The Formation of Sex Cells

•  You simply cannot combine an egg and sperm if they contain the normal number of chromosomes!

•  The chromosome # must be reduced to half! This is Meiosis.

I. Overview of Meiosis A.   All organisms inherit two complete sets of genes,

one from each parent!•  In animals, mom’s set of genes comes from an egg cell;

dad’s set of genes comes from a sperm cell!

•  In plants, the genes come from pollen and ovule!

B. !Sex cells, or gametes contain half of the normal amount of chromosomes!

!1. !Meiosis is the process of creating haploid (N) gametes from diploid (2N) cells!

•  occurs in eukaryotic cells only!

•  required for sexual, but not asexual, reproduction!

II. Chromosome Number A.   Eukaryotic sexual reproduction results in organisms

with two sets of chromosomes!

!1. the two sets are said to be homologous!

! !a. chromosomes in one set have a version of themselves in !the other set!

B.  A cell with both sets of homologous chromosomes is said to be diploid (2N) !

!1. Ex. somatic cells (liver cell, heart cell, skin cell etc.)!

C.  A cell with one set of chromosomes is said to be haploid (N)!

!1. Ex. gametes (egg and sperm or ovule and pollen)!

Each body cell of a fruit fly has 8 chromosomes, 4 from the male and 4

from the female

Each body cell of a human has 46

chromosomes, 23 from the male and 23 from the

female

Homologous Chromosome Pairs

III. Phases of Meiosis Overview: Diploid cells destined to become gametes

! ! !go through two rounds of cell division!

A. DNA replication !

1. occurs before the first round of cell division (recall the S phase of mitosis)!

B. Meiosis I!

1. newly synthesized chromosomes pair up with their homologes forming a tetrad!

•  Ex. The chromosome containing the gene for eye color from mom will pair up with the chromosome containing the gene for eye color from dad!

•  This is different from mitosis!! Tetrad

B. Meiosis I cont. !

2. the tetrads may exchange portions of their chromatids, in a process called crossing-over!

•  genes are exchanged between homologous chromosomes!

•  this introduces variation!!

3. homologous chromosomes separate and the cell divides (recall cytokinesis)

4. two new diploid (2N) cells are formed, each having a unique set of genes

Images of Meiosis I note the similarities and differences to mitosis

C. Meiosis II!

!1. Performed by the two new cells !produced in Meiosis I!

•  each of the new haploid cell’s chromosomes contains 2 unique chromatids!

!2. DNA does not replicate!

!3. Prophase through cytokinesis !occurs!

!4. Four unique haploid (N) cells !are formed!

•  two from each of the cells produced in meiosis I!

Males vs.

Females

there is a difference!

For animals, meiosis in males produces 4 sperm from every germ

cell vs. one egg for females

3 polar bodies donate cell materials

(except DNA) and disintegrate

Link to Meiosis

Animation

Compare Mitosis and

Meiosis

IV. Mitosis vs. Meiosis 1. Mitosis!

•  Asexual reproduction!

•  Begins with a diploid cell (2N)!

•  Results in the formation of 2 diploid cells (2N) that are genetically identical to each other and to the original cell!

2.  Meiosis!•  Required for sexual reproduction!

•  Begins with a diploid cell (2N)!

•  Results in the formation of 4 haploid cells (N) that are genetically different from each other and the original cell!