Meiosis

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Meiosis

• Sexual Reproduction

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Meiosis What is the point?

• Cell division that produces a sperm cell or an
  egg cell (Gametes).
• Starts with one cell that contains two copies of
  each chromosome (Diploid). One chromosome from
  female parent and one from male parent.
• Ends with 4 Haploid (only one set of chromosomes)
  daughter cells. 4 cells all genetically different.

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Vocabulary

• Meiosis- process of reduction division in which
  the number of chromosomes per cell is cut in half
  
• Homologous- chromosomes that each have a
  corresponding chromosome from the opposite-sex
  parent
• Diploid a cell that contains both sets of
  homologous chromosomes

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More Vocab

• Haploid- a cell that contains only a single set
  of chromosomes. (single set of genes)
• Tetrad- Structure containing 4 chromatids that
  form during meiosis
• Crossing over- homologous chromosomes exchange
  portions of their chromatids during meiosis

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Mendels principles of genetics require at least
two things

• Each organism must inherit a single copy of every
  gene from both its parents.
• When an organism produces its own gametes, those
  two sets of genes must be separated from each
  other so that each gamete contains just one set
  of genes.

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Lets Look at a Fruit Fly Drosophila

• A fruit fly has 8 chromosomes
• 4 of the chromosomes came from the fruit flys
  male parent and 4 came from the female parent.
• These two sets of chromosomes are homologous.
  Meaning that each of the 4 chromosomes that came
  from the male parent has a corresponding
  chromosome from the female parent
• A cell that contains both sets of homologous
  chromosomes is said to be diploid. (two sets)

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Diploid Cell

• A diploid cell is sometimes represented by the
  symbol 2N.
• What is the diploid number for the Drosophilia
  fly?
• Your answer should be 8
• 2N 8

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Gametes (sperm egg cells)

• The gametes of sexually reproducing organisms,
  contain only a single set of chromosomes and
  therefore only a single set of genes.
• These are called Haploid cells (one set)
• Haploid is represented as N.
• What is the Haploid number for Drosophilia?
• Your answer should be N 4

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Why are all 4 daughter cells different?

• When homologous chromosomes form tetrads in
  meiosis I, they exchange portions of their
  chromatids in a process called crossing over.
• Crossing-over produces new combinations of
  alleles.

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Crossing Over

• Segments of homologous chromosomes break and
  reattach at similar locations.
• Results in new genetic combinations of offspring.
• This is the main advantage of sexual reproduction

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Why is this an advantage?

• Offspring has more variety
• If an environment undergoes frequent changes it
  is better for the offspring to have more variety
  so the species will not die

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• Video

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Meiosis I

• Prior to Meiosis I, each chromosome is
  replicated.
• The cells then begin to divide similar to
  mitosis.
• In mitosis, the 4 chromosomes line up
  individually at the center of the cell.
• The 2 chromatids that make up each chromosome
  then separate from each other.

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• http//www.sumanasinc.com/webcontent/animations/co
  ntent/meiosis.html

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The Difference

• In prophase of Meiosis I, however, each
  chromosome pairs with its corresponding
  homologous chromosome to form a structure called
  a tetrad.
• There are 4 chromatids in a tetrad.
• This pairing of homologous chromosomes is the key
  to understanding meiosis.

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How are Mitosis and Meiosis the same?

• Division of cells
• DNA replication occurs before the division of the
  nucleus.

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Summary

• Similar in many ways to mitosis
• Several differences however
• Involves 2 cell divisions
• Results in 4 daughter cells with 1/2 the normal
  genetic information.
• The 4 cells are genetically DIFFERENT.
• Gametes or cells resulting from meiotic division
  will have HALF as many chromosomes.

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Overview of Meiosis
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Phases of Meiosis

• Meiosis II

Meiosis II
Telophase I and Cytokinesis I
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Prophase II
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• Gamete Formation
• In male animals, meiosis results in four
  equal-sized gametes called sperm.

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• In many female animals, only one egg results from
  meiosis. The other three cells, called polar
  bodies, are usually not involved in reproduction.

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Video square dance
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Practice questions

• Q What is the cell process that occurs only in
  organisms that reproduce sexually?___________
• Q.A sperm cell of an alligator has 16
  chromosomes. What is the total number of
  chromosomes normally present in a stomach cell of
  this alligator? _________

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