Meiosis Notes

Meiosis- cell division that producing reproductive cells called gametes (or egg and sperm) Meiosis involves replicating DNA once and dividing the nucleus two times in steps called Meiosis I and Meiosis II. By dividing twice, the chromosome number is halved in gametes. Slide 1

- 2 parents - offspring genetically similar to parents (combination of parent chromosomes) (offspring not identical) Gamete: Sex cell Female: egg Male:sperm Zygote:Union of sperm and egg n n n (Diploid) (Haploid) Gonads: Male – Testes Female – Ovaries Slide 2

Meiosis:Process of cell division used to produce gametes (sex cells) 46 SC 46 DC 23 DC 23 DC Interphase PMATPMAT 23 SC 23 SC 23 SC 23 SC PMATPMAT IIIIIIII II Primary Sex Cell Secondary Sex Cells 2n nn n n n n Slide 3

What happens in PMAT is the same in meiosis as in mitosis EXCEPT in……. Prophase I Anaphase I Slide 4

Prophase I of Meiosis I Sister chromatids pair with their homologous pair, forming a tetrad *Double chromosomes made during interphase* Slide 5

Homologous Chromosomes Humans have 23 chromosome pairs, each chromosome in the pair contains the same kind of traits, like eye color; they are called homologous chromosomes. (Humans have 23 homologous chromosomes for a total of 46 chromosomes Having two copies of every gene can be an advantage. For example, if the one gene from Dad was damaged, Mom’s good gene could take over. Slide 6

Sister Chromatid Slide 7

Tetrad Maternal chromosome replicated Paternal chromosome replicated Slide 8

Prophase I tetrad Joining of homologous chromosome at the centromere forming a tetrad Slide 9

A Way Meiosis Makes Lots of Different Sex Cells Crossing-Over Slide 10

Crossing Over: - increases variation -changes chromosome package -(not same as either parent) Pieces of homologous chromosomes switch locations P Bld Blu Fair M Brn Drk P Bld Blu Drk M Brn Fair Slide 11

Metaphase I of Meiosis I Homologous pairs (tetrads) line up in the center of the cell. Slide 12

Anaphase I of Meiosis I Spindle fibers pull the homologues (tetrad) away from each other during disjunction. Slide 13

Telophase I of Meiosis I The chromosome number in these cells divides in half; starting with an initial four (2n) and ending with two (n) in each new cell. Slide 14

Meiosis II : Similar to Mitosis In Meiosis II each chromosome lines up and sister chromatids separate from each other. Prophase II Metaphase II Anaphase II Telophase II Slide 15

Meiosis in malesMeiosis in female Occurs in testesOccurs in ovaries 2n nn n n nnnn n nn 4 sperm 1 egg polar bodies 1:4 1:1 n 3 polar bodies Slide 16

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 good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells. Slide 17

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. Genetic diversity is important for the evolution of populations and species. Slide 18

Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes What if Meiosis doesn’t happen as planned?!?! Slide 19

Meiosis – division error Chromosome pair Slide 20

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome Slide 21

Trisomy 21– Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female? Slide 22

Anaphase I Disjunction: Separation of tetrad at the centromere Nondisjunction: Uneven separation of a tetrad during disjunction Ex. Down’s Syndrome (nondisjunction of 21 st chromosome tetrad) Polyploidy: Nondisjunction of all tetrads -gamete has complete extra set of chromosomes Animal cells – fatal Plant cells – larger more vigorous variety Slide 23

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome Slide 24

Boy or Girl? The Y Chromosome “Decides” Slide 25

How does the image in the first picture become the person in the last picture? Slide 26

Type of cells that perform: Uses: Number of cell divisions: Chromosome counts: Genetic Variation?: Number of daughter cells: Mitosis: Meiosis: Somatic 1 2n  2n None Gametes Sexual Reproduction 2 2n  n Yes (crossing over) Growth, Repair, Asexual reproduction 2 4 sperm/ 1 egg Slide 27

- Organism that normally contains both sexes (ovaries and testes) - Usually does not self fertilize Example: earthworm -Development of an unfertilized egg to a complete organism Example: male drones bees - Fertilized eggs give rise to queen and female workers Slide 28

Union of the sperm and the egg forming a zygote Occurs outside body of female Occurs inside body of female Produces more eggs (1000+)Produces less eggs (1-12) Occurs in water Secretes fluid to provide watery environment Fish Amphibians Birds Reptiles Mammals Slide 29