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Meiosis Chromosomes • The human body has 23 pairs of chromosomes • Cells that have 46 chromosomes are called somatic cells • In these cells there is 23 pairs of matching chromosomes • These chromosomes are a set and can be referred to as homologous chromosomes Chromosomes • In certain instances during a cell’s life cycle they may alternate the number of chromosomes that are present • A diploid cell contains pairs of homologous chromosomes • This is often abbreviated as 2n • That means two times the normal chromosome number Chromosomes • A haploid cell Is a cell that contains one copy of each chromosome • This is often abbreviated as n • This means that these cells only carry one copy of each chromosome Gametes • Why might a cell only want to carry half the chromosomes needed for life? • Normally it is because these cells are gametes • Gametes are cells that can combine to create offspring during sexual reproduction Gametes • That means that the gametes are combining to create offspring with non identical chromosomes • One chromosome will be donated from the male • One chromosome will be donated from the female Gametes • In a human life cycle this means that a sperm (n male gamete) and an egg (n female gamete) must combine to create an offspring • The initial 2n cell that is created from the combination is called a zygote Alternating Stages • This means that all sexual life cycles alternate between n and 2n stages • During most phases the life cycle is 2n • However the combination of haploid cells that are n must happen for there to be sexual reproduction Meiosis • Mitosis creates two identical daughter cells • However, if a cell is to go from 2n to n the has to be a process to create a different type of cell • Meiosis is a type of cell division that creates haploid cells in a diploid organism Meiosis • Meiosis and mitosis are very similar • They both contain the stages prophase, metaphase, anaphase and telophase • However, meiosis performs two distinct cell divisions • This means that cells that undergo meiosis will undergo PMAT twice Meiosis • During Meiosis I cells undergo prophase I, metaphase I, anaphase I and telophase I • Immediately after the end of the first cytokinesis there is a second round of PMAT • This means the cells enter prophase II, metaphase II, anaphase II and telophase II Meiosis I • We can call the first PMAT Meiosis I and the second PMAT Meiosis II • During Meiosis I there two main differences from Mitosis • All of the other steps are very similar to mitosis Meiosis I • During prophase I there is one main distinct difference • Crossing over is a period in prophase I where sections of homologous chromosomes will switch • This means the chromosomes do not remain the same during meiosis I Meiosis I • During metaphase I chromosomes do not line up directly in the center of cells • The chromosomes line up in pairs at the center of the cell • This means that during anaphase I there is no separation of centromeres Meiosis II • Meiosis II is very similar to mitosis • The steps in PMAT apply to most of the steps of PMAT II • However, remember that there is already 2 cells • The result of these steps will be 4 non identical cells