1. Meiosis

2. What is Meiosis and Why is it Important?
To really understand basic concepts of genetics, you need to know how hereditary traits are passed from parents to offspring.
Sexual reproduction – 2 parts:
Fertilization – genetic material from parents brought together to form new genetic identity of offspring
Meiosis – nuclear division helping to diversify genetics of the offspring
Occurs in sex cells – sperm and egg cells

3. The Basics Genes located on chromosomes in the cell nucleus
Chromosomes – each species has certain chromosome numbers per somatic (body) cells which are diploid (2n)
Cats – 38, humans – 46, goldfish – 94, corn plants – 10
In every diploid cell, each chromosome has a partner resembling each other = homologous pairs/homologues. One homologue comes from one parent and the other comes from the other parent
Diploid cells contain two complete sets of chromosomes and two complete sets of genes – one from “mom” and one from “dad”

4. The Basics (continued)
Sex cells (gametes – egg and sperm) are “special” in that they have exactly half the number of chromosomes as the somatic cells – thus, sex cells are haploid (n)
Gametes have one allele for each gene
Cats – 19, humans – 23, goldfish – 47, corn plants – 5
Why haploid?
When a male gamete (sperm) fuses with a female gamete (egg), the diploid number is restored = counterbalances effects of fertilization

5. The Phases of Meiosis There are two nuclear divisions during meiosis:
Meiosis I
Meiosis II
Meiosis forms gametes and is a process where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell – 2n to n

6. Overview of Meiosis

7. Stages in Meiosis – Meiosis I
Meