L4 MEIOSIS I & MEIOSIS II

Somatic cells vs Sex cells

Somatic cells are also known as diploid or 2n (chromosomes from mom (n) chromosomes from dad);

sex cells are haploid or 1n (one copy of each chromosome); when sperm and egg combine, they produce a 2n zygote)

Meiosis vs. Mitosis

- meiosis has 2 cell divisions, mitosis only has one

- in meiosis homologous chromosomes pair up on cell's equator, in mitosis homologous chromosomes never pair up

- in anaphase 1 of meiosis sister chromatids are still paired, in anaphase in mitosis, sister chromatids are separated

- meiosis results in a haploid cell, mitosis results in a diploid

Meiosis creates 4 daighter cells, Mitosis only 2

Meiosis I Stages

Prophase I

Metaphase I

Anaphase I

Telophase I and cytokinesis

thre events of meiosis l:

1- Synapsis and crossing over in prophase I

2- At the metaphase plate, there are

paired homologous chromosomes (tetrads) (::-:: ::-::) ::-::)

3- At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate (:: :: ::) (:: :: :: )

Prophase I stages

1. Leptotene

2. Zygotene

3. Pachytene

4. Diplotene

5. Diakinesis

Prophase I: Leptotene

-chromosomes thicken and become visible, but the chromatids remain invisible.

- Centrosomes begin to move toward opposite poles

Prophase I: Zygotene

1. Homologous chromosomes enter synapsis.

2. The synaptonemal complex forms (Each pair of chromosomes forms a tetrad, a group of four chromatids)

Prophase I: Pachytene

synapsis is complete, crossing over occurs

Prophase I: Diplotene

Crossing over is completed. Synaptonemal complex dissolves. Nuclear envelope continues to break down.

Prophase I: Diakinesis

chromatids thicken and shorten, at the end of prophase I, the nuclear membrane breaks down and the spindle begins to form

Metaphase I

Pairs of homologous chromosomes move to the equator of the cell. (::-:: ::-:: ::-::)

Anaphase I

Homologous chromosomes separate (:: :: :: ) (:: :: :: )

Telophase I and Cytokinesis

Chromosomes gather at the poles of the cells. the cytoplasm divides.

Meiosis II

the second phase of meiosis consisting of chromatids separating, along with the two diploid cells splitting in two

Metaphase II

Chromosomes line up at the equator. (:: :: ::) (:: :: :: )

Anaphase II

sister chromatids separate (: : :) (: : :) ( : : :) (: : :)

Spermatogenesis

4 sperm cells are

produced from each

primary spermatocyte.

Oogenesis

the production, growth, and maturation of an egg, or ovum. one Egg and three polar bodies

spermatogenesis vs oogenesis

Spermatogenesis: 4 gametes made, no polar bodies; Oogenesis: 1 gamete and three polar bodies

genetic variation

• Independent assortment of chromosomes

• Crossing over

• Random fertilization

independent assortment

the random distribution of the pairs of genes on different chromosomes to the gametes.

2^n