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Meiosis and Sexual Lifecycles

Meiosis and Sexual Lifecycles. Chapter 5.6. Objectives. Understand the meaning of “SEX” Understand the process of meiosis Understand the need to undergo meiosis as sexual organisms. Need For Sex. Sex: results in the new combinations of genetic information

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Meiosis and Sexual Lifecycles

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  1. Meiosis and Sexual Lifecycles Chapter 5.6

  2. Objectives • Understand the meaning of “SEX” • Understand the process of meiosis • Understand the need to undergo meiosis as sexual organisms

  3. Need For Sex • Sex: results in the new combinations of genetic information • Organisms with sexual lifecycles have a greater range of genetic variation that may result in a greater ability to adapt to an environment • Meiosis is key to enhancing this variation

  4. Sex Terms • Diploid: having 2 of each chromosome type • Haploid: having a single representative of each chromosome type • Homologous chromosomes (homologues): chromosomes from different parents that are of the same type (contain similar genetic information) • Sister Chromatids: 2 “identical” strands of DNA held together by a kinetochore that make up each member of a homologous pair (have identical versions (alleles) of the genetic information)

  5. Karyotype • Allows the visualisation of the chromosome pairs • Humans have 23 pairs of homologous chromosomes • 22 pairs of autosomal chromosomes • 1 pair of sex chromosomes

  6. Mitosis produces 2 daughter cells daughters are clones of the parent daughter cells are diploid growth, replacement, repair Meiosis produces up to 4 daughter cells daughters are very different from parents daughter cells are haploid generation of gametes (sex cells): egg & sperm Comparison

  7. Why is meiosis necessary? • Keep the chromosomal number constant between generations • Introduce new genetic combinations

  8. Meiosis (the process) • Meiosis: nuclear division with a reduction in chromosome number by half • Two parts to the process • Meiosis I: separation of homologous pairs(reduction division) • Meiosis II: separation of sister chromatids (just as in mitosis)

  9. Meiosis I • Prophase I: same as mitosis • Metaphase I: both members of a homologous pair arrange themselves as a tetrad along the metaphase plate (cross over may occur) • Anaphase I: Kinetochore remains intact and homologous pairs are separated as spindle fibers shorten • Telophase I: same as mitosis (includes cytokinesis)

  10. Meiosis II • Same process as mitosis except starting from telophase I instead of Interphase • no doubling of DNA has occurred • Result is up to four daughter cells

  11. Genetic Variation • Variation in the DNA of a zygote can be induced by three different methods occuring the during sexual cycle • Random fertilization: 8+ million possible combinations of sperm and egg = 70,368,744,000,000 possible combinations • Independent Assortment • Crossing over

  12. Independent Assortment (Random Alignment) • There are about 8 million possible combinations of chromosomes based on random assortment of the different types in the human genome (8,388,744). • This occurs during Anaphase I of meiosis as homologous pairs are separated • Genes that do not reside on the same chromosome may sort independent of one another

  13. Crossing Over • During tetrad formation (Prophase I) chromosomes may touch each other and exchange pieces (cross over) • Sometimes genetic information may be exchanged between homologues further shuffling the genetic information of the eventual gamete (4,951,760,200,000,000,000,000,000,000 different zygotes with a single crossover)

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