the other cell division: making sex cells
DESCRIPTION
The Other Cell Division: Making Sex Cells. Meiosis – A Source of Distinction. Ever wonder why you don’t look exactly like either your mother or father? Or why you and your siblings are not identical? It’s all in MEIOSIS!. 2 Major Roles of Meiosis. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/1.jpg)
The Other Cell Division: Making Sex Cells
![Page 2: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/2.jpg)
Meiosis – A Source of Distinction
Ever wonder why you don’t look exactly like either your mother or father? Or why you and your siblings are not identical?
It’s all in MEIOSIS!
![Page 3: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/3.jpg)
2 Major Roles of Meiosis
1. Make GAMETES (egg and sperm); diploid (2n) cells make haploid (n) cells
2. Genetic Variation
![Page 4: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/4.jpg)
GAMETESMeiosis takes a cell with two copies
of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
This change (diploid haploid) is critical if two gametes combine to make a new individual
In meiosis, one diploid cells produces four haploid cells.
![Page 5: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/5.jpg)
Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. Increases genetic diversity (accomplished through independent assortment and crossing-over).Genetic diversity is important for the evolution of populations and species.
Genetic Variation
![Page 6: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/6.jpg)
MeiosisParent cell – chromosome pair
Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes
![Page 7: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/7.jpg)
Homologous ChromosomesHomologous Chromosomes are:
1. The same size
2. The same shape
3. Have the same genes
4. Different forms of gene
![Page 8: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/8.jpg)
Meiosis I : Separates Homologous Chromosomes●Interphase
●Each of the chromosomes replicate
●The result is two genetically identical sister chromatids which remain attached at their centromeres
![Page 9: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/9.jpg)
Prophase I●Each pair of sister chromatids
move to their homologous pair and join together (synapsis) in a group of four called a tetrad.
●This is when crossing over can occur.
●Crossing Over is the exchange of segments during synapsis.
![Page 10: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/10.jpg)
![Page 11: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/11.jpg)
![Page 12: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/12.jpg)
Metaphase I
●The chromosomes line up at the equator (metaphase 1 plate) attached by their centromeres to spindle fibers from centrioles.
●Still in homologous pairs
![Page 13: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/13.jpg)
Anaphase I
●The spindle guides the movement of the chromosomes toward the poles●Sister chromatids remain
attached●Move as a unit towards the
same pole●The homologous
chromosomes are separated to opposite poles
![Page 14: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/14.jpg)
Telophase I●This is the end of Meiosis I. ●The cytoplasm divides, forming two
new daughter cells. ●Each of the newly formed cells has
half the number of the parent cell’s chromosomes (23 unique chromosomes)
●but each chromosome is already replicated (sister chromatids) and the daughter cells are ready for Meiosis II
![Page 15: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/15.jpg)
Cytokinesis
●Occurs simultaneously with Telophase I●Forms 2 daughter cells●Plant cells – cell plate●Animal cells – cleavage furrows
●NO FURTHER REPLICATION OF GENETIC MATERIAL (S Phase) PRIOR TO THE SECOND DIVISION OF MEIOSIS
![Page 16: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/16.jpg)
![Page 17: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/17.jpg)
Meiosis II : Separates sister chromatids
●There is no Interphase.●Results in 4 haploid daughter cells
(gametes)
![Page 18: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/18.jpg)
Prophase II
●Each of the daughter cells forms a spindle, and the sister chromatids move toward the equator
![Page 19: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/19.jpg)
Metaphase II
●The chromosomes are positioned on the metaphase plate in a mitosis-like fashion
![Page 20: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/20.jpg)
Anaphase II
●The sister chromatids finally separate
●The sister chromatids of each pair move toward opposite poles●Now individual chromosomes,
we no longer call them chromatids!
![Page 21: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/21.jpg)
Telophase II and Cytokinesis
●Nuclei form at opposite poles of the cell and cytokinesis occurs
●After completion of cytokinesis there are four daughter cells ●All are haploid (n)●This is the whole point of
meiosis!
![Page 22: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/22.jpg)
Figure 13.7 The stages of meiotic cell division: Meiosis II
![Page 23: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/23.jpg)
One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent AssortmentIndependent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
That’s a lot of diversity by this mechanism alone.
In humans, n = 23 and 223 = 8388608.
![Page 24: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/24.jpg)
![Page 25: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/25.jpg)
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.
![Page 26: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/26.jpg)
![Page 27: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/27.jpg)
The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis The first (and distinguishing) division of meiosis
![Page 28: The Other Cell Division: Making Sex Cells](https://reader035.vdocuments.site/reader035/viewer/2022062410/568162af550346895dd33423/html5/thumbnails/28.jpg)