meiosis: formation of gametes utilizing biocoach activity “meiosis”
TRANSCRIPT
Meiosis: Formation of gametes
Utilizing http://www.biology.comBioCoach 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
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
Meiosis consists of two successive nuclear divisions:meiosis I meiosis II
Each division consists of : Prophase Metaphase Anaphase Telophase
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 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 I
Nucleolus disappears Meiotic spindle forms Nuclear envelope disappears
Prophase I consumes 90% of the time for meiosis
Crossing Over During Meiosis
crossing-over results in a physical exchange of equivalent segments of maternal and paternal homologous chromosomes
Crossing Over
During Meiosis
Prophase I
Meiotic prophase 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
Meiotic metaphase 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
Meiotic anaphase 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
Meiotic telophase 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
There is no interphase between the two stages (chromosomes are not duplicated).
Meiosis II: Prophase II
Centrioles duplicate Nuclear envelope breaks down Spindle apparatus forms
Prophase II
Meiotic prophase II
Comparing Prophase Iand Prophase II
Meiosis II: Metaphase II
Spindle apparatus attaches to each chromosome
Single chromosomes align along
center of each cell
Metaphase II
Meiotic metaphase II
Comparing Metaphase Iand Metaphase II
Meiosis II: Anaphase II
Centromeres separate Sister chromatids — now individual
chromosomes — move toward the opposite poles of the cell.
Anaphase II
Meiotic anaphase II
Comparing Anaphase Iand Anaphase 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
Meiotic telophase II
Comparing Telophase Iand Telophase 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
Mitosis and Meiosis Reviewed
Compare meiosis to mitosis
•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
How are they alike?
Compare meiosis to mitosis
MeiosisTwo cell divisions
Sexual Reproduction --makes gametes
Results in 4 haploid cells
MitosisOne cell division
Asexual reproduction--makes body cellsAlso for growth and
regenerationResults in 2 diploid
cells
How are they different?
Compare meiosis to mitosis
MeiosisTwo parts - eight
phasesDNA in daughter cells
differs from parent cell
Crossing over occurs
MitosisOne part – four
phases DNA in daughter cells is same as parent cell
Crossing over doesn’t occur
How are they different?
Compare anaphase in mitosis to anaphase I in meiosis
Mitotic Anaphase Meiotic Anaphase I
How are they different?