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http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html. Unit 6: Meiosis. 1 asexual reproduction. In unicellular organisms such as bacteria and protists , cell division is also reproduction called binary fission. Offspring are identical to the parents.
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http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.htmlhttp://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html
1 asexual reproduction • In unicellular organisms such as bacteria and protists, cell division is also reproduction called binary fission. • Offspring are identical to the parents. • Division for unicellular organisms can happen very quickly.
2 asexual reproduction Can occur in multicellular organisms. So, if an amoeba has two chromosomes, how many chromosomes do the two daughter cells have? By what process did they divide?
3 Sexual Reproduction • When offspring are created from the union of two cells from two different parents, often called sex cells or gametes. • Offspring inherit genetic material from two parents.
4 slower able to adapt in a changing environment genetically unique offspring two parents must find one another to mate • faster • take full advantage of a good environment • genetically identical offspring • only one parent
5 Sexual Reproduction • If a fruit fly has 8 chromosomes, how many chromosomes do the offspring have? • So, could there be 8 chromosomes in the egg and sperm cells? Why or why not? • So, how many chromosomes should be in each egg and sperm for a fruit fly? • Could the egg and sperm be created by mitosis? • Somehow, we have to do what to the chromosome number?
6 Chromosome Number • Most cells are diploid, meaning that they have two sets of chromosomes (“di” means two). • So if you have 46 chromosomes, how many pairs do you have? • If a fruit fly has 8 chromosomes, how many pairs do they have? • The pairs of chromosomes are called homologous pairs. • The diploid number is often referred to as 2N.
7 Karyotypes • Show the chromosome pairs for an organism.
8 Cutting the chromosome # in half • For gametes, though, we want half the chromosome number, or only one chromosome from each pair. • This is called the haploid number and is shown as 1N or N. • What would the haploid number be for a fruit fly or a human? • What do we call the process of producing gametes are haploid?
9 Meiosis • Occurs in two stages, Meiosis I and Meiosis II. • Within each stage, there is a prophase, metaphase, anaphase and telophase. • Meiosis I yields two daughter that have half the chromosomes. • Those two cells enter Meiosis two, where they divide again and sister chromatids separate, creating a total of four haploid daughter cells.
So N =? How many daughter cells are made by the end of Meiosis I? How many chromosomes does each daughter cell have? 11 How many chromosomes does the cell start with? 2N = ? How many homologous pairs?
12 Meiosis Goals • For Meiosis 1 – split chromosome number in half • For Meiosis 2 – split sister chromatids in half • So what must have happened to the DNA before Meiosis occurred?
13 Tetrads • During Prophase 1 of Meiosis 1, a tetrad is formed when the homologous pair comes together. • A tetrad contains four chromatids: 2 chromosomes, each with its own copy.
14 Crossing Over • When the tetrads form, some genetic variety is created from crossing over. • Crossing over is done when chromatids cross over one another and the crossed sections are exchanged.
Mitosis vs. Meiosis http://www.pbs.org/wgbh/nova/miracle/divide.html
