P1254413744GDhuv

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Meiosis

• Chapter 12

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• Heredity of Genes

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Heredity

• Living organisms are distinguished by their
  ability to reproduce
• Heredity- transmission of traits from one
  generation to the next
• Offspring share characteristics with parents
  (resemble their parents more than they do other
  individuals)
• Variation shows that offspring differ somewhat in
  appearance from parents and siblings

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Genetics

• Scientific study of heredity and hereditary
  variation
• Mechanisms of heredity and variation
• Work at organism, cell and molecular levels

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Inheritance of Genes

• Offspring acquire genes from parents by
  inheriting chromosomes
• Genes
• Are the units of heredity
• Are segments of DNA
• Each gene in an organisms DNA
• Has a specific locus on a certain chromosome
• We inherit
• One set of chromosomes from our mother and one
  set from our father

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AsexualReproduction

• In asexual reproduction
• One parent produces genetically identical
  offspring by mitosis
• Produce clones
• Amoebas, hydras

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Sexual Reproduction

• Offspring that have unique combinations of genes
  inherited from the two parents
• Offspring vary from their parents and from their
  siblings
• Variations on a common theme
• Process of cell reproduction called meiosis
• Produces gametes

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The Life Cycle of Cells

• A life cycle is the generation-to-generation
  sequence of stages in the reproductive history of
  an organism
• In humans each somatic cell ( body cell) has 46
  chromosomes, made up of two sets (23 sets)
• One set of chromosomes comes from each parent

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Homologous Chromosomes

• Different versions of a gene are called alleles
• Chromosomes of the same type, with the same genes
  in the same locations, are homologous chromosomes

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A Human Karyotype
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Fertilization and Meiosis

• In sexual reproduction, gametes unite to form a
  new individual, a process called fertilization
• Body cells 2n ( two sets of chromosomes)
• Gametes n ( one set of chromosomes)
• Meiosis is nuclear division that precedes
  formation of gametes and results in the halving
  of the chromosome number

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• Meiosis

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Meiosis

• Before meiosis begins, each chromosome in
  replicated
• When replication is complete, each chromosome
  consists of two identical sister chromatids
  attached at the centromere
• Sister chromatids are identical and attached

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Meiosis
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Meiosis

• Meiosis I- Homologous chromosomes of each
  chromosome pair separate
• Sister chromatids still intact
• Meiosis II- sister chromatids separate
• At the end of meiosis II, there are four haploid
  gametes, each containing one copy of each
  chromosome.

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The Phases of Meiosis I

• Early Prophase I The homologous chromosome pairs
  come together, a process called synapsis
• Late Prophase I The non-sister chromatids begin
  to separate. Crossing over between homologous
  non-sister chromatids occurs during this stage.
• Metaphase I The pairs of homologous chromosomes
  line up at the metaphase plate.

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The Phases of Meiosis I

• Anaphase I The paired homologs separate and
  migrate to opposite ends of the cell.
• Telophase I The homologs finish migrating to the
  poles of the cell.
• At the end of meiosis I, the cell divides to form
  two haploid daughter cells.

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Phases of Meiosis II

• At the start of meiosis II, each daughter cell is
  haploid but each replicated chromosome is
  composed of two identical sister chromatids
• Prophase II The spindle apparatus forms. If a
  nuclear envelope formed at the end of meiosis I,
  it breaks apart
• Metaphase II Replicated chromosomes, consisting
  of two sister chromatids, line up at the
  metaphase plate.

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Phases of Meiosis II

• Anaphase II Sister chromatids separate. The
  unreplicated chromosomes that result begin moving
  to opposite sides of the cell.
• Telophase II Chromosomes finish moving to
  opposite sides. Nuclear envelope forms
• At the end of meiosis II, each cell divides, and
  four haploid daughter cells are produced

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Meiosis vs Mitosis
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Meiosis vs. Mitosis
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Meiosis vs Mitosis
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• Genetic Variation and Inheritance

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Genetic Variation and Meiosis

• Mutations are the original source of variation
• Create different versions of genes
• Reshuffling of genetic material in meiosis
• Produces genetic variation
• The behavior of chromosomes during meiosis and
  fertilization is responsible for most of the
  variation that arises each generation in sexually
  reproducing organisms

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Genetic Variation and Meiosis

• Homologous pairs of chromosomes
• Orient randomly at metaphase I of meiosis
• In independent assortment
• Each pair of chromosomes sorts its maternal and
  paternal homologues into daughter cells
  independently of the other pairs
• Random Fertilization
• The fusion of gametes will produce a zygote with
  any of about 64 trillion diploid combinations

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Independent Assortment
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Genetic Variation and Meiosis

• Crossing Over produces recombinant chromosomes
  that carry genes derived from two different
  parents

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Genetic Variation and Evolution

• Populations evolve through differential
  reproductive success from variation
• Those individuals best suited for an environment
  will leave offspring and transmit their genes
• If environment changes variations in the genes
  allow a population to adapt to it

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• Life Cycles

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The Human Life Cycle
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Three Types of Sexual Life Cycles

• Alternation of meiosis and fertilization is
  common to most organisms
• Timing of the two events in the life cycle varies
  depending on species
• Three main types
• Animal type, plant type and fungal types

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Animal Sexual Life Cycle

• In animals, gametes are the only haploid cells
• Meiosis occurs during production of gametes and
  the gametes undergo no mitotic division before
  fertilization

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Plant Sexual Life Cycle

• Alternation of generations
• Life cycle includes both diploid and haploid
  stages
• Sporophyte, diploid stage (plant)
• Gametophyte, haploid stage (unfertilized seed)

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Fungal and Algal Sexual Life Cycle

• Meiosis produces haploid cells that give rise to
  a haploid multicellular adult organism
• The haploid adult carries out mitosis, producing
  cells that will become gametes

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• Mistakes in Meiosis

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Mistakes in Meiosis

• Mistakes in Meiosis I can lead to extra
  chromosomes
• Can lead to conditions such as Down syndrome, an
  extra copy of chromosome 21.
• Failure of homologous chromosomes to separate is
  called nondisjunction

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Nondisjunction

• May occur in as many as 10 of meiotic divisions
• Aneuploid zygotes typically do not survive to
  produce viable offspring.

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Chromosome Breakage

• Chromosome breakage can lead to rearrangements
  that can produce genetic disorders or cancer
• Chromosomal changes in a somatic cell can cause
  cancer

• A chromosomal translocation in the bone marrow is
  associated with chronic myelogenous leukemia