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Science 9. 6.1 - Meiosis. Objectives. By the end of the lesson you should be able to: Describe the process of meiosis Compare and contrast meiosis and mitosis Explain why meiosis is needed. MEIOSIS. Sexual reproduction – 2 parents – genetic differences
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Science 9 6.1 - Meiosis
Objectives By the end of the lesson you should be able to: • Describe the process of meiosis • Compare and contrast meiosis and mitosis • Explain why meiosis is needed
MEIOSIS • Sexual reproduction – 2 parents – genetic differences • The process of Meiosis results in the production of special cells called gametes • Gametes have half the number of chromosomes as body cells
MEIOSIS • Cell division occurs twice in Meiosis • Once at the end of Meiosis I and again at the end of Meiosis II • In Meiosis I, matching chromosomes called homologous chromosomes separate • In Meiosis II, sister chromatids separate
MEIOSIS • The process of meiosis shuffles genetic information and results in variation of the gametes • Results in genetic diversity
IMPORTANT Terms: • Haploid: half the total number of chromosomes found in gametes (in humans = 23) • Diploid: total number of chromosomes found in body cells (in humans = 46)
Meiosis • AKA: Germ cell division or sex cell division • Purpose: to produce gametes (i.e. sex cells: sperm and egg) • Products: 4 haploid cells each genetically different this creates genetic variability
Fertilization • 2 gametes come together; in humans an egg and sperm come together • Each has 23 chromosomes, thus together there are 46 in the zygote • The zygote will grow to become the developing embryo 23 + Gametes: why is the sperm smaller than the egg? 23 46 Zygote: will grow (by mitosis) into a baby in ~9 months
Number of DNA STRANDS Sequence 46 Parent Cell 92 46 46 Daughter Cells 23 23 23 23 All genetically different
CHROMOSOME SEQUENCE Meiosis Video
Meiosis – Prophase I • Chromosomes coil and become visible. • Nuclear membrane disappears. • Spindle fibers form. • Homologous chromosomes move towards each other and pair.
Meiosis – Metaphase I • Pairs of homologous chromosomes move to the midline of the cell. • These pairs are aligned on either side of the metaphase plate.
Meiosis – Anaphase I • Homologous chromosome pairs are pulled away from each other towards opposite ends of the cell. • Paired chromatids remain attached.
Meiosis - Telophase I • Paired chromatids may uncoil and become less visible. • Nuclear membranes may form around each chromosome cluster. • Spindle fibers disappear. • Cytokinesis occurs.
Meiosis II • Phase II of meiosis is the mitotic division of haploid cells. • Although there will be four new non-identical cells at the end of meiosis II, all other processes for this division are similar to mitosis.
Meiosis II – Prophase II • Paired chromatids coil. • Spindle fibers form. • Nuclear membrane disappears.
Meiosis II – Metaphase II • Paired chromatids line up in the middle of the cell. • The chromatids are attached to spindle fibers.
Meiosis II – Anaphase II • Each pair of chromatids split at the centromere to form two independent chromosomes. • Chromosomes move to opposite ends of the cell.
Meiosis II – Telophase II • Chromosomes uncoil and become less visible. • Nuclear membranes form around four new nuclei. • Spindle fibers disappear. • Cytokinesis occurs. • Meiosis Square Dance
Meiosis - Cytokinesis • Same process as in mitosis. • Four non-identical cells are formed. • These cells are called haploid cells (n). • # chromosomes = half the # of chromosomes of parent cell.
Meiosis Events Meiosis I • Matching chromosome pairs (homologous chromosomes) move to opposite poles of the cell - two daughter cells result. Meiosis II • Chromatids of each chromosome are pulled apart - the end result is four haploid cells, each with half the number of chromosomes. These develop into gametes.
CROSSING OVER • Important event that occurs between each chromosome pair in meiosis I • There is an exchange of segments of DNA and picks up new genetic information
INDEPENDENT ASSORTMENT • This occurs when homologous pairs of chromosomes separate at the equator and move towards opposite poles of the cell. • Video • Mitosis vs Meiosis
FERTILIZATION • There are more than 8 million combinations possible for these 23 pairs in any egg or sperm cell • When fertilization occurs, 70 trillion different zygotes are possible from the combination of one sperm cell and one egg cell!
Gamete Formation • Males produce 4 sperm for every round of meiosis • Females produce one egg for each round of meiosis: 3 of the 4 “eggs” produced die off - called polar bodies • All the cytoplasm, nutrients and energy gets put into one egg
Chromosome Mutations Sometimes occur spontaneously • Chromosome changes during meiosis can cause changes in the genetic information. • Parts of chromosomes can be inverted, deleted, duplicated or moved to another spot. Occur because of mutagens • Chromosome changes, sometimes leading to genetic disease or death, can be caused by mutagens such as radiation or chemicals. Can occur if there is failed separation of chromosomes in meiosis • Failed separation means that a gamete may end up with no chromosome or too many of a chromosome. • Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders.
DIAGNOSING GENETIC DISORDERS • A geneticist can prepare a different type of picture for individuals that show all chromosomes called karyotype
DIAGNOSING GENETIC DISORDERS • By analyzing karyotypes, geneticists can determine whole chromosome mutations • Can diagnose genetic disorders or syndromes • Down syndrome – most frequent genetic disorder • Caused by an extra 21stchromosome