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Meiosis: Formation of gametes. Utilizing http://www.biology.com BioCoach Activity “Meiosis”. Meiosis occurs only in sexually reproducing organisms. In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes.

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meiosis formation of gametes

Meiosis: Formation of gametes

Utilizing http://www.biology.com

BioCoach Activity

“Meiosis”

introduction to meiosis
Meiosis occurs only in sexually reproducing organisms.

In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes.

Some plants, algae and fungi have more complex cycles involving both mitosis and meiosis.

Introduction to Meiosis
introduction to meiosis3
A two-step cell-division process in sexually reproducing eukaryotes.

Results in cells (typically gametes) with one-half the chromosome number of the original parental cell.

One diploid cell (2N) results in 4 haploid cells (N).

Introduction to Meiosis
introduction to meiosis4
Introduction to Meiosis
  • Meiosis consists of two successive nuclear divisions:

meiosis I

meiosis II

  • Each division consists of :
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
premeiotic interphase in animal cells
Premeiotic Interphase in Animal Cells
  • All chromosomes are duplicated
  • The two centriole are produced by the duplication of a single centriole
meiosis i prophase i
MeiosisI: Prophase I
  • Chromosomes become visible as tetrads
    • A tetrad is a pair of homologous chromosomes (ex. both copies of chr #3)
  • Crossing-over occurs
    • Homologous chromosomes exchange portions of their chromatids
    • Produces new combinations of alleles
meiosis i prophase i7
MeiosisI: Prophase I
  • Nucleolus disappears
  • Meiotic spindle forms
  • Nuclear envelope disappears

Prophase I consumes

90% of the time for meiosis

crossing over during meiosis
Crossing Over During Meiosis

crossing-over results in a physical exchange of equivalent segments of maternal and paternal homologous chromosomes

prophase i
Prophase I

Meiotic prophase I

meiosis i metaphase i
Meiosis I: Metaphase I
  • The centrioles are at opposite poles of the cell
  • The pairs of chromosomes (tetrads) are arranged along the center of cell
  • Spindle fibers are attached to one chromosome of each pair
metaphase i
Metaphase I

Meiotic metaphase I

meiosis i anaphase i
Meiosis I: Anaphase I
  • Chromosomes of each tetrad separate and move to opposite poles

Note: Sister chromatids remain attached and move together to the pole

anaphase i
Anaphase I

Meiotic anaphase I

meiosis i telophase i
Meiosis I: Telophase I
  • Homologous chromosome pairs reach the poles of the cell
  • Nuclear envelopes form around them
  • Cytokinesis follows to produce two cells
telophase i
Telophase I

Meiotic telophase I

product of meiosis i
Product of Meiosis I

Two cells with a haploid number of chromosomes is produced. Each cell has two copies of each chromatid.

The two cells have sets of chromosomes that are different from each other and from the parent cell that entered meiosis.

Animation

meiosis ii
Meiosis II

There is no interphase between the two stages (chromosomes are not duplicated).

meiosis ii prophase ii
Meiosis II: Prophase II
  • Centrioles duplicate
  • Nuclear envelope breaks down
  • Spindle apparatus forms
prophase ii
Prophase II

Meiotic prophase II

meiosis ii metaphase ii
Meiosis II: Metaphase II
  • Spindle apparatus attaches to each chromosome
  • Single chromosomes align along center of each cell
metaphase ii
Metaphase II

Meiotic metaphase II

meiosis ii anaphase ii
Centromeres separate

Sister chromatids — now individual chromosomes — move toward the opposite poles of the cell.

Meiosis II: Anaphase II
anaphase ii
Anaphase II

Meiotic anaphase II

meiosis ii telophase ii
Meiosis II: Telophase II
  • Nuclear envelope forms around each set of chromosomes
  • Cytokinesis occurs
  • Four daughter cells are produced, each with a haploid set of chromosomes.
telophase ii
Telophase II

Meiotic telophase II

product of meiosis ii
Product of Meiosis II

Four cells with a haploid number of chromosomes is produced. Each cell has a single copy of each chromatid.

The four cells have sets of chromosomes that are genetically different from each other and from the parent cell that entered meiosis.

Animation

compare meiosis to mitosis
Compare meiosis to mitosis

How are they alike?

  • In interphase prior to both, replication of DNA occurs
  • Both are methods of cell division
  • Both include Prophase, Metaphase, Anaphase and Telophase
  • Cytokinesis occurs in both
compare meiosis to mitosis35
Compare meiosis to mitosis

How are they different?

Meiosis

Two cell divisions

Sexual Reproduction

--makes gametes

Results in 4 haploid cells

Mitosis

One cell division

Asexual reproduction

--makes body cells

Also for growth and regeneration

Results in 2 diploid cells

compare meiosis to mitosis36
Compare meiosis to mitosis

How are they different?

Meiosis

Two parts - eight phases

DNA in daughter cells differs from parent cell

Crossing over occurs

Mitosis

One part – four phases

DNA in daughter cells is same as parent cell

Crossing over doesn’t occur

compare anaphase in mitosis to anaphase i in meiosis
Compare anaphase in mitosis to anaphase I in meiosis

Mitotic Anaphase

Meiotic Anaphase I

How are they different?