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Mitosis, Meiosis and Fertilization

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Mitosis, Meiosis and FertilizationMitosis, Meiosis and Fertilization1I. IntroductionWhen you fall and scrape the skin off your hands or knees, how does your body make new skin cells to replace the skin cells that were scraped off? How does each new cell get a complete set of chromosomes? How does a baby get his or her genes? To answer these questions, we need to begin by reviewing what chromosomes and genes are.Chromosomes and GenesEach chromosome contains a long molecule of DNA. This molecule of DNA contains many genes. A gene is a segment of the DNA molecule that gives the instructions for making a protein. For example, one gene gives the instructions for making a protein enzyme which helps to make the pigment melanin,a molecule that contributes to our skin and hair color. Each cell in your body has two copies of each chromosome. These two copies are called a pair of homologous chromosomes. The DNA in both homologous chromosomes contains the same genes at the same locations in the chromosome. However, the two homologous chromosomes may have different versions of a gene. Different versions of the same gene are called alleles. Different alleles give the instructions for making different versions of the protein, and different versions of the protein can result in different characteristics (see examples in the following table).Allele Protein If both homologous chromosomes have this allele, the person has:Anormal enzyme for melanin productionnormal skin and hair coloradefective enzyme for melanin productionvery pale skin and hair color (albino)Rnormal protein that responds to an odor can smell that odor rprotein that does not respond to the odor can not smell that odor Snormal hemoglobinnormal bloodssickle cell hemoglobinsickle cell anemia (sickle shaped red blood cells that can block blood flow, causing pain and other problems) Questions1. Circle everything in the above table that is part of a DNA molecule.2. The following diagram illustrates just three of the many alleles on a pair of homologous chromosomes in a girl named Tiffany. __________________________________________________________________ (___| a allele| __ | r allele| ____| S allele|__ _) _________________________________________________ (___| a allele| __ __| R allele| ___ | S allele|__ _) Is Tiffany an albino? ___yes ___ noDoes Tiffany have ___ sickle cell anemia or ___ normal blood? 1 by Drs. Ingrid Waldron, Jennifer Doherty, R. Scott Poethig, and Lori Spindler, Department of Biology, University of Pennsylvania, © 2013; Teachers are encouraged to copy this student handout for classroom use. A Word file (which can be used to prepare a modified version if desired), and Teacher Preparation Notes are available at http://serendip.brynmawr.edu/sci_edu/waldron/. 13. Fill in the blanks of the following sentences.A chromosome contains one long __________ molecule. Each gene in this molecule givesthe instructions for making a __________________.Both chromosomes in a pair of ________________________ chromosomes have the same ____________, but the two chromosomes may have different _________________. Chromosomes that are not homologous have different ______________ which give the instructions for making different proteins.Challenge Question Use the information on page 1 to explain why a person with aa alleles has very pale skin and hair color. Include the words protein, enzyme and melanin in your explanation.Each human cell has 23 pairs of homologous chromosomes. Each of these pairs of homologous chromosomes has its own unique set of genes. For example, chromosome 11 has the genes shown in question 2. There are over 1000 additional genes on chromosome 11. In this activity, you will use model chromosomes that represent chromosome 11 and chromosome 4. Chromosome 4 has over 1000 genes, including a gene that can result in dwarfism.II. Mitosis – How Your Body Makes New CellsEach of us began as a single cell, so one important question is:How did that single cell develop into a body with more than a trillion cells?The production of such a large number of body cells is accomplished by many, many repeats of a cycle of cell division in which one cell divides to form two cells, then each of these cells divides resulting in atotal of four cells, etc. Thus, repeated cell division is needed for growth. 4. Even in a fully grown adult, cells still undergo cell division. Why is this useful? Think about your skin, for example.The type of cell division that produces almost all the cells in our bodies is called mitosis. In mitosis, one cell divides to produce two identical daughter cells. (It may seem odd, but the cells produced by cell division are called daughter cells, even in boys and men.) Each of the daughter cells needs to 2have a complete set of chromosomes containing an exact copy of all the DNA in the original cell. 3Mitosis – How Each Daughter Cell Gets a Complete Set of ChromosomesThe cell shown below has a pair of homologous chromosomes; one chromosome is shown as dark or striped to indicate that it has different alleles from the other homologous chromosome.4Preparation for MitosisTo get ready for mitosis, the cell makes a copy of the long strand of DNA in each chromosome (DNA replication). The two copies of each DNA molecule are attached to each other. (You can't see the two copies until the beginning of mitosis which is shown in the next diagram.)Beginning of MitosisEach copy of the DNA is wound tightly into a compact chromatid, and the two chromatids in each chromosome are attached to each other at a centromere. The two chromatids are called sister chromatids because they are identical. At the beginning of mitosis, each chromosome consists of a pair of sister chromatids, and the chromosomes are linedup in the center of the cell.Mitosis continuesNext, the two sister chromatids of each chromosome are separated. After they separate, each chromatid is an independentchromosome. The cell begins to pinch together in the middle (cytokinesis). Cytokinesis proceeds to produce two separate daughtercells.Mitosis completedTwo new daughter cells have been formed. Each daughter cell has received one copy of each chromosome, so each daughter cellhas a complete copy of all the DNA in the original cell.


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