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?