Cell Divison

 Each of your body cells has 46 chromosomes, or 23 pairs.  Each pair is made of HOMOLOGOUS CHROMOSOMES Homo = same These chromosomes contain the same genes in the same order

 Half of your chromosomes (23) came from your mom  Half of your chromosomes (23) came from your dad Each parent gave you one chromosome from a homologous pair.

gene From Mom From Dad From Mom From Dad

 When your BODY cells (autosomal) divide, they make exact copies  Results = two cells with the same type and number of chromosomes as the parent cell.  Many animals reproduce ASEXUALLY – through mitosis ex: flatworms, sponges, jellyfish

If humans were created through mitosis: 1 st generation 46 from mom + 46 from dad = 92 chromosomes for baby then 2 nd generation 92 from mom + 92 from dad = 184 chromosomes for baby And so on, and so on, and so on…

 A second type of cell division that occurs in the SEX CELLS (gametes: sperm and egg)  Division creates four cells with half the number of chromosomes as the parent cell.

 Body cells (autosomal cells) are DIPLOID  DI = 2; Each cell has 2 copies of each chromosome  These are considered “2n” ◦ “n” is the chromosome number Sex cells are HAPLOID Each sex cell has HALF the normal number of chromosomes These are considered “n”

 Sperm are the sex cells in males  Eggs are the sex cells in females Both are produced through MEIOSIS Any reproduction that involves SPERM and EGGS is considered SEXUAL REPRODUCTION.

 Prophase I  Metaphase I  Anaphase I  Telophase I  Prophase II  Metaphase II  Anaphase II  Telophase II First Division Second Division **Meiosis contains TWO divisions whereas Mitosis only has one! ** Similar to Mitosis…Meiosis does experience interphase. **Chromosomes are replicated during “S” phase.

Prophase I  Chromatin coil and condense, chromosomes are visible.  Spindle form  Homologous chromosomes line up gene by gene to form a tetrad. ◦ Tetrad: two homologous chromosomes made of two sister chromatids paired tightly ◦ Crossing over occurs.

 Occurs between two non-sister chromatids.  Can happen many times or not at all  Average: 2 to 3 crosses per each pair of homologues.  Results in genetic variation ◦ Explains why siblings look alike but not identical.

Interphase: All 23 pairs of chromosomes duplicate. Prophase begins.

 Metaphase I ◦ Spindle fibers attach to centromere of each chromosome. ◦ Tetrads pulled to the center ◦ Line up beside each other  In Mitosis they line up end to end…not paired.

Meiosis: Chromosome line up beside their homologous pairs. Mitosis: Chromosomes line up end to end

Anaphase I  Homologous chromosomes separate ◦ Head to opposite sides of cell  Centromeres do not split ◦ Unlike mitosis  Ensures that each new cell will receive one chromosome from each homologous pair

Meiosis: Sister Chromatids stay together Mitosis: Sister Chromatids separate

Telophase I  Prophase Reversed  Cytokenisis Occurs  Still has one chromosome from each homologous pair  Start the division process over again!

Meiosis: Still have one chromosome from each homologous pair **need to separate them in order for each sex cell to have ½ of the genetic material.

Prophase II  Chromosomes Condense  Spindle Form and attach to centromere ◦ NOTICE….did not undergo inerphase therefore the chromosomes did not replicate again! Metaphase II ◦ Chromosomes line up at the equator  End to end this time….like mitosis Anaphase II ◦ Sister chromatids split apart and move to opposite poles Telophase II ◦ Nuclei reform ◦ Spindle Breaks down Cytokenisis

Meiosis I Meiosis II

 Four haploid cells  Contains 1 chromosome of each kind 2n n n n n n n n n

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