Chapter 13 Meiosis notes
Chapter 13 Meiosis notes BIOL 2311
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Date Created: 07/15/16
Chapter 13 – Meiosis MingHan Lu A male reproductive cell—a sperm A female reproductive cell – an egg o Both unite to form a new individual. The process of uniting sperm and egg is called fertilization. Question that arose: How can the chromosomes from a sperm cell and an egg cell combine, but form an offspring that has the same chromosome number as its mother and its father? o During the formation of gametes—reproductive cells such as sperm and eggs—there must be a distinctive type of cell division that leads to a reduction in chromosome number. If the sperm and egg contribute an equal number of chromosomes to the fertilized egg, they must each contain half of the usual number of chromosomes. Then, when sperm and egg combine, the resulting cell has the same chromosome number as its mother’s cells and its father’s cells have. Meiosis is nuclear division that leads to a halving of chromosome number and ultimately to the production of sperm and egg. 13.1 How Does Meiosis Occur? Important observation: Each organism has a characteristic number of chromosomes. Chromosomes Come in Distinct Sizes and Shapes Females lack a Y chromosome but contain a pair of X chromosomes. The X and Y chromosomes are called sex chromosomes and are associated with an individual’s sex. Nonsex chromosomes, such as chromosomes 24 in Drosophilia, are autosomes. 1 Chapter 13 – Meiosis MingHan Lu Chromosomes that are the same size and shape are called homologous chromosomes, or homologs, and the pair is called a homologous pair. o Homologous chromosomes are similar in content as well as in size and shape o Homologous chromosomes carry the same genes. A gene is a section of DNA that influences some hereditary trait in an individual. Influences eye color, wing size and shape, and bristle size. Versions of a gene found on homologous chromosomes may differ. o Allele: different versions of the same gene. Homologous chromosomes carry the same genes, but each homolog may contain different alleles. The Concept of Ploidy karyotype – the number and types of chromosomes present o Insects, humans, oak trees, and other organisms that have two versions of each type of chromosome are called diploid. Diploid organisms have two alleles of each gene. One allele is carried on each of the homologous pairs of chromosomes. Although a diploid individual can carry only two different alleles of a gene, there can be many different alleles in a population. Organisms whose cells contain just one of each type of chromosome – for chromosomeswo sets of example, bacteria, archaea, and many algae and fungi—are called haploid. Haploid organisms have only one copy of each chromosome and just one allele of each gene. Notation: The letter n stands for the number of distinct types of chromosomes in a given cell and is called the haploid number. If sex chromosomes are present, they are counted as a single type in the haploid number. In humans, n is 23. To indicate the number of chromosome sets observed, a number is placed before the n. Thus, a cell can be n, or 2n, 3n, and so on. The combination of the # of sets and n is termed the cell’s ploidy. Diploid cells or species are designated 2n, because two (sets of) chromosomes of each type are present – one (set) from each parent. A maternal chromosome comes from the mother, and a paternal chromosome comes from the father. 2 Chapter 13 – Meiosis MingHan Lu Humans are diploid; 2n is 46. Haploid cells or species are labeled simply n, because they have just one set of chromosomes—no homologs are present. To summarize, the haploid number n indicates the number of distinct types of chromosomes present. In contrast, a cell’s ploidy (n, 2n, 3n, etc) indicates the number of each type of chromosome present. Instead of having two homologous chromosomes per cell, polyploidy species have three or more of each type of chromosome in each cell. An Overview of Meiosis Cells replicate each of their chromosomes before undergoing meiosis. At the start of meiosis, chromosomes are in the same state they are in before mitosis. Each chromosome will consist of two identical sister chromatids. The trick is to recognize that unreplicated and replicated chromosomes are both considered single chromosomes—even though the replicated chromosome contains two sister chromatids. Whether there is a copy present, the amount of unique information is the same. Meiosis Comprises Two Cell Divisions Meiosis consists of two cell divisions, called meiosis I and meiosis II. o Occurs consecutively but differ sharply. During meiosis I, the homologs in each chromosome pair separate from each other. o One homolog goes to one daughter cell the other homolog goes to the other daughter cell. At the end of meiosis I, each of the two daughter cells has one of each type of chromosome instead of two, and thus half as many chromosomes as the parent cell had. During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells. Notice, however that each chromosome still consists of two sister chromatids – meaning that chromosomes are still replicated at the end of meiosis I. During meiosis II, sister chromatids from each chromosome separate. o One sister chromatid (now called a daughter chromosome) goes to one daughter cell. The cells produced by meiosis II also have one of each type of chromosome, but now the daughter chromosomes are no longer replicated. Sister chromatids separate into daughter chromosomes during