MEIOSIS INTRODUCTION

 What is a chromosome?  A piece of coiled DNA  How many chromosomes do human cells have?  46 (in non-sex cells)  Do ALL humans have the same number of chromosomes in their cells?  YES! (Except people with genetic disorders causing extra chromosomes or missing chromosomes)  Where did the 46 chromosomes come from? CHROMOSOMES

 If we get our chromosomes from our parent, why do we not have a total of 92 (46 from mom and 46 from dad)?  How does each parent manage to only give 23 chromosomes to their child? TURN TO A FRIEND AND DISCUSS…

 YOU were made from one cell from your mom and one cell from your dad. So, how is it the same? Shouldn’t you have double the number of chromosomes?  No, the number is the same because the egg and sperm cells only have half the number of chromosomes, so when they combine they have the full 46.  SO, sex cells have how many chromosomes?  23 chromosomes SO…

 How are gametes (sex cells) formed, if they only have 23 chromosomes?  Through a process called MEIOSIS  Meiosis is similar to mitosis but has some differences. SEX CELLS -- GAMETES

MEIOSIS  Living things inherit a single copy of each gene from each of their parents. The genes are on chromosomes  Homologous chromosomes from the male parent correspond(pair) to the chromosomes from the female parent.

HAPLOID Gametes are haploid(n)- they contain only a single set of chromosomes. Humans have 23 chromosomes in our gametes.

DIPLOID When gametes join, a diploid(2n) zygote is formed. It contains two copies of each chromosome. A human zygote has 46 chromosomes

MEIOSIS SUMMARIZED  The process of nuclear division where the number of the chromosomes is halved.  Produces four daughter cells  All daughter cells are haploid  Chromosomes are shuffled in process called CROSSING OVER so that each daughter cell has a unique combination used to create gametes (sperm and egg)

CROSSING-OVER  Crossing Over- when homologous chromosomes are paired up during Prophase I they can exchange genetic information leading to diversity.

 There are two stages of Meiosis.  Meiosis I results in two haploid (N) daughter cells.  Each with half the number of chromosomes as the original cell.

MEIOSIS II  The two cells produced by meiosis I enter a second meiotic division, meiosis II.  Unlike meiosis I, neither cell goes through a round of chromosome replication before entering meiosis II.  Each of the cell’s chromosomes has 2 chromatids.

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 Mitosis results in…  Two IDENTICAL daughter cells—no genetic diversity  same number of chromosomes as parent  Meiosis results in…  FOUR daughter cells—that are different from parent cells, resulting in genetic diversity  Chromosome number half that of the parent cell. MITOSIS VS. MEIOSIS—PRIMARY DIFFERENCE