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Chapter 13 Sexual Reproduction & Meiosis Dr. Joseph Silver Slide 2 In this chapter we will study 1. why we reduce the chromosomes in sex cells 2. importance of homologous pairing 3. the steps in meiosis 4. errors in meiosis 5. comparison to mitosis Slide 3 genetics = study of heredity heredity = how we pass on of traits to next generation Slide 4 gametes = eggs and sperm eggs and sperm = haploid (half a set) somatic cells = diploid (full set) what do you inherit? genes what are genes = messages for what a cell should do where are genes located? on chromosomes Slide 5 what do we pass on to the next generation? genes mutations variations of combinations it is as if you took a few thousand genes which occur as pairs and you rearranged them randomly and for the fun of it you see what happens Slide 6 you get chromosomes from mom containing genes for certain traits and you get chromosomes from dad containing genes for the same traits these are homologous chromosomes they contain genes for the same traits but the genes do not have to be the same Slide 7 sister chromosomes = 2 copies of the same chromosome homologous chromosomes = mom and dads same chromosome see fig. 13.3 Slide 8 Slide 9 what is the purpose of meiosis 1. to reduce the chromosome number 2. to rearrange the genes 3. to make a new arrangement of genes 4. to preserve the species Slide 10 Slide 11 For the first few years of life every cell, tissue, and organ in our bodies only go through mitosis they live, grow, and do their thing Slide 12 then our brains release some hormones in our brains which initiate the process of puberty Slide 13 Slide 14 so what is or better yet what are the functions of puberty and meiosis??????????????? Slide 15 to produce functional eggs and sperms to make our bodies ready for making babies to produce functional offsprings/babies with functional parents ???? Slide 16 we will spend the rest of this class talking about meiosis See fig. 13.8 Slide 17 Slide 18 1. double the chromosomes (4 become 8) 2. form tetrads 3. exchange genes 4. line up at midplate 5. pull apart chromosomes so sister chromatids are intact 6. cell becomes 2 cells(each with 4 chromosomes) 7. sister chromatids in 2 cells line up at midplate 8. sister chromatids pulled apart 9. 2 cells become 4 cells each with 2 chromosomes Slide 19 when the chromosomes double we now have 2 sets of moms genes and 2 sets of dads genes each is called a sister chromatid Slide 20 Slide 21 Slide 22 when we studied mitosis we learned that cohesions hold chromosomes together and their kinetochore region and that cohesions are broken down in order to allow the chromosomes to separate from each other Slide 23 look at pages 258 and 259 and 261 to see the steps of meiosis Slide 24 at steps where sister chromatids or tetrads separate from each other the cohesions are broken down by the action of the APC complex to activate a group of enzymes called separases Slide 25 the pair of moms genes and dads genes line up next to each other as a tetrad during prophase I 4 chromosomes next to each other Slide 26 Slide 27 mom and dads chromosomes which contain genes for the same traits are homologous chromosomes Slide 28 during prophase I homologous chromosmes pair up so we now have a complex of 4 chromosomes each having genes for the same traits this pairing is known as synapsis and this structure is known as a tetrad Slide 29 what is the function of a tetrad? Slide 30 to rearrange the genes to produce a new combination of genes to make us different from our parents to produce a diverse population crossing over see fig.13.9 Slide 31 Slide 32 Slide 33 now that the genes have been rearranged and we have created chromosomes which never existed before we now proceed with making eggs and sperms Slide 34 Slide 35 metaphase I - the tetrads line up at the equatorial area - spindle fibers are attached to sister chromatids - centrioles are at opposite poles of cell - how the tetrads line up is totally random - which side sister chromatids go to is totally random independent assortment Slide 36 anaphase I sister chromatids separate from homologous pairs moms pair are going one way and dads pair are going the opposite way Slide 37 Slide 38 Slide 39 the process began with a cell with 4 chromosomes now in telophase I we are producing 2 cells each with 4 chromosomes made up of 2 sister chromatids Slide 40 - the chromosomes thin out - the nuclear membrane may reform - the spindles release the chromosomes - the center of the cell pinches in to form 2 cells Slide 41 Slide 42 cytokinesis I splits the cell into 2 cells each with the same number of chromosomes which were present in the original cell and then quickly proceeds to prophase II Slide 43 Slide 44 Slide 45 metaphase II spindle fibers shorten sister chromatids pulled apart Slide 46 Slide 47 anaphase II - nuclear membrane begins to reform spindles are reabsorbed chromosomes begin to unwind organelles reform cell membrane forms cleavage furrow 2 cells with 2 sister chromatids (4 chromosomes) become 4 cells each with 2 chromosomes (1/2 of parent cells) Slide 48 Slide 49 telophase II & cytokinesis II nuclear membrane forms chromosomes unwind organelles function spindles gone result of meiosis is four (4) different haploid cells Slide 50 Slide 51 meiosis 1 rearrangement of genes 2 cell divisions 2 independent assortments results in haploid cells with a new combination of genes producing an increase in species diversity Slide 52 meiosis summary chromosome duplication homologue pairing crossing over 1 st separation sister chromatids intact interphase II DNA replication suppressed 2 nd separation sister chromatids separated production of haploid cells production of cells which are not identical Slide 53 meiosis produces eggs and sperms which are different is it possible that 2 eggs or sperms are identical yes but is it likely - no