Chromosomes free essay sample

Meiosis is the second important kind of nuclear division. It resembles mitosis in many ways but the consequences of meiotic divisions are very different from those of mitotic divisions. While mitotic division may occur in almost any living cell of an organism, meiosis occurs only in special cells. In animals, meiosis is restricted to cells that form gametes (eggs and sperm). Each species has a characteristic number of chromosomes per somatic cell. Fruit flies have 8; normal humans have 46. They exist as homologous pairs (partners) that are similar in size and shape and carry the same kinds of genes. Thus humans have 23 homologous pairs. The full complement of 46 chromosomes is referred to as the diploid number (referring to the fact that each kind of chromosome is represented twice). In higher organisms when an egg is fertilized the egg and sperm fuse to form a single cell called a zygote which develops into a new organism. We will write a custom essay sample on Chromosomes or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page If the egg and sperm were both diploid (46 chromosomes each in the case of humans) then the resulting zygote would be tetraploid. This would be an intolerable situation, so a mechanism has evolved to insure that each gamete (egg or sperm) contains only one representative of each homologous pair (or half the diploid number). This is referred to as the haploid number. The mechanism that makes this possible is meiosis. Meiosis consists of two divisions, Meiosis I and Meiosis II, and can potentially result in the production of four cells. However the DNA is only synthesized once (prior to Meiosis I). The subdivisions of meiosis are named like the subdivisions of mitosis (prophase, metaphase, anaphase, telophase) but as we shall see the events are somewhat different. To understand the physical processes involved in meiosis, we will use pipe cleaners as models of chromosomes. The first step will be to determine the types of chromosomes that exist and the genotype and phenotype of the cell. We will then manipulate the chromosomes to stimulate Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis). At the end of Meiosis I we will determine the genotypes carried by each daughter cell produced. Then we will stimulate Meiosis II (Prophase II, Metaphase II, Anaphase II Telophase II and Cytokinesis). At the end of Meiosis II we will stimulate fertilization by trading gametes with other lab groups and fusing two gamete nuclei to form diploid offspring. After fertilization, we will determine the genotype and phenotype of our new offspring. Each lab group will receive a â€Å"nucleus† (plastic bag) with 8 duplicated chromosomes (pipe cleaners). Each chromosome consists of two identical chromatids. Except for the two sex chromosomes, each chromosome is marked to show the location of certain genes. Since we are stimulating a diploid organism, each nucleus has two homologous chromosomes of each type. A Phenotype is the outward, physical manifestation of the organism. These are the physical parts, the sum of the atoms, molecules, macromolecules, cells, structures, metabolism, energy utilization, tissues, organs, reflexes and behaviors; anything that is part of the observable structure, function or behavior of a living organism. A genotype is the internally coded, inheritable information carried by all living organisms. This stored information is used as a blueprint or set of instructions for building and maintaining a living creature. These instructions are found within almost all cells. They are written in a coded language (the genetic code), they are copied at the time of cell division or reproduction and are passed from one generation to the next (inheritable). These instructions are intimately involved with all aspects of the life of a cell or an organism. Conclusion: The Genotype, carried by all living organisms, holds the critical instructions that are used and interpreted by the cellular machinery of the cells to produce the Phenotype of the organism. Meiosis I. Procedure: We the used a sheet of paper to represent the cell undergoing meiosis. We demonstrated Prophase and Metaphase I by aligning the chromosomes along the equator, homologous beside each other. Then we will demonstrate Anaphase I by separating homologous chromosomes into opposite sides. This is called reduction division because we reduced the number of different chromosomes in each nucleus. We will then demonstrate Telophase I and Cytokinesis by forming two new nuclei and cells. This will determine the Genome of each daughter cell. We then list the traits observed for genome A and genome B: Blood type, Insulin, Eye color, Hair Color, Hair Style, hemoglobin, and sex. After we recorded the results, we compared our daughter cells to those from other lab groups. Hypothesis Mitosis is the process of cell division in eukaryotes, in which the parental chromosome number is conserved in each of the daughter cells, while meiosis is a two-cell-division process in sexually reproducing eukaryotes that results in cells (typically gametes) with one-half the chromosome number of the original parental cell. At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasma. The nucleolus disappears during prophase I. In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions. In Metaphase 1, the centrioles are at opposite poles of the cell. The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids). Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle. In anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate. In Telephase 1 the homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes. Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Conclusion Segregation means that when these alleles go through meiosis to create gametes, they will segregate from one another, and each of the haploid gametes will end up with only one allele. Independent assortment comes into play when you are looking at how the alleles of two genes separate. As long as each gene lies on a different chromosome, then the alleles of these genes will assort themselves independently of one another when the haploid gametes are formed in meiosis. Each haploid gamete can end up with a different combination of alleles of these two genes. Meiosis II. Procedure After a period of metabolic activity (but no DNA duplication), the germ cell enters Meiosis II. We demonstrated Prophase II and Metaphase II for each cell by aligning the chromosomes at the equators of your two new cells. We demonstrated Anaphase II by separating the two sister chromatids and placing them in opposite ends of their cells. Then we demonstrated Telophase II and Cytokinesis by forming new nuclei and cells from each of your daughter cells. Hypothesis Meiosis involves two successive divisions of a diploid eukaryotic cell of a sexually reproducing organism that result in four haploid progeny cells, each with half of the genetic material of the original cell. Through the mechanisms by which paternal and maternal chromosomes segregate, and the process of crossing-over, genetic variation is produced in the haploid cells. In meiosis, one parent cell produces four daughter cells. Results Our group ended up with four haploid gamete cells. Each cell has only 1 copy of each type of chromosome. When comparing our four cells with the other groups four cells we realized that each of the four daughter cells is genetically unique. This uniqueness arises in part from independent assortment of chromosomes in meiosis. Thought independent assortment, each daughter cell randomly receives a maternally or paternally derived homolog from each chromosome pair. Independent assortment can yield 2^23 or 8,388,608 unique ways to arrange 23 pairs of chromosomes. So when comparing our four daughter cells with another groups four daughter cells, independent assortment can yield 2^4 or 16 unique ways to arrange the 8 available pairs chromosomes. Discussion While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. In Prophase II The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. Then, the two pairs of centrioles separate into two centrosomes. The nuclear envelope breaks down, and the spindle apparatus forms. In Metaphase II, each of the daughter cells completes the formation of a spindle apparatus. The single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole. Following that, Anaphase II the centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes. Lastly, in Telephase II a nuclear envelope forms around each set of chromosomes. Cytokinesis takes place, producing four daughter cells (gametes, in animals), each with a haploid set of chromosomes. Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental chromosomes. Conclusion One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes. When comparing our two daughter cells to other groups it demonstrates that every diploid cell has two alleles for every gene. Fertilization. Procedure We found a group of the opposite sex and traded 1 gamete cell with them. We united one of the remaining gamete cells we obtained from our â€Å"partner† group. Then we aligned the chromosomes, homologous by homologue and we saw the genetic make-up of our zygote. We then determined the genotype and phenotype of your new â€Å"offspring† using the same characteristics as before. Hypothesis The shuffling of genetic material produces genetically unique gametes, each of which can then fuse with another unique gamete during fertilization to produce a unique zygote of the next generation. Results Trait:Genotype:Phenotype: Blood Type:IA IB AB Insulin:1A 1BNormal Eye Color:BbBrown Hair Color:BbBrown Hair Style:CCCurly Hemoglobin:CCSickle Discussion Traits passed from parents to offspring by gene transmission. Genes are located on chromosomes and consist of DNA. They are passed from parents to their offspring through reproduction. The principles that govern heredity were discovered by Gregor Mendel in the 1860s. One of these principles is now called Mendels law of segregation. At fertilization, the sperm binds to a receptor on the surface of the egg and fuses with the egg plasma membrane, initiating the development of a new diploid organism containing genetic information derived from both parents. Not only does fertilization lead to the mixing of paternal and maternal chromosomes, but it also induces a number of changes in the egg cytoplasm that are critical for further development. These alterations activate the egg, leading to the completion of oocyte meiosis and initiation of the mitotic cell cycles of the early embryo. Following completion of oocyte meiosis, the fertilized egg (now called a zygote) contains two haploid nuclei, one derived from each parent. In mammals, the two pronuclei then enter S phase and replicate their DNA as they migrate toward each other. As they meet, the zygote enters M phase of its first mitotic division. The two nuclear envelopes break down, and the condensed chromosomes of both paternal and maternal origin align on a common spindle. Completion of mitosis then gives rise to two embryonic cells, each containing a new diploid genome. These cells then commence the series of embryonic cell divisions that eventually lead to the development of a new organism. Conclusion When we traded our daughter cells we ended up changing a good bit of the zygote’s genes. This proves the shuffling of genetic material produces genetically unique gametes, can produce a unique zygote even if only one chromosome is different, it can change the entire genetic makeup of the future organism.