Key Concepts:

•What happens during the process of meiosis?

•How is meiosis different than mitosis?

Vocabulary• Homologous—chromosome from mom and

corresponding chromosome from dad

Vocabulary• Diploid—Cells with two

sets of chromosomes

(2N)

– Code for same genes

in somatic

• Haploid—Cells with one

set (N)

– In the sex cells

(gametes)

– These will join with

sex cells (ex: ova

joins with pollen)

Eye Color is not as simple as it appears in this figure!

Each Species contains a characteristic number of chromosomes

Organism Body Cell (2N) Gamete (N)

Human 46 23

Garden Pea 14

Fruit Fly 8

Dog 39

Chimpanzee 24

Leopard Frog 13

Corn 20

Apple 17

Adder’s Tongue Fern 1260

7

4

78

48

26

10

34

630

• Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm.

• Meiosis appears much more complicated than mitosis…

– It is really just two divisions in sequence

• Recall that the purpose of mitosis was for growth

and repair of damaged tissues.

• What is the purpose of meiosis?

To make sex cells!!!!

Aka- gametes

Aka sperm or egg

Aka ova or pollen

INTERPHASE• Interphase in meiosis is identical

to interphase in mitosis

• Meiotic division will only occur in cells associated with male or female sex organs.

MEIOSIS I

• Meiosis I, the first of the two divisions, is often called

reduction division

– It is here that the chromosome number is reduced from

2N (diploid) to 1N (haploid).

• Ex: Humans: diploid number = 46 (haploid = 23)

PROPHASE I• Prophase I is similar to

prophase in mitosis

– Chromatin condenses into

chromosomes

– Spindle begins to develop

– Breakdown of the nuclear

envelope

Prophase I—Important Events

• Homologous chromosomes pair up to form a tetrad

• The sister chromatids press together at points along their length

• It is during this alignment that chromatid arms may overlap and temporarily fuse resulting in crossing over

Result: chromosomes (consisting of 2 sister chromatids) are no longer just from “mom” or “dad”—the chromosomes are a mixture!!!

Vocabulary

• Tetrad—each chromosome pairs with

homologous chromosome

– 4 sister chromatids!

• Crossing over– when chromosomes are in tetrads, they

may exchange parts

This happens in Prophase I

Result–increased genetic variety in offspring (a good

thing)

METAPHASE I

• Here is where the critical difference occurs

between Metaphase I in meiosis and

metaphase in mitosis.

METAPHASE I

• In Mitosis, all the chromosomes line up on the metaphase plate in no particular order.

• In Metaphase I (of Meiosis), the homologous pairs are alignedon either side of the metaphase plate.

ANAPHASE I

• During Anaphase I the

spindle fibers contract

– The homologous pairs

are pulled away from

each other

TELOPHASE I• Cleavage is followed by cytokinesis, but…

• The nuclear membrane (envelope) usually is not reformed and the chromosomes do not disappear.

At the end of Telophase I

• Each daughter cell has a single set of chromosomes

• Half the total number in the original cell where the chromosomes were present in pairs.

• While the original cell was diploid (2n), the daughter cells

are now haploid (1n).

• This is why Meiosis I is often called reduction division.

– Diploid = 4 Haploid = ?

– Diploid = 36 Haploid = ?

It was 4, now it will be 2 in

each cell!!

MEIOSIS II

• Meiosis II is quite simply a mitotic division of each of

the haploid cells produced in Meiosis I.

• There is no Interphase between Meiosis I and Meiosis

II and the latter begins with…

PROPHASE II

• A new set of spindle fibers

forms

• The chromosomes begin to

move toward the equator

of the cell.

METAPHASE II

• All the chromosomes in

the two cells align with the

metaphase plate. (Like in

mitosis….)

ANAPHASE II

• The centromeres split

• The spindle fibers shorten

– Drawing the chromosomes

toward each pole of the cell.

– (Like in mitosis….)

TELOPHASE II

• Cleavage is followed by cytokinesis

• The nuclear membrane (envelope) reforms

• The chromosomes begin to fade, replaced by the granular chromatincharacteristic of interphase.

• Total of four daughter cells, each with half the total number of chromosomes as the original cell.

• Male structures

– All four cells will eventually

develop into typical sperm

cells.

• Female life cycles in “higher”

organisms

– Three of the cells will typically

abort

– Leaving a single cell to develop

into an egg cell

• Usually much larger than a

typical sperm cell.

Gametes

Meiosi

s II

http://www.biologycorner.com/bio4/notes/meiosis.php

NOVA video

http://www.pbs.org/wgbh/nova/baby/divi_flash.html

https://www.youtube.com/watch?v=qCLmR9-YY7o