Meiosis

Is a form of cell division that halves the number of chromosomes when forming

specialized reproductive cells (gametes).

Meiosis

• Involves two divisions of the nucleus – meiosis I and Meiosis II

• Starts with one diploid cell, and ends with four haploid cells.

Review of Terms

• Haploid (n)-- one set chromosomes – (23 chromosomes)

• Diploid (2n)-- two sets chromosomes – (46 chromosomes)

• Chromatid – Two exact copies of DNA (each single copy called a chromatid) makes up a chromosome

• Centromere – point at which the two chromatids are attached

• Homologous chromosomes – chromosomes similar in size, shape, and genetic content.

Meiosis

Meiosis I

Prophase I Metaphase I Anaphase I

Telophase I

Prophase I

• Homologous Chromosomes synapse (pair together) Each pair is called

a tetrad because there are 4 chromatids.• Crossing – over occurs when portions on one homologous chromosome are

broken and exchanged with the corresponding portion on one of the chromatids of the other homologous chromosome.

Crossing - Over

Meiosis IIProphase II Metaphase II Anaphase II Telophase II

Between Meiosis I & II

• The chromosomes do NOT replicate between Meiosis I and Meiosis II.

Start of Meiosis I Start of Meiosis II

DNA copy made during Interphase

Prophase I & II

• A new spindle forms around the chromosomes

Spindles – are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division

Centrioles - two cylindrical cellular structures that form at the poles of a cell during meiosis.

Metaphase I & II

• Chromosomes line up in the center of the cell.• In metaphase I, chromosome tetrads line up,

each with their sister chromatids attached.

Anaphase I & II

• Centromeres divide• Chromatids move to opposite poles of the cell• In anaphase I, the tetrad chromosomes with

attached sister chromatide moves to poles.

* Remember centromeres are what hold the two chromatids together to make a chromosome.

Telophase I

• Nuclear membrane forms around each set of chromosomes

• Cytokinesis begins -division of cytoplasm• Left with two identical haploid daughter cells.

Telophase II

• Nuclear membrane forms around each set of chromosomes

• Cytokinesis begins• Left with four haploid cells.

Male vs Female Meiosis

• Spermatogenesis• Oogenesis

Spermatogenesis (Males)

• Cytoplasm divided equally between all 4 cells during cytokinesis

• Results in 4 haploid sperm cells

Oogenesis (Female)

• Cytoplasm divided unequally between 4 cells during cytokinesis; 1 cell receives more cytoplasm than other cells

• Results in 1 mature egg and 3 polar bodies

Sexual vs. Asexual Reproduction

Sexual Reproduction

• 2 Parents required• Offspring are similar to

parents (USES MEIOSIS)

• Advantages-genetic variation.

- Allows species to rapidly adapt to changing environment.

Asexual Reproduction

• Only 1 parent required • Offspring identical (NO MEIOSIS)

– Ex: Binary Fission & Budding

• Advantages - Less energy required- Faster reproduction time