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11–4  Meiosis

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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…
  • 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)

  • 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
  • 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?


What is the haploid number for these cells?


meiosis i
Meiosis I
  • 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
crossing over
Crossing Over
  • 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…
  • 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 
  • 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
  • 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.

Notice the HUGE size difference.

comparing mitosis and meiosis
Comparing Mitosis and Meiosis
  • 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
  • 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.
  • Makes sex cells (gametes)
  • a diploid cell four haploid (N) cells
  • These cells are genetically different from the diploid cell and from one another.