Meiosis and Mendel

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Meiosis and Mendel

• Biology
• Chapter 6
• Dr. Altstiel

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Meiosis

• Objectives
• Summarize the events that occur during meiosis.
• Relate crossing-over, independent assortment, and
  random fertilization to genetic variation.
• Compare spermatogenesis and oogenesis.

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Number and structure of Chromosomes affect
development

• Humans have 2 copies of 23 different chromosomes
  (46 total)
• Homologous chromosomes are similar in size, shape
  and genes
• Each one in a set comes from mom other from dad

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Chromosome Number

• Karyotype a set of chromosomes. Humans have 23
  pairs of chromosomes 46.
• Diploid when cells in the body contain 23 pairs
  of chromosomes. Practically all cells in the
  body are diploid.
• Diploid Number 2n
• Humans, 2n 46
• Haploid cells that contain only one copy of
  each chromosome.
• Haploid Number n
• Humans, n 23
• Zygote fusion of two haploid gametes by a
  process called fertilization a diploid zygote
  is a fertilized egg.

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• Diploid cells (somatic body cells) contains two
  sets of chromosomes
• Haploid cells (sex cells) contains one set of
  chromosomes
• Two haploid sex cells come together during
  fertilization and make a diploid zygote

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Sex Chromosome

• Determines whether you are male or female. Sex
  chromosomes designated as either X or Y.
• Female XX
• Can only produce eggs that contain an X
  chromosome.
• Male XY
• Can produce sperm with either an X or Y
  chromosome. Therefore, males sperm is sex
  determiner.
• Autosomes chromosomes not directly involved in
  determining the sex of an individual (22 pairs).

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Formation of Haploid Cells

• Meiosis a form of cell division that halves the
  number of chromosomes when forming specialized
  reproductive cells, such as gametes and spores.
• Involves two divisions of the nucleus meiosis I
  and meiosis II.

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Meiosis

• Meiosis is a two-stage form of nuclear division
  in which the chromosome number is halved.
• Meiosis I reduces the chromosome number by
  half.
• Meiosis II separates identical copies of
  chromosomes.

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Steps of Meiosis

• Prophase I chromosomes condense, and nuclear
  envelope breaks down. Crossing-over may occur.
• Crossing-over when a portion of a chromatid on
  one homologous chromosome are broken and
  exchanged with the corresponding chromatid
  portions of the other homologous chromosome.
• Metaphase I Pairs of homologous chromosomes
  moved by the spindle to the equator of the cell.
• Anaphase I Homologous chromosomes separate.
• Telophase I Individual chromosomes gather at
  each of the poles. Cytokinesis occurs resulting
  in two cells.

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Steps of Meiosis Cont.

• Prophase II a new spindle forms around the
  chromosomes.
• Metaphase II Chromosomes line up along the
  equator.
• Anaphase II the centromeres divide, and
  chromatids move to opposite poles of the cell.
• Telophase II a nuclear envelope forms around
  each set of chromosomes. Spindle breaks down,
  and cytokinesis occurs.
• The result of meiosis is 4 haploid cells.

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Meiosis I

• Prophase I
• DNA replication precedes the start of meiosis I.
• Homologous chromosomes pair and form synapses, a
  step unique to meiosis.
• The paired chromosomes are called bivalents
• A bivalent has two chromosomes and four
  chromatids, with one chromosome coming from each
  parent.

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Meiosis I Continued

• Metaphase I
• Bivalents align at the metaphase plate.
• The orientation is random, with either parental
  homologue on a side. This means that there is a
  50-50 chance for the daughter cells to get either
  the mother's or father's homologue for each
  chromosome.

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Meiosis I Continued

• Anaphase I
• Chromosomes, each with two chromatids, move to
  separate poles.
• Each of the daughter cells is now haploid (23
  chromosomes), but each chromosome has two
  chromatids.

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Meiosis I Continued

• Telophase I
• Nuclear envelopes may reform, or the cell may
  quickly start meiosis 2.

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Meiosis I Continued

• Cytokinesis
• Analogous to mitosis where two complete daughter
  cells form.

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Meiosis II
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Meiosis II Continued
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Meiosis II Continued
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Meiosis II Continued
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Meiosis II Continued
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Phases of Meiosis
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Class Work

• Directed Reading Meiosis

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Meiosis and Genetic Variation

• Three Mechanisms that make key contributions to
  genetic variation
• Independent assortment
• Crossing-over
• Random fertilization

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Independent Assortment

• Each human gamete receives one chromosome from
  each of the 23 pairs of homologous chromosomes.
• Which of the two chromosomes passed to the
  offspring is a matter of chance.
• Independent Assortment the random distribution
  of homologous chromosomes during meiosis
• Each of the 23 pairs of chromosomes segregate
  independently.
• Possibility of 223 different gene combinations.
  (8,388,608 possible combinations)

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Crossing-Over

• DNA exchange that occurs during crossing-over
  adds more recombination to independent assortment
  of chromosomes. Number of possible genetic
  combinations that can occur among gametes
  virtually unlimited.

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Crossing-Over
DNA is exchanged between homologous chromosomes,
resulting in genetic recombination.
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Random Fertilization

• The zygote that forms a new individual is created
  by the random joining of two gametes (each
  produced independently).
• Number of possible genetic outcomes is (223)2
  7.04 X 1013 .

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Meiosis and Gamete Formation

• Gametogenesis meiosis is the primary event in
  the formation of gametes.
• Spermatogenesis process by which sperm is
  produced in male animals.
• Occurs in the testes.
• After completion of meiosis, four haploid cells
  are produced.
• Sperm four cells change in form and develop a
  tail.
• Oogenesis process by which gametes are produced
  in female animals.
• Occurs in the ovaries.
• Cytokinesis following meiosis I cytoplasm
  divides unevenly resulting in a large cell (will
  become the ovum) and three polar bodies.

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Homework

• Correct Directed Reading for Section 1

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Sexual and Asexual Reproduction

• Asexual Reproduction a single parent passes
  copies of all of its genes to each of its
  offspring no fusion of haploid cells such as
  gametes.
• Clone an organism genetically identical to its
  parents.
• Sexual Reproduction two parents each form
  reproductive cells that have ½ the number of
  chromosomes.
• Offspring have traits of both parents.

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Alternation of Generations

• Life-cycle that regularly alternates between a
  haploid and a diploid phase.
• Plants, algae, some protists.
• Sporophyte - diploid phase in the life-cycle of
  plants the name of the spores that are
  produced.
• Spore a haploid reproductive cell produced by
  meiosis give rise to multicellular individual
  called a gametophyte w/o joining with another
  cell.
• Gametophyte haploid phase produces gametes by
  mitosis. Gametes fuse and give rise to the
  diploid phase.