Cell Cycle There are two types of Cell Division •  1. Mitosis-­‐ occurs in soma;c (body) cells •  2. Meiosis-­‐ cell division which reduces the chromosome number by half. Where does Meiosis occur? •  Meiosis occurs in human sex cells which are also called gametes (eggs and sperm) Chromosomes •  Cells with the full number of chromosomes (46 in humans) are called diploid. •  Cells with half the number of chromosomes (23 in humans) are called haploid •  Think half-­‐loid •  “Di” means 2, or 2x as much Chromosomes •  Humans have 46 chromosomes in their soma;c (body cells) •  Sex cells however, have only 23 chromosomes •  Egg and sperm cells are considered haploid A sex cell would only have one of each chromosome. Quick Check •  Why do sperm and egg cells only have half the number of chromosomes? Quick Check •  Why do sperm and egg cells only have half the number of chromosomes? •  During fer;liza;on an egg and sperm cell need to combine to form a full 46 chromosome cell that will later develop into an embryo. •  23 chromosomes from mom and 23 from dad=46 Phases Meiosis • 
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Prophase Metaphase Anaphase Telephase •  Unlike mitosis, meiosis goes through these phases twice before dividing Meiosis •  ASer meiosis 1, the cell divides into 2 daughter cells •  ASer meiosis 2, those two cells divide into 2 •  At the end of meiosis, you have a total of 4 haploid cells Crossing Over •  Some interes;ng things can occur will the chromosomes are divided up in the cell •  When two chromosomes are close enough they “swap” part of their genes Sex Chromosomes •  All humans have two sex chromosomes (one from mom and one from dad) •  X is the female chromosome •  Y is the male chromosome Sex Chromosomes •  Women typically have two X chromosomes (XX) •  Men typically have an X and Y chromosome (XY) •  Everyone inherits an X chromosome from their mother. Which ever chromosome their father’s sperm cell has determines the sex of the child. Sex Chromosomes •  Since men have on X and one Y, there is a 50/50 chance the child will be male or female •  This is what ensures we have about the same number of males and females on earth (cri;cal for survival).