WHAT IS MEIOSIS?

 • I. Organisms can reproduce asexually (DNA from only one parent, identical)

or sexually (DNA from two parents)

Asexual Sexual

 

• 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 diploidIn humans, body cells are diploid (2n) 

• 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. MeiosisMeiosis 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 chromosomesThese 46 cells are arranged in 23 pairsEach of these pairs is called a homologous pair with one exception, each pair contains chromosomes that are exactly alike in size 

Homologous Pair

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

• III. Phases of Meiosis Before meiosis begin 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 otherhomologous chromosomes align with one another to form a homologous pair

*each homologous pair consists of four chromotids 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 an pulled to opposite sides of the cell

Meiosis I• Telophase I:

Cytokinesis occurs resulting in 2 haploid cells(each chromosomes still consists of two sister chromatids) (which member of the homologous pair that gets into each gamete is random= random 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 IIcytokinesis occurs

Four haploid cells are produced

Meiosis Provides GeneticVariation Two Ways:

 1. Random Assortment of chromosomes 

 

Meiosis Provides Genetic Variation

• 2. Crossing Overmeiosis 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 

Mitosis vs. Meiosis

Differences Between Mitosis and Meiosis

 1. meiosis: produces 4 daughter cells with half the number of chromosomes (haploid)mitosis: produces 2 daughter cells with the same number of chromosomes as the parent cell. 2. meiosis: daughter cells are not genetically identical to parent cellmitosis: daughter cells are genetically identical to the parent cells 

3: meiosis: cell division takes place twicemitosis: cell division takes place once.