meiosis II. (same as mitosis) o Meiosis II is actually equivalent to mitosis occurring in a haploid cell. 3 Chapter 13 – Meiosis MingHan Lu In meiosis I, sister chromatids stay together. This sets meiosis I apart from both mitosis and meiosis II. As in mitosis, chromosome movements during meiosis I and II are coordinated by microtubules of the spindle apparatus that attach to kinetochores located at the centromere of each chromosome. Meiosis I is a Reduction Division The outcome of meiosis I is a reduction in chromosome number. For this reason, meiosis I is known as a reduction division. Reduction is another important way in which meiosis I different from meiosis II and mitosis. In plants and animals, the original cell entering meiosis is diploid and the four final daughter cells are haploid. o In animals, the haploid daughter cells, each containing one of each homologous chromosome, eventually go on to form egg cells or sperm cells via a process called gametogenesis. When two haploid gametes fuse during fertilization, a full complement of chromosomes is restored. (both sets of chromosomes are restored) o The cell that results from fertilization is diploid and is called a zygote. In this way, each diploid individual receives a haploid chromosome set from its mother and a haploid set from its father. 4 Chapter 13 – Meiosis MingHan Lu Life cycle – the sequence of events that occurs over the life span of an individual, from fertilization to the production of offspring. The Phases of Meiosis I Meiosis begins after chromosomes have been replicated during S phase. Before the start of meiosis, chromosomes are extremely long structures, just as they are during interphase of the normal cell cycle. Early Prophase I During early prophase I, the nuclear envelope begins to break down, chromosomes condense and the spindle apparatus begins to form. Then a crucial event occurs: Homologous chromosome pairs come together. The end result of this process is called synapsis. The structure that results from synapsis is called a bivalent (2) or tetrad (4). A bivalent consists of paired homologous chromosomes, with each homolog consisting of two sister chromatids. Chromatids from different homologs are referred to as nonsister chromatids. o Redcolored chromatids are nonsister chromatids with respect to the bluecolored chromatids. Late Prophase I During late prophase I, the nuclear envelope breaks down and microtubules of the spindle apparatus attach to kinetochores. Nonsister chromatids begin to separate at many points along their length. They stay joined at certain locations, however, each of which forms an Xshaped structure called a chiasma. The chromatids that meet to form a chiasma are nonsister chromatids. At each chiasma there is an exchange of parts of chromosomes between paternal and maternal homologs. o These reciprocal exchanges between different homologs create nonsister chromatids that have both paternal and maternal segments. This process of chromosome exchange is called crossing over. o When crossing over occurs, the chromosomes that result have a mixture of maternal and paternal alleles. Crossing over is a major way that meiosis creates genetic diversity. 5 Chapter 13 – Meiosis MingHan Lu Metaphase I The next major stage in meiosis I is metaphase I. o This is when kinetochore microtubules move the pairs of homologous chromosomes (bivalents) to a region called the metaphase plate in the middle of the spindle apparatus. o Two key points about chromosome movement: Each bivalent moves to the metaphase plate independently of the other bivalents, The alignment on one side or the other of the metaphase plate is random for maternal and paternal homologs from each chromosome. Anaphase and Telophase I Sister chromatids of each chromosome remain together. During anaphase I, the homologous chromosomes in each bivalent separate and begin moving to opposite poles of the spindle apparatus. Meiosis I concludes with telophase I, when the homologs finish moving to opposite sides of the spindle. o When meiosis I is complete, cytokinesis occurs and two haploid daughter cells form. Meiosis I: A Recap The end result of meiosis I is that one chromosome of each homologous pair is distributed to a different daughter cell. A reduction division has occurred: The daughter cells of meiosis I are haploid, having only one copy of each type of chromosome. The sister chromatids remain attached in each chromosome, however, meaning that the haploid daughter cells produced by meiosis I still contain replicated chromosomes. The chromosomes in each cell are a random assortment of maternal and paternal chromosomes as a result of (1) crossing over and (2) the random distribution of maternal and paternal homologs during metaphase. ** Chromosome movement takes place as microtubules that are attached to the kinetochore dynamically assemble and disassemble. When meiosis I is complete, the cell divides and two haploid daughter cells are produced. The Phases of Meiosis II Throughout meiosis I, sister chromatids remained attached. Because no chromosome replication occurs between meiosis I and meiosis II, each chromosome consists of two sister chromatids at the start of meiosis II. o And because only one member of each homologous pair of chromosomes is present, the cell is haploid. 