WHAT IS MEIOSIS?

  I. When Less Is More organisms can reproduce asexually (DNA from only one parent, identical) or sexually (DNA from two parents) Body cells of every species have a characteristic number of chromosomes. Human have 46 chromosomes (23 pairs) in each body cell  

A cell that contains two complete sets of chromosomes (2n) are diploid In humans, body cells are diploid (2n)  

Human sex cells (gametes) (egg/sperm) combine to produce offspring Human sex cells (gametes) (egg/sperm) combine to produce offspring. Gametes contain 23 individual chromosomes (half the number of a diploid body cell)

A cell with only one complete set of chromosomes is called haploid   Why is it important that gametes be haploid?   to keep the number of chromosomes stable from generation to generation. The fusion of haploid gametes creates offspring with diploid cells (n+n=2n)  

II. Meiosis Meiosis is a type of cellular reproduction in which the number of chromosomes is reduced by half. Cells produced by meiosis are haploid

Chromosomes Pairing   Body cells contain 46 chromosomes These 46 chromosomes are arranged in 23 pairs Each of these pairs is called a homologous pair   with one exception, each pair contains chromosomes that are exactly alike in size  

Only one pair differs (23rd pair): in females this pair is alike XX in males this pair is different XY

III. Phases of Meiosis   Before meiosis begins the cell is in interphase (resting phase), just like in mitosis the DNA is replicated during this phase, the cell is then ready to begin meiosis   Meiosis consists of two stages: Meiosis I and Meiosis II

Meiosis I   Prophase I replicated chromosomes attach to each other, homologous chromosomes align with one another to form a homologous pair *each homologous pair consists of four chromatids called a tetrad        

Meiosis I Metaphase I: homologous chromosomes line up in the middle of the cell

Meiosis I Anaphase I: homologous pairs are separated and pulled to opposite sides of the cell

Meiosis I Telophase I: Cytokinesis occurs resulting in 2 haploid cells (each chromosome still consists of two sister chromatids) which member of the homologous pair that gets into each gamete is random = independent assortment            

Meiosis II (mitotic division) Prophase II: chromosomes condense again    

Meiosis II (mitotic division) Metaphase II: chromosomes line up along the center of the cell (metaphase plate)

Meiosis II (mitotic division) Anaphase II: chromatids are separated and pulled to opposite sides of the cell  

Meiosis II (mitotic division) Telophase II cytokinesis occurs Four haploid cells are produced

Meiosis Provides Genetic Variation Two Ways:   1. Independent Assortment - Depending on how the chromosomes arrange themselves during metaphase I, the cells formed can have a variety of different chromosome combinations.    

Meiosis Provides Genetic Variation 2. Crossing Over meiosis allows for the shuffling of chromosomes through crossing over

crossing over - the exchange of genetic material between homologous pairs of chromosomes   Remember that genetic variation is important to species survival, especially in a changing environment  

Differences Between Mitosis and Meiosis  1. meiosis: produces 4 daughter cells with half the number of chromosomes (haploid) mitosis:   2. meiosis: daughter cells are not genetically identical to parent cell mitosis:   3: meiosis: cell division takes place twice mitosis:____________  

Prophase II Prophase I Metaphase I Metaphase II Anaphase II Anaphase I Telophase I Telophase II

Reflection Questions What type of cells are formed by meiosis? What type of cells are formed by Mitosis? How many daughter cells are formed from one egg during meiosis? Are they all functional? Why is it important that the number of chromosomes is reduced by half during meiosis? What is the advantage of sexual reproduction to a population whose environment changes?