1. Sexual reproduction
How many chromosomes do we have in body cells?
How many chromosomes do we have in sex cells?
If your parents have 46 chromosomes in their cells, do you have 92 chromosomes in your cells? Why?

2. Review
Hereditary information is contained in genes, located in the chromosomes of each cell.

3. Diploid cells
Chromosomes come from both the male parent and female parent
Homologous: each of the chromosomes coming from one parent have corresponding chromosomes from the other parent
Diploid: a cell that contains both sets of homologous chromosomes = “two sets” (2N)
Diploid cells contain two complete sets of chromosomes and two complete sets of genes
Human somatic (body) cells have 46 chromosomes or 23 homologous pairs

4. Haploid cells
Haploid: contain only one set of chromosomes and one set of genes = “one set” (N)
Gametes of sexually reproducing organisms are haploid containing one complete sets of chromosomes and one complete sets of genes
Human gametes (sex cells) have
23 chromosomes
Sperm (23) + Egg (23) = Zygote (46)
How are haploid gamete
produced from diploid cells?

5. Because of MEIOSIS this doubling does not occur and the ZYGOTE (fertilized egg) has the same number of chromosomes as the parent. HOW DOES THIS OCCUR ?

6. Meiosis
The process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

7. Meiosis Meiosis consists of 2 phases: Meiosis I Meiosis II
To begin, you have one diploid cell
In the end, you have four haploid cells
Meiosis

8. Meiosis I Cells begin to divide in a way that looks similar to mitosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad – there are 4 chromatids in a tetrad
The homologous pairs may exchange portions of their chromatids in a process called crossing-over which results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

9. Meiosis I Homologous chromosomes separate and two new cells are formed
Each new cell has equal numbers of chromatids but neither have the same complete set of chromosomes or alleles as each other or the original diploid cell

10. Figure 11-15 Meiosis Meiosis I Interphase I Section 11-4 Prophase I
Metaphase I
Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
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11. Meiosis II The two cells enter a second meiotic division
Neither cell goes through chromosomal replication, so each chromosome has 2 chromatids
Pairs of chromatids separate
Each daughter cell produced receives equal amounts of chromatids
The four daughter cells contain haploid (N) chromosomes

12. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II
Metaphase II
Anaphase II
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
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13. Gamete formation Male: sperm (4 equal sized gametes produced)
Female: egg/polar bodies (1 egg cell produced which contains most of the cytoplasm, and 3 polar bodies which are usually not involved in reproduction)

14. Mitosis vs. Meiosis
Mitosis – produces 2 genetically identical diploid cells; ONE division; autosomes (body cells); chromosome # maintained; asexual reprod
Meiosis – produces 4 genetically different haploid cells; TWO divisions; gametes (sex cells); chromosome # halved; sexual reprod

15. MITOSIS VS. MEIOSIS MITOSIS:
THE PROCESS BY WHICH A DIPLOID CELL(2n) PRODUCES TWO DIPLOID (2n) CELLS
INCLUDES ONE CELL DIVISION
ASEXUAL REPRODUCTION
MEIOSIS:
THE PROCESS BY WHICH A DIPLOID CELL (2n) PRODUCES FOUR HAPLOID (1n) CELLS (GAMETES)
INCLUDES TWO CELL DIVISIONS
SEXUAL REPRODUCTION
Compare mitosis and meiosis on the graphic organizer with your seat partner…