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Meiosis: Making haploid reproductive cells

Meiosis: Making haploid reproductive cells. Goal of Meiosis. The purpose is to make haploid cells so that when 2 haploid cells combine you have a new diploid offspring that has an original combination of genes. + =

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Meiosis: Making haploid reproductive cells

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  1. Meiosis:Making haploid reproductive cells

  2. Goal of Meiosis The purpose is to make haploid cells so that when 2 haploid cells combine you have a new diploid offspring that has an original combination of genes. + = Haploid cell + Haploid cell = Diploid cell (2n) (1n) (1n) that turns into a muticellular offspring

  3. Meiosis Overview • Makes 4 haploid cells • Takes place in sex cells (gametes) such as egg & sperm cells • Location: Testes (male) and ovaries (female) • The Goal: • To make 4 haploid daughter cells for reproduction that are genetically different from the parent cells. • Each haploid daughter cell will contain a mixture of the parents chromosomes, so that the 4 daughter cells should be different.

  4. Meiosis • Involves 2 cell divisions • The cell will go through interphase (G1, S, G2)…but then will divide twice  resulting in 4 haploid cells • There are 2 parts in Meiosis: • Meiosis 1 • Meiosis 2

  5. Both go through each of the phases of mitosis (PMAT) After each one cytokinesis occurs The goal is to reduce chromosome number to haploid so that 2 cells can combine to make a diploid organism Meiosis I and Meiosis II

  6. DNA coils into chromatids Spindle fibers appear Nucleus/nucleolus disappear *Homologous chromosomes pair up into tetrads of 4 chromatids, called synapsis *Crossing over may occur causing genetic recombination Meiosis 1: Prophase 1

  7. What is crossing over? • When a homologous pair bumps into each other during prophase I and they exchange portions one chromatid  leads to genetic recombination

  8. Metaphase I: Tetradsline up randomly in the center. Spindle fibers attach to each end of the homologous pair Anaphase I: Homologous pairs split randomly, called independent assortment Metaphase I & Anaphase I

  9. Independent Assortment The random lining up of homologous pairs during metaphase I. This allows for different gametes to be possible and also leads toward genetic recombination

  10. More independent assortment

  11. What is genetic recombination? • …when the combination of genes in an offspring are different from the gene combination found in the parents • Occurs in 2 ways in meiosis 1. Crossing over where homologous pairs exchange portions of one chromatid 2. Independent assortment where the homologous pairs line up randomly or without influences like which parent they came from or whether they are dominant or recessive.

  12. Chromosomes reach poles Cytokinesis occurs forming 2 new cells. Nuclear membrane may or may not reform before meiosis II Telophase I and Cytokinesis I

  13. Meiosis II (2nd cell division, similar to mitosis steps, PMAT) • Prophase II: Spindle fibers form • Metaphase II: Chromosomes move to center (along equator) • Anaphase II: Chromatids separate toward the poles • Telophase II: Nuclear membrane reforms. Spindle fibers disassemble. • Cytokinesis occurs forming 4 haploid cells with 1 copy of each chromosome.

  14. How does Meiosis differ in males vs. females? Since meiosis is producing sex cells (sperm & eggs) it takes place in the testes (males) & the ovaries (female) • In males: • Spermatogenesis - the formation of 4 spermatids (haploid cells) which will turn into spermatozoa (sperm cells). • In females: • Oogenesis- the formation of 1 egg cell (ova) with more cytoplasm than the other 3 cells. The remaining 3 cells are called polar bodies and will disintegrate

  15. The phases of Meiosis

  16. Diagrams

  17. Mitosis makes IDENTCIAL cells to the parent Mitosis make DIPLOID cells Mitosis occurs in BODY cells Mitosis makes 2 DAUGHTER CELLS Meiosis make DIFFERENT cells from the parent Meiosis makes HAPLOID cells Meiosis occurs in REPRODUCTIVE cells Meiosis makes 4 DAUGHTER CELLS Let us compareMitosis VS. Meiosis

  18. Animations Johnkyrk.com http://www.lewport.wnyric.org/jwanamaker/animations/meiosis.html http://www.stolaf.edu/people/giannini/ • Meiosis square dance

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