Meiosis and sexual lifecycles
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Meiosis and Sexual Lifecycles. Chapter 13. 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 recombination of genetic information

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Meiosis and Sexual Lifecycles

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Meiosis and sexual lifecycles

Meiosis and Sexual Lifecycles

Chapter 13


Objectives

Objectives

  • Understand the meaning of “SEX”

  • Understand the process of meiosis

  • Understand the need to undergo meiosis as sexual organisms


Need for sex

Need For Sex

  • Sex: results in the recombination 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


Sex terms

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 information)

  • Sister Chromatids: 2 “identical” strands of DNA connected by a centromere that contains a kinetochore. Makes up each member of a homologous pair


Comparison

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 distinct from parents

daughter cells are haploid

generation of gametes (sex cells): egg & sperm

Comparison


Meiosis the process

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

    • Meiosis II: separation of sister chromatids (just as in mitosis)


Meiosis i

Meiosis I

  • Prophase I: same as mitosis

  • Metaphase I: both members of a homologous pair arrange themselves as a tetrad (synapsis) along the metaphase plate; crossover (chiasma) may occur

  • Anaphase I: Kinetochore remains intact and homologous pairs are separated as spindle fibers shorten


Meiosis ii

Meiosis II

  • Telophase I: same as mitosis (includes cytokinesis)

  • Meiosis II is the 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


Genetic variation

Genetic Variation

  • Variation in the DNA of a zygote can be induced by three different methods occurring 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


Independent assortment

Independent Assortment

  • 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


Crossing over

Crossing Over

  • During tetrad formation (Prophase I) chromosomes may cross (chiasma).

  • 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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