Vocabulary:Homologous

Diploid

Haploid

Meiosis

Tetrad

Crossing-over

Key Concept:•What happens during the process of meiosis?

•How is meiosis different than mitosis?

Vocabulary

• Homologous—chromosome from mom has corresponding chromosome from dad

• Diploid—two sets of chromosomes– Code for same genes (ex: hair color)

• Haploid—one set– In the sex cells– These will join with sex cells from the opposite

sex (ex: egg joins with sperm)

• 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– each one of which has strong similarities to

mitosis.

INTERPHASE

• Interphase in meiosis is identical to interphase in mitosis

• There is no way, by simply observing the cell, to determine what type of division the cell will undergo when it does divide.

• 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– involving the appearance of

the chromosomes– the development of the

spindle apparatus– 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: chomosomes (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 variety in

offspring (a good thing)

Result: INCREASED genetic variety!!!

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

chromosome pairs are aligned on either side of the metaphase plate.

ANAPHASE I

• During Anaphase I the spindle fibers contract– the homologous pairs

are pulled away from each other • toward each pole of

the cell.

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 = 2

It was 4, now it will be 2 in each cell!!

MEIOSIS II• Meiosis II is quite simple in that it is 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.

ANAPHASE II

• The centromeres split

• The spindle fibers shorten– drawing the chromosomes

toward each pole of the cell.

TELOPHASE II

• Cleavage is followed by cytokinesis • The nuclear membrane (envelope)

reforms• The chromosomes begin to fade,

replaced by the granular chromatin characteristic 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

NOVA video

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