Meiosis – A Source of Distinction Why 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 Distinction Meiosis 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 Distinction The 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 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 KM8

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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)