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Class Objectives:.  Distinguish between haploid and diploid cells and its relation to meiosis and fertilization. D istinguish the various phases of meiosis due to the position of the chromosomes at that particular phase.

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class objectives
Class Objectives:

Distinguish between haploid and diploid cells and its relation to meiosis and fertilization.

Distinguish the various phases of meiosis due to the position of the chromosomes at that particular phase.

 Explain the reason meiosis reduces the number of chromosomes by one half so that when gametes are fertilized the number of chromosomes is restored.

 Compare and contrast the cells produced by both mitosis and meiosis and the fate of the cells from each cycle.

haploid vs diploid
Haploid vs. Diploid
  • Diploid (2n)
    • A cell with two of each kind of chromosome
    • ex) somatic cells
  • Haploid (n)
    • A cell with one kind of chromosome
    • ex) germ cells
  • In mitosis, cells are dividing to form diploid cells
  • In meiosis, cells are dividing to form haploid cells
the phases of meiosis

The Phases of Meiosis

Meiosis I & Meiosis II

meiosis i
Meiosis I
  • Consists of:
    • Prophase I
    • Metaphase I
    • Anaphase I
    • Telophase I
    • Cytokinesis I
  • Before meiosis begins, what must happen to the cell’s DNA/chromosomes?
prophase i
Prophase I

Like prophase of mitosis:

  • The nucleus and nuclear envelope disappears.
  • Centrioles move to the opposite poles and the spindle fibers form between the centrioles.
  • Chromosomes condenses into sister chromatids.

Unlike prophase of mitosis:

  • Pairs of homologous chromosomes form a tetrad.
  • Crossing over may occur.
    • Exchange of genetic material
metaphase i
Metaphase I
  • Spindle fibers attach to centromeres.
  • Tetrads line up at the spindle’s equator.

Unlike mitosis:

  • Homologous chromosomes are lined up side by side as tetrads.
anaphase i
Anaphase I
  • Tetrads, which consist of two homologous chromosomes separate.
telophase i cytokinesis i
Telophase I/Cytokinesis I
  • The nucleus and nuclear envelope reappears. disappears.
  • The spindle is broken down and the chromosomes uncoil.
  • Cytokinesis, the division of the cytoplasm and the organelles begins and ends forming two cells.
mitosis ii
Mitosis II
  • Consists of:
    • Prophase II
    • Metaphase II
    • Anaphase II
    • Telophase II
  • The purpose of meiosis II is to separate sister chromatids of each chromosome into separate cells.
prophase ii
Prophase II
  • The nucleus and nuclear envelope disappear.
  • Spindle fibers and centrioles form.
  • Chromosomes condenses into sister chromatids.
metaphase ii
Metaphase II
  • Sister chromatids attach to the centromere.
  • Sister chromatids line up at the equator.
anaphase ii
Anaphase II
  • Sister chromatids separate into two chromosomes.
  • Chromosomes move to the opposite poles.
telophase ii cytokinesis ii
Telophase II/Cytokinesis II
  • The nucleus and nuclear envelope reforms.
  • The spindles breakdown.
  • The cytoplasm divides
  • 4 haploid cells are formed.
fertilization
Fertilization
  • The 4 haploid cells formed in meiosis will become gametes.
  • Gametes are:
    • Sperm in males (haploid)
    • Eggs in females (haploid)
  • Gametes transmit the genes they contain to their offspring.
  • When a sperm fertilizes an egg a zygote is formed  this is called sexual reproduction
  • A zygote is a diploid cell.
    • Sperm (haploid) + Egg (haploid) = zygote (diploid)
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