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11–4  Meiosis. The making of sex cells!. Making Sex Cells. Meiosis is the process of making sex cells (gametes) Gametes are sex cells Ex: sperm, egg, pollen, etc…. Female Gametes - Eggs. Human Eggs. Male Gametes - Sperm. Human Sperm. Chromosomes.

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11 4 meiosis

11–4  Meiosis

The making of sex cells!


Making sex cells
Making Sex Cells

  • Meiosis is the process of making sex cells (gametes)

  • Gametes are sex cells

    • Ex: sperm, egg, pollen, etc…




Chromosomes
Chromosomes

  • A body cell in an adult fruit fly has 8 chromosomes, as shown in the drawing on the right.

  • Four of the chromosomes came from the fruit fly's male parent, and 4 came from its female parent.


Homologous chromosomes
Homologous Chromosomes

  • chromosomes that each have a corresponding chromosome from the opposite-sex parent

    • Example: Fruit-Fly (Drosophila) Chromosomes -

      8 chromosomes (4 from mom and 4 from dad)


Different types of cells
Different Types of Cells

  • A cell that contains both sets of homologous chromosomes is said to be diploid (“two sets”)

  • a diploid cell is sometimes represented by the symbol 2N

    • for Drosophila, the diploid number is 8, which can be written 2N = 8

  • Diploid cells = body cells (somatic cells)


HaploidCells = a cell that contains only a single set of chromosomes

Therefore only a single set of genes

a haploid cell is sometimes represented by the symbol N

for Drosophila, the haploid number is 4, which can be written N = 4

Haploid cells = sex cells (gametes)



Meiosis
Meiosis cells?

  • a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.


Phases of meiosis
Phases of Meiosis cells?

  • two distinct divisions, called meiosis I and meiosis II

  • By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.


What is the diploid number for these cells? cells?

2

What is the haploid number for these cells?

1


Meiosis i
Meiosis I cells?

  • Before meiosis I, each chromosome is replicated.

  • Division looks similar to mitosis

  • Prophase I, Metaphase I, Anaphase I, Telophase I

  • prophase of meiosis I, however, each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad


Tetrads
Tetrads cells?


Crossing over
Crossing Over cells?

  • As homologous chromosomes pair up and form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over

  • results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

  • Alleles = alternative forms of the same gene (ex: blue eyes vs. brown eyes)


After crossing over
After Crossing Over… cells?

  • homologous chromosomes separate

  • two new cells are formed

  • New cells:

    • each pair of homologous chromosomes was separated

    • neither of the daughter cells has the two complete sets of chromosomes (they have been shuffled and sorted)

    • The new cells are DIFFERENT from each other


Meiosis ii
Meiosis II  cells?

  • two cells produced by meiosis I now enter a second meiotic division

  • NO DNA replication before Meiosis II

  • Prophase II, Metaphase II, Anaphase II, Telophase II

  • paired chromatids separate

  • Produces: haploid cells (N)

    • Haploid cells are DIFFERENT from each other


Gamete formation
Gamete Formation cells?

  • Males gametes = sperm (pollen in plants)

    • All sperm the same size at the end of Meiosis

    • 4 sperm produced for each round of meiosis

  • Female gametes = eggs

    • One egg produced and 3 polar bodies (egg is MUCH larger in size)

    • The one egg receives the most cytoplasm

    • One egg produced for each round of meiosis (and 3 polar bodies which can’t be fertilized)


Fertilization of a human egg by sperm. cells?

Notice the HUGE size difference.


Comparing mitosis and meiosis
Comparing Mitosis and Meiosis cells?

  • Mitosis and Meiosis sound alike but are VERY different!

  • Mitosis results in the production of two genetically identicaldiploid cells

  • Meiosis produces four genetically differenthaploid cells


Mitosis
Mitosis cells?

  • Makes body cells (somatic cells)

  • diploid cell two diploid (2N) daughter cells.

  • Daughter cells are identical to each other & the original parent cell

  • Mitosis allows an organism's body to grow and replace cells.


Meiosis1
Meiosis cells?

  • Makes sex cells (gametes)

  • a diploid cell four haploid (N) cells

  • These cells are genetically different from the diploid cell and from one another.


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