PPT 7: Cell Division and Meiosis PDF

1/5/2023 PPT 7 Cell Division and Meiosis Meiosis What advantage does sex provide?  Answer has to do with the genetic variation caused by creating egg and sperm  Sexual reproduction is best when the environment is variable Internal fertilizatio...

1/5/2023 PPT 7 Cell Division and Meiosis Meiosis What advantage does sex provide?  Answer has to do with the genetic variation caused by creating egg and sperm  Sexual reproduction is best when the environment is variable Internal fertilization:  Enables egg to meet sperm in a dry environment  Independence from water  More efficient 1  1/5/2023 Meiosis Reproductive System:  Sexual – males and females  Cyclical Fig 13.5 Fig 13.5 Concept of diploid versus haploid In animals Reduction division – ovaries or testes Cells that divide by meiosis must be diploid Diploid means having a full set of chromosomes – haploid means half a set Fig 16.23 In humans: 2n = diploid=46 n = haploid=23 Fig 13.3, 13.4, 13.5, 13.7 Similar to mitosis: must replicate first But then: Divide twice MEOSIS I and MEOSIS II Results in: 4 daughter cells 2  1/5/2023 Meiosis Fig 16.23 Meiosis Fig 13.3 3  1/5/2023 Meiosis Fig 13.4 Meiosis Fig 13.7 4  1/5/2023 Meiosis PHASES of MEOSIS I Interphase:  Replication similar to mitosis Prophase I:  Chromosomes condense and homologous pair up  Centrioles migrate to poles  Nuclear membrane breaks down  A process called crossing over occurs  Tetrads form  Non- sister chromatids cross over exchanging genetic information – chiasmata Meiosis Fig 13.8a 5  1/5/2023 Meiosis Crossing over is 1st source of variation in meiosis  Centrioles still moving to poles  Spindle fibers forming (microtubules)  Nuclear membrane completely gone  Kinetochores form on chromosomes  Can last for days, 90% of meiosis is spent here Meiosis Metaphase I:  Homologous chromosomes arranged in pairs on metaphase plate  Kinetochore microtubules are attached to one chromosome from each tetrad – opposite poles Independent Assortment is a 2nd source of variation in meiosis 6  1/5/2023 Meiosis Meiosis Anaphase I: Spindle guides WHOLE chromosomes to poles (Disjunction) Unlike mitosis – sister chromatids move to same pole (Compare mitosis) Non – disjunction causes major illness Examples of Aneuploidy:  Down syndrome Fig 15.16  Turner syndrome (see fig in ppt)  Klinefelter syndrome (see fig in ppt) There are many possible combinations of linkage groups 2n 7  1/5/2023 Meiosis Fig 15.16 Meiosis Klinefelter XXY 8  1/5/2023 Meiosis Turner’s Syndrome X __ Meiosis Telophase I / Cytokinesis:  Pairs of homologous chromosomes reach poles  Each pole now has a haploid set (still with 2 sis chromatids)  Cytokinesis occurs at same time  Cleavage furrows form  There is no further replication although chromosomes will divide again at MEOSIS II  In some species nuclear membrane reforms 9  1/5/2023 Meiosis Fig 13.8a Meiosis Fig 13.8b 10  1/5/2023 Meiosis PHASES MEOSIS II Centrosomes replicate Another round of division occurs –sis chromatids finally separate Results in 4 daughter cells Prophase II:  Spindle reforms and the chromosomes move to metaphase II plate Metaphase II:  Chromosomes are lined up along plate Meiosis Anaphase II:  Centromeres begin separation of sister chromatids  Sister chromatids become individual chromosomes  Chromosomes move towards opposite poles Telophase II / Cytokinesis:  Separation of cytoplasm – cells completely split  Nuclei reform – spindle fibers break up  Now have 4 daughter cells that have independently assorted 11  1/5/2023 Meiosis Daughter cells will be unique Possible combinations 2n = 223 = about 8 million Review Fig 13.9 Fertilization is a 3rd source of variation but not directly part of meiosis Which egg and sperm is used for offspring is random and all combinations are possible Meiosis Fig 13.11 12  1/5/2023 Meiosis Fig 13.11 Meiosis Fig 13.11 13  1/5/2023 MALE VERSUS FEMALE MEIOSIS DIFFERENCES Practice labelling with the terminology – no need to use the lines provided just draw your own lines in 14