is removed from the placenta. Because the fetus and placenta both develop from the same fertilized egg, their cells contain the same genetic composition. Can be done several weeks earlier in the pregnancy, usually between the 10th and 12th weeks.
- 50. Nondisjunction The unequal distribution of chromosomes during meiosis Error of cell division that creates a gamete with zero or two copies of a chromosome rather than a single copy
- 51. 6.18 Life is possible with too many or too few sex chromosomes.
- 55. XYY Males
- 56. XXX Females
Editor's Notes
- Section 6.3 Opener Human gametes: sperm attempting to fertilize an egg.
- Our bodies have a problem to solve relating to cell division. We are sexually reproducing organisms; that is, when offspring are created, they carry the genetic material from two individuals. But think about the difficulties this presents. If reproductive cells were produced through mitosis, both parents would contribute a full set of genes—that is, 23 pairs of chromosomes in humans—to create a new individual; the new offspring would inherit 46 pairs of chromosomes in all. And when that individual reproduced, if she contributed 46 pairs of chromosomes and 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. That wouldn’t work at all. At the very least, within a few generations cells would be so overloaded with chromosomes that they would explode. Figure 6-16 Sexual reproduction.
- Sexually reproducing organisms have evolved a way to avoid the inevitable chromosome overload. It is called meiosis, a process that enables organisms, prior to fertilization, to make special reproductive cells called gametes that have only half as many chromosomes as the rest of the cells in the body. In other words, in anticipation of combining one individual’s genome with another’s, meiosis reduces each individual’s genome by half. In humans, for example, gamete cells have only one set of 23 chromosomes, rather than two sets. In studying genetics, the term diploid refers to cells that have two copies of each chromosome, 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 reproduction, two haploid cells, each with one set of 23 chromosomes, are brought together creating a new individual with the proper diploid genome of 46 chromosomes. And when the time comes to reproduce, this new individual also will produce haploid gametes through meiosis 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. This repeats perpetually and maintains a stable genome size in a species. There are some variations on this pattern of alternation between the haploid and diploid state. Most multicellular animals, nonetheless, produce simple haploid cells for reproduction. And after two of those gametes come together as a diploid fertilized egg, multiple cell divisions via mitosis produce a diploid multicellular animal again.
- Meiosis achieves more than just a reduction in the amount of genetic material in gametes. As a diploid individual, you have two copies of every gene: one from your mother and one from your father. When making cells that are haploid from cells that are diploid, an individual creates cells that have one allele for each trait rather than two. Which of these two alleles are included in each gamete? Each egg or sperm is produced with a varied combination of maternal and paternal alleles so that all of the gametes an individual makes carry a unique set of alleles. As a consequence, all of the offspring an individual produces from different gametes inherit a slightly different assemblage of that parent’s alleles. All offspring resemble their parents, yet none resemble them in exactly the same way.
- In all, meiosis has two important features: 1. It reduces the amount of genetic material in gametes. 2. It produces gametes that all differ from each other with respect to the combinations of alleles they carry. In the next sections, we examine the exact steps by which these things occur and investigate the implications of all the variation that meiosis and sexual reproduction generate.
- Mitosis is an all-purpose process for cell division. It occurs all over the body, all the time. Meiosis, on the other hand, is a special purpose process. It occurs in just a single place: the gonads (the ovaries and testes in sexually reproducing animals). And it occurs for just a single reason: the production of gametes (reproductive cells). To better understand meiosis, let’s examine in detail the steps by which a diploid cell can create numerous haploid cells that all differ from each other.
- Meiosis starts with a diploid cell—not just any diploid cell, but one of the specialized diploid cells in the gonads capable of undergoing meiosis.
- Thus, for humans meiosis starts with a cell that has two copies (a maternal copy and a paternal copy) of each of the 23 chromosomes. Together, the maternal and paternal copies of a chromosome are called a homologous pair, or homologues. This means that there are actually 46 DNA molecules in the cell. Figure 6-18 Chromosome vocabulary: homologous and sister chromatids.
- The first thing that happens is that all of these 46 chromosomes are duplicated. Each strand and its identical duplicate are called a pair of sister chromatids, and they are held together at the centromere. So now, instead of having just a maternal and paternal copy of chromosome #1, we have a pair of sister chromatids for the maternal copy of chromosome #1 and a pair of sister chromatids for the paternal copy of chromosome #1. This duplication of the chromosomes occurs before cell division has “officially” begun, during the interphase portion of the cell cycle.
- Unlike mitosis, which has only one cell division, cells undergoing meiosis divide twice. There are two major parts to meiosis. In the first division, the homologues are separated. In other words, for each of the 23 chromosome pairs, the maternal sister chromatids and the paternal sister chromatids are separated into two new cells. In the second division, each of the two new cells divides again, separating the sister chromatids into two even newer cells. At the end of meiosis, there are four new cells, each of which has 23 strands of DNA (23 chromosomes).
- Figure 6-19 Meiosis reduces the genome by half in anticipation of combining it with another genome.
- 1. Prophase I This is by far the most complex of all of the phases of meiosis. It begins with all of the replicated duplicate material condensing. As the sister chromatids become shorter and thicker, the homologous chromosomes come together. That is, the maternal and paternal sets of sister chromatids for chromosome #1 come together so there are four versions of this chromosome lined up together. The maternal and paternal sets of chromosomes #2, #3, #4, and so on, also line up. Under a microscope, the two pairs of sister chromatids appear as pairs of X’s laying on top of each other. At this point, the sister chromatids that are next to each other do something that leads to every sperm or egg cell being genetically unique: they swap little segments. In other words, when this is happening inside of you, some of the genes that you inherited from your mother get swapped onto the strand of DNA you inherited from your father, and the corresponding bit from your father gets inserted into the DNA strand from your mother. This event is called crossing over and it can take place at dozens or even hundreds of spots. As a result of crossing over, every sister chromatid ends up having a unique mixture of the genetic material that you received from your two parents. We explore crossing over in more detail in the next section. The remaining steps of meiosis are relatively straightforward.
- 2. Metaphase I After crossing over has occurred, each pair of homologous chromosomes (that is, the pairs of X’s lying on top of each other) moves to the center of the cell. Remember that each pair of homologous chromosomes is the maternal and paternal version of a chromosome and the replicated copy of each, four strands in all.
- 3. Anaphase I This is the beginning of the first cell division that occurs during meiosis. In this phase, the homologues are pulled apart toward opposite sides of the cell. One of the homologues goes to the top “pole,” the other to the bottom. At this point, something else occurs that contributes, along with crossing over, to making all of the products of meiosis genetically unique. The maternal and paternal sister chromatids are pulled to the ends of the cell in a random fashion. That is, the maternal sister chromatids do not all go to one end of the cell while all of the paternal sister chromatids go to the other. Imagine all the different combinations that can occur in a species with 23 pairs of chromosomes. For chromosome pair #1, perhaps the maternal homologue goes to the top and the paternal to the bottom. And for pair #2, perhaps the maternal homologue also goes to the top and the paternal to the bottom. But maybe for pair #3, it is the paternal that goes to the top and the maternal to the bottom. For each of the 23 pairs, it is random which goes to the top and which goes to the bottom. There is a huge number of possible combinations.
- 4. Telophase I and Cytokinesis After the chromatids arrive at the two poles of the cell, the nuclear membrane re-forms, then cytokinesis occurs and the cytoplasm divides, and the cell membrane pinches the cell into two daughter cells, each with its own nucleus containing the genetic material.
- Genetically speaking, there are two ways to be unique. The obvious way is for an organism to display a trait that has never been seen in another individual. Alternatively—but no less uniquely—an individual can exhibit a collection of traits, none of which is unique, that have never before occurred together in the same individual. Both types of novelty introduce important variation to a population of organisms. The process of crossing over (Figure 6-23 Swapping DNA), which we’ll examine here, creates a significant amount of the second type of variation.
- It’s important to keep in mind that meiosis has just one overriding purpose: producing the gametes that are necessary for sexual reproduction. And gametes have two features that suit them to this purpose: 1) They only have half as much genetic material, and 2) they are genetically varied, with each containing a unique combination of alleles. This genetic variability is greatly enhanced via crossing over. Crossing over occurs when the sister chromatids of the homologous chromosomes all come together. Once the sister chromatids of the homologous chromosomes line up, regions that are close together can swap segments. Every time a swap of DNA segments occurs, an identical amount of genetic material is exchanged, so all four chromatids still contain the complete set of genes that make up the chromosome. It’s just that the specific alleles on each strand end up being different. This means that the combination of traits that are linked together is new. And when a gamete (let’s say it’s an egg) carrying a new combination of traits is fertilized by a sperm, the developing individual will carry a completely novel set of alleles. And so without creating any new traits (i.e., yellow eyes), crossing over still creates gametes with collections of traits that may never have existed together. Figure 6-23 Crossing over and meiosis: creating many different combinations of alleles.
- There are fundamentally different ways that cells and organisms can reproduce. On one hand there is mitosis and asexual reproduction via binary fission. On the other hand, there is meiosis and sexual reproduction. Is one method better than the other? It depends. In fact, the more appropriate question is: what are the advantages and disadvantages of each and under what conditions do the benefits outweigh the costs? Figure 6-24 Crossing over and meiosis: creating many different combinations of alleles.
- Let’s evaluate sexual reproduction first. Advantages? Sexual reproduction leads to offspring that are all genetically different from each other and from either parent in three different ways. 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. As we saw, 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. Recall that as the homologues for each chromosome are pulled to opposite poles of the cell during the first division of meiosis, maternal and paternal homologues are randomly pulled to each pole. This means that there is an extremely large number of different combinations of maternal and paternal homologues that could end up in each gamete.
- The variability among the offspring produced by sexual reproduction enables populations of organisms to better cope with changes in their environment. After all, if the environment is gradually changing from one generation to the next, individuals producing many offspring increase the likelihood that one of their offspring will carry a set of genes particularly suited to the new environment. It’s like buying lottery tickets—the more different tickets that you buy, the more likely one of them will be a winner. Over time, populations of sexually reproducing organisms can quickly adapt to changing environments.
- In the yellow dung fly (Scathophaga stercoraria), males will sometimes wrestle with each other for mating access to a female (Figure 6-25 Drowning in dung). She awaits the outcome of the battle in the same pile of dung. Occasionally, though, females drown in the dung pile as they wait. This is but one illustration that there is a downside to sexual reproduction. There are several others. First, when one parent reproduces, only half of its offsprings’ alleles will come from that organism. The other half will come from the other parent. With asexual reproduction, parents produce identical offspring—their genetic makeup is virtually identical to the parent’s—a very efficient transfer of genetic information from one generation to the next. It also takes time and energy to find a partner. This is energy that asexual organisms can devote to additional reproduction. Moreover, 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 reproduction.
- Advantages? 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 6-26 Pluses and minuses). It 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. Disadvantages? 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 using asexual reproduction and large numbers, including the vast majority of animals and plants, that reproduce sexually. It seems that the conditions favoring each occur in the great diversity of habitats of the 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.
- Section 6.4 Opener XY and XX: Only one of the 23 chromosome pairs determines sex in humans.
- In humans the sex of a baby is determined by its father. The complex sequence of events involved in this process is instigated by one special pair of chromosomes called the sex chromosomes. The sex chromosomes carry information that directs a growing fetus to develop as either a male or a female, and an individual’s sex depends on the sex chromosomes that they inherit from its parents.
- Let’s take a closer look at the sex chromosomes. In humans, we noted that there are 23 pairs of chromosomes in every somatic cell. These can be divided into two different types: There is one pair of sex chromosomes and there are 22 pairs of non-sex chromosomes. The human sex chromosomes are called the X and Y chromosomes (Figure 6-27 X and Y: the human sex chromosomes). How do the X and Y chromosomes differ from the other chromosomes? As we described earlier, all of the genetic information is stored on the chromosomes in all cells of an organism’s body. But most of this information is not sex specific. That is, if you are building an eye or a neuron or a skin cell or a digestive enzyme, it doesn’t matter whether it is for a male or for a female. The instructions are the same. Some genetic information, however, instructs the body to develop into one sex or the other. That information—which does differ dramatically depending on whether it is the genetic information for building a female or a male—is found on the sex chromosomes. An individual has two copies of all of the non-sex chromosomes. One copy is inherited from their mother, one from their father. Individuals also have two copies of the sex chromosomes, but they vary a bit in how they do this. Males have one copy of the X chromosome and one copy of the Y chromosome. Females, on the other hand, don’t have a Y chromosome but instead have two copies of the X chromosome.
- XXX Females Sometimes called “metafemales,” individuals with three X chromosomes occur at a frequency of about one per 1,000 women. Very few studies of this condition have been completed, although initial observations suggest that some XXX females are sterile but otherwise have no obvious physical or mental problems. Figure 6-35 part 4 Characteristics of individuals with too many or too few sex chromosomes.