Meiosis- The Formation of Sex Cells

I. Introduction to Meiosis

A. Purpose - to make sex cells for reproduction.

B. Why can’t mitosis do this?

1. Mitosis would make too many chromosomes.

2. Ex. If a fruit fly has 4 chromosomes, each of its children would need to have 4 chromosomes.

A. Mom and dad each give 4 chromosomes, offspring has 8 and would die.

3. The organism would have double the number of chromosomes which would cause the organism to die.

4. Mom gives 2 and dad gives 2, new organism has 4 just like parents.

C. Diploid Chromosome - the total number of chromosomes the individual should have.

1. Ex. Human = 46

Ex. Lettuce = 18

2. Diploid number = 2n

D. Haploid Chromosome Number - half the number of chromosomes an organism should have.

1. Ex. Human = 23

2. Ex. Lettuce = 9

3. Haploid Chromosome # = n

II. Workings of MeiosisA. Meiosis is - the formation of

sex cells (eggs and sperm)

1. Gametes - another name for sex cells.

A. Gametes have haploid chromosome number

B. Meiosis is the making of eggs and sperm with half the number of chromosomes as the parent cell.

1. Female egg = 23

2. Male sperm = 23

C. Two Main Stages of Meiosis

1. Meiosis I a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I

2. Meiosis II

a. Prophase II

b. Metaphase II

c. Anaphase II

d. Telophase II

D. There is only one interphase

III. Meiosis I

A. Interphase

1. Replication of chromosomes occurs.

2. This is the only interphase.

B. Prophase I

1. Chromatids appear

2. Centrosome splits into two centrioles.

3. Centrioles move to the opposite poles.

4. Spindle fibers appear

5. Nucleolus disappears

6. Nuclear membrane disappears

7. Tetrads form - two pairs of homologous chromosomes joining together.

8. Crossing-over occurs here - genes are exchanged between homologous chromsomes.

a. Creates variation in new offspring

C. Metaphase I

1. Tetrads line up at the equator.

D. Anaphase I

1. Tetrads separate.

2. Chromatids are pulled to the opposite poles.

E. Telophase I

1. Chromatids still visible.

2. Nucleolus and Nuclear Membrane reforms.

3. Centrioles replicate

4. Two new cells.

IV. Meiosis IIA. No interphase begins this

phase. 1. 2 cells with the diploid

chromosome number begin this stage.

2. Meiosis II will always have two cells in each phase.

B. Prophase II 1. Nuclear membrane and

nucleolus disappear. 2. Centrosome splits into two

centrioles. 3. Centrioles move to the

poles.

4. Spindle fibers form.

5. Chromatids are visible and attach to spindle fibers.

C. Metaphase II

1. Chromatids line up on the equator.

D. Anaphase II

1. Centromeres separate.

2. Chromosomes are pulled to opposite poles.

E. Telophase II

1. Centrioles replicate

2. Nuclear membrane and nucleolus reappear.

3. Spindle fibers disappear.

4. 4 new cells are formed

a. Only have half the number of chromosomes as the parent cell.

b. Males - all 4 remain alive and viable.

c. Females - one grows rest die off.

V. Meiosis-Mitosis Comparison

A. Mitosis

1. 4 phases

2. Produces 2 diploid cells

3. Daughter cells identical to parent

B. Meiosis

1. 8 phases

2. 4 haploid cells produced

3. Daughter cells not identical to parent

4. Contain tetrads and crossing-over.