6 Chapter 13 – Meiosis MingHan Lu During prophase II, a spindle apparatus forms in both daughter cells. Microtubules attach to kinetochores on each side of every chromosome and begin moving the chromosomes toward he middle of each cell. In metaphase II, the chromosomes are lined up at the metaphase plate. The sister chromatids of each chromosome separate during anaphase Ii and move to different daughter cells during telophase II. Once they are separated, each chromatid is considered an independent daughter chromosome. o Meiosis II results in four haploid cells, each with one daughter chromosome (sister chromatid) of each type in the chromosome set. o Like meiosis I, meiosis II is continuous. These stages are essentially those of mitosis. Prophase II – The spindle apparatus forms. If a nuclear envelope formed at the end of meiosis I, it breaks apart. Metaphase II – Replicated chromosomes, consisting of two sister chromatids, are lined up at the metaphase plate. Anaphase II – Sister chromatids separate. The daughter chromosomes that result begin moving to opposite poles of the spindle apparatus Telophase II – Chromosomes finish moving to opposite poles of the spindle apparatus. A nuclear envelope forms around each haploid set of chromosomes. When meiosis II is complete, each cell divides to form two daughter cells. Because meiosis II occurs in both daughter cells of meiosis I, the process results in a total of four daughter cells from each original, parent cell. o Summary: 1 diploid cell with replicated chromosomes four haploid cells with unreplicated chromosomes. A key difference between the two processes (mitosis vs meiosis) is that homologous chromosomes pair early in meiosis but do not pair at all during mitosis. Because homologs pair through synapsis in prophase of meiosis I, they can migrate to the metaphase plate together and then separate during anaphase of meiosis I, resulting in a reduction division. A Closer Look at Synapsis and Crossing Over Step 1 – sister chromatids are held together along their full length by proteins known as cohesins. At the entry to prophase I, chromosomes begin to condense. Step 2 – Homologs pair. In many organisms, pairing begins when a break is made in the DNA of one chromatid. This break initiates a crossover between nonsister chromatids. Step 3 – A network of proteins forms the synaptonemal complex, which holds the two homologs tightly together. 7 Chapter 13 – Meiosis MingHan Lu Step 4 – The synaptonemal complex disassembles in late prophase I. The two homologs partially separate and are held together only at chiasmata. Attachments at chiasmata are eventually broken to restore individual, unconnected chromosomes. At a chiasma, the nonsister chromatids from each homolog have been physically broken at the same point and attached to each other. As a result, corresponding segments of maternal and paternal chromosomes are exchanged. 13.2 Meiosis Promotes Genetic Variation Thanks to the independent shuffling of maternal and paternal chromosomes and crossing over during meiosis I, the chromosomes in one gamete are different from the chromosomes in another gamete and different from the chromosomes in the parental cells. Fertilization brings haploid sets of chromosomes from a mother and father together to form a diploid offspring. The chromosome complement of this offspring is unlike that of either parent. It is a random combination of genetic material for each parent. ** Changes in chromosome sets occur only during sexual reproduction—NOT during asexual reproduction. ** Asexual reproduction is any mechanism of producing offspring that does not involve the production and fusion of gametes. Asexual reproduction in eukaryotes is based on mitosis. The chromosomes in cells produced by mitosis are identical to the chromosomes in the parental cell. Sexual reproduction is the production of offspring through the production and fusion of gametes. Sexual reproduction results in offspring that have chromosome complements unlike those of their siblings or their parents. Chromosomes and Heredity The changes in chromosomes produced by meiosis and fertilization are significant because chromosomes contain the cell’s hereditary material. Chromosomes store genes, and identical copies of chromosomes are distributed to daughter cells during mitosis. o Thus, cells that are produced by mitosis are genetically identical to the parent cell, and offspring produced during asexual reproduction are genetically identical to one another as well as to their parent. Offspring produced by asexual reproduction are clones—or exact copies—of their parent. 8 Chapter 13 – Meiosis MingHan Lu In contrast, the offspring produced by sexual reproduction are genetically different from one another and unlike either their mother or their father. The Role of Independent Assortment Each somatic cell in your body contains 23 homologous pairs of chromosomes and 46 chromosomes in total. o Each chromosome contains genes, and genes influence particular traits. When pairs of homologous chromosomes line up during meiosis I and the homologs separate, a variety of combinations of maternal and paternal chromosomes can result. Each daughter cell gets a random assortment of maternal and paternal chromosomes. o This phenomenon is known as the principle of independent assortment. The appearance of new combinations of alleles is called genetic recombination. With each additional pair of chromosomes, the number of combination doubles. n In general, a diploid organism can produce 2 combinations of maternal and paternal chromosomes, where n is the haploid chromosome number. This means that you (n=23) produce 2 = 8.4 million gametes that differ in their combination of maternal and paternal chromosome sets. o Generates an impressive