The Other Cell Division: Making Sex Cells

Meiosis – A Source of Distinction

Ever wonder why you don’t look exactly like either your mother or father? Or why you and your siblings are not identical?

It’s all in MEIOSIS!

2 Major Roles of Meiosis

1. Make GAMETES (egg and sperm); diploid (2n) cells make haploid (n) cells

2. Genetic Variation

GAMETESMeiosis takes a cell with two copies

of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).

This change (diploid haploid) is critical if two gametes combine to make a new individual

In meiosis, one diploid cells produces four haploid cells.

Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. Increases genetic diversity (accomplished through independent assortment and crossing-over).Genetic diversity is important for the evolution of populations and species.

Genetic Variation

MeiosisParent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Homologous ChromosomesHomologous Chromosomes are:

1. The same size

2. The same shape

3. Have the same genes

4. Different forms of gene

Meiosis I : Separates Homologous Chromosomes●Interphase

●Each of the chromosomes replicate

●The result is two genetically identical sister chromatids which remain attached at their centromeres

Prophase I●Each pair of sister chromatids

move to their homologous pair and join together (synapsis) in a group of four called a tetrad.

●This is when crossing over can occur.

●Crossing Over is the exchange of segments during synapsis.

Metaphase I

●The chromosomes line up at the equator (metaphase 1 plate) attached by their centromeres to spindle fibers from centrioles.

●Still in homologous pairs

Anaphase I

●The spindle guides the movement of the chromosomes toward the poles●Sister chromatids remain

attached●Move as a unit towards the

same pole●The homologous

chromosomes are separated to opposite poles

Telophase I●This is the end of Meiosis I. ●The cytoplasm divides, forming two

new daughter cells. ●Each of the newly formed cells has

half the number of the parent cell’s chromosomes (23 unique chromosomes)

●but each chromosome is already replicated (sister chromatids) and the daughter cells are ready for Meiosis II

Cytokinesis

●Occurs simultaneously with Telophase I●Forms 2 daughter cells●Plant cells – cell plate●Animal cells – cleavage furrows

●NO FURTHER REPLICATION OF GENETIC MATERIAL (S Phase) PRIOR TO THE SECOND DIVISION OF MEIOSIS

Meiosis II : Separates sister chromatids

●There is no Interphase.●Results in 4 haploid daughter cells

(gametes)

Prophase II

●Each of the daughter cells forms a spindle, and the sister chromatids move toward the equator

Metaphase II

●The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

Anaphase II

●The sister chromatids finally separate

●The sister chromatids of each pair move toward opposite poles●Now individual chromosomes,

we no longer call them chromatids!

Telophase II and Cytokinesis

●Nuclei form at opposite poles of the cell and cytokinesis occurs

●After completion of cytokinesis there are four daughter cells ●All are haploid (n)●This is the whole point of

meiosis!

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent AssortmentIndependent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.

That’s a lot of diversity by this mechanism alone.

In humans, n = 23 and 223 = 8388608.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in

Meiosis

Mitosis The first (and distinguishing) division of meiosis