Meiosis – A Source of DistinctionWhy do you share some, but not all, of the characteristics of each of your parents?

At one level, the answers lie in meiosis.

Meiosis – A Source of DistinctionMeiosis does two things -

1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a necessary if you’re going to combine two cells to make a new organism. You don’t want to double the amount of information in the cell each generation! This trick is accomplished by halving the chromosome number during two divisions.In meiosis, one diploid cell produces four haploid cells.

Meiosis – A Source of DistinctionThe second accomplishment of meiosis:

2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.

This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

Keys to Understanding Meiosis

Chromosomes are paired.(One from each parent)

Chromosomes carry genes.

The gene forms on a pair of chromosomes may be identical Brown eyes

Brown eyes

Tall

Tall

Brown eyes

Blue eyes

Tall

Short

or different.

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.

Meiosis• A process of reduction division. We would not

survive if we received 46 chromosomes from each parent. So….we receive 23 from each parent- each a complete set of instructions.

• Diploid – 2 sets of chromosomes• Haploid – 1 set of chromosomes• Homologous – chromosomes that each have a

matching chromosome from the opposite sex parent (Mom and Dad!)

Meiosis Stages

• Meiosis involves 2 distinct stages– Meiosis I– Meiosis II

• Each stage has phases – similar to mitosis

Meiosis KM 8

Meiosis KM 9

Interphase• Just like mitosis, the cell is

growing & getting ready to divide• Chromatin still duplicates!!

Prophase I• Sister chromatids pair up and attach• Homologous pairs get together• Spindle-fibers appear• Centrioles begin to move to poles• Nuclear membrane disintegrates • Crossing-over may occur here

Metaphase I

• Spindle fibers attach to the chromosomes at the centromeres

• Homologous pairs line up in the middle!

Anaphase I

• The spindle fibers pull the homologous chromosomes toward opposite ends of the cell. (sister chromatids still attached at the centromeres- move as a pair)

Telophase I & Cytokinesis

• Nuclear membranes form.• The cell separates into 2 unique cells.

Meiosis I(In Action)

Not Done Yet - Prophase II• No new duplication occurs (no Interphase II) • Now we have two different cells to follow.• Pairs already attached • Nuclear membranes dissolve• Centrioles move to poles• Spindle fibers form

Metaphase II

• The chromosomes line up similar to metaphase in mitosis.

Anaphase II

• Sister chromatids separate and move to opposite ends of the cell.

Telophase II

• Meiosis II results in 4 unique haploid cells.

Gamete Formation

• In males, meiosis results in 4 sperm cells• In females, meiosis results in 1 egg cell

and three polar bodies, which are not used in reproduction.

What’s the Point?• The former zygote

~ 1 year after fertilization.....

Cute, multicellular, unique and diploid...thanks to meiosis followed by LOTS of mitosis!

• (Instructions not included)