Meiosis https://www.khanacademy.org/science/biology/cellular-molecular-biology/meiosis/v/chromosomal-crossover-in-meiosis-i

What is Meiosis Exactly? Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells such as gametes or spores There are 2 stages of meiosis, Meiosis 1 and meiosis 2 Meiosis 1 and 2 create 4 haploid cells all together

IV. Meiosis – Reduction Division

Stages of Meiosis 1

Stages of Meiosis 2

Interphase before replication

Interphase after replication

I. Meiosis (Reduction Division) A. Meiosis I 1. Prophase I a. chromosomes become distinct b. nucleolus and nuclear membrane disappear and spindle fibers appear

Prophase I c. spindle fibers appear d.synapsis–homologous chromosomes Line up together

Prophase I e. crossing over may occur 1) portions of chromatid from one parent break off and attach to a homologous chromatid from the other parent 2) results in genetic recombination

2. Metaphase I a. chromosomes line up along the midline b. sister chromatids do not separate

3. Anaphase I a. at random, one member of each homologous pair moves to the opposite poles (independent assortment

4. Telophase I and Cytokinesis I a. chromosomes reach opposite poles b. cytokinesis begins

Telophase I c. resulting cells have the n or haploid number of chromosomes 1) one member of each homologous pair with two attached chromatids d. each new cell contains ½ the the number of chromosomes as the original cell

B. Meiosis II 1. Prophase II a. spindle form and chromosomes begin to move toward the mid-line of the cell

. Metaphase II a. chromosomes move to the mid-line of the dividing cell

3. Anaphase II a. chromatids separate and move to the opposite poles of the cell

4. Telophase II a. nuclear membrane forms around the nucleus in each cell b. each resulting cell contains the n number of chromosomes

Meiosis 1 and Meiosis 2

Animation of meiosis

C. Chart Comparing Mitosis and Meiosis 2 cells result One division 2n number of chromosomes in resulting cells Meiosis 4 cells result Two divisions n number of chromosomes in resulting cells

D. Genetic Recombination and Variation 1. independent assortment 2. crossing over 3. random fusion of sperm and egg

E. Formation of Gametes 1. spermatogenesis – results in 4 viable sperm 2. oogenesis – results in 1 egg and 3 polar bodies

Spermatogenesis

Oogenesis

Fertilization [sperm (n) + egg (n)  zytote (2n) ]

F. Sexual vs Asexual Reproduction One parent Identical genetic material Mitosis, budding, binary fission Sexual Two parents Different genetic material Meiosis + fertilization

Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

Karyotype 1. picture of homologous chromo- somes and sex chromosomes 2. used to detect chromosome abnormalities

Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues.

Human Chromosomes

In humans … 23 chromosomes donated by each parent (total = 46 or 23 pairs). Gametes (sperm/ova): Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid number “n” = 23 in humans). Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Only gametes are produced by meiosis.

In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!!

Karyotyping- normal

Down's Syndrome

some of the previous slides came from the following website http://cchs.churchill.k12.nv.us/marshk/Notes/meiosis.ppt#352,39,Slide 39

www.biology.Arizona.edu/