amount of genetic variation among gametes. The Role of Crossing Over Thus, crossing over produces new combinations of alleles within a chromosome —combinations that did not exist in either parent. This phenomenon is known as recombination. Crossing over is an important source of genetic recombination. Genetic recombination is important because it dramatically increases the genetic variability of gametes produced by meiosis. o The independent assortment of homologous chromosomes during meiosis generates varied combinations of chromosomes in gametes; genetic recombination due to crossing over varies the combinations of alleles along each chromosome that is involved in a crossover. With crossing over, the number of genetically different gametes that you can produce is much more than 8.4 million (limitless). How Does Fertilization Affect Genetic Variation? Crossing over + independent assortment of maternal and paternal chromosomes ensure that each gamete is genetically unique. Even if two gametes produced by the same individual fuse to form a diploid offspring – in which case selffertilization (selfing)– the offspring are very likely to be genetically different from the parent. Selfing is common in many plant species, and it happens with hermaphrodites (contain both male & female sex organ). o Selffertilization is rare/nonexistent. Instead, gametes from different individuals combine to form offspring. (woman + man) 9 Chapter 13 – Meiosis MingHan Lu This process is called outcrossing. Increases genetic diversity of offspring even further because it combines chromosomes from different individuals. These chromosomes are likely to contain different alleles. Two parents can potentially produce 8.4 million x 8.4 million = 70.6 x 10 genetically distinct offspring, even without crossing over. 13.3 What happens When Things Go Wrong in Meiosis? Down syndrome o The presence of an extra copy of chromosome 21 Situation is called trisomy (threebodies). How do Mistakes Occur? For a gamete to get one complete set of chromosomes, 2 steps in meiosis must be perfectly executed: 1. The chromosomes in each homologous pair must separate from each other during the first meiotic division, so that only one homolog ends up in each daughter cell. 2. Sister chromatids must separate from each other and move to opposite poles of the dividing cell during meiosis II. If both homologs in meiosis I or both sister chromatids in meiosis II move to the same pole of the parent cell, the products of meiosis will be abnormal. o This sort of meiotic error is referred to as nondisjunction, because the homologs or sister chromatids fail to separate, or disjoin. Gametes that contain an extra chromosome are symbolized as n+1; gametes that lack one chromosome are symbolized as n1. o If an n+1 gamete is fertilized by a normal n gamete, the resulting zygote will be 2n+1. This situation is trisomy. o If the n1 gamete is fertilized by a normal n gamete, the resulting zygote will be 2n1. This situation is called monosomy. Cells that have too many/ too few chromosomes of a particular type are said to be aneuploid (“withoutform”). Most of the errors result from the failure of a homologous pair to separate in the anaphase of meiosis I; less often, sister chromatids stay together during anaphase of meiosis II. It also can’t really produce any viable offspring Mistakes in meiosis are common and are the leading cause of spontaneous abortion (miscarriage) in humans. Why do Mistakes Occur? 10 Chapter 13 – Meiosis MingHan Lu Trisomy and other meiotic mistakes are random errors that occur during meiosis. Maternal age is an important factor in the occurrence of trisomy. o Incidence increases after mothers turn 35 years old. o Spindle apparatus function and ability to separate chromosomes properly appear to decline after many years. Successful meiosis is critical to the health of offspring. 13.4 Why Does Meiosis Exist? Sexual reproduction is common among multicellular organisms Organisms in most lineages of the tree of life undergo asexual reproduction. The Paradox of Sex Asexual reproduction is much more efficient than sexual reproduction because no males are produced. The Purifying Selection Hypothesis If a gene is damaged or altered in a way that causes it to function poorly, it will be inherited by all of that individual’s offspring when asexual reproduction occurs. An allele that functions poorly and lowers the fitness of an individual is said to be deleterious. Sexually individuals are likely to have offspring that lack the deleterious alleles that are present. Natural selection against deleterious alleles is called purifying selection. Purifying selection should reduce the numerical advantage of asexual reproduction. The ChangingEnvironment Hypothesis Offspring that are genetic clones of their parents are unlikely to thrive if the environment changes. If a new strain of diseasecausing agent evolves, then all the asexually produced offspring are likely to be susceptible to that new strain. But if the offspring are genetically varied, then it is likely that at least some offspring will have combinations of alleles that enable them to fight off the new strain of pathogen or parasite and produce offspring of their own. Sexual reproduction is helpful for two reasons: (1) Offspring are not doomed to inherit harmful alleles, and (2) the production of genetically varied offspring means that at least some may be able to resist rapidly evolving pathogens and parasites. 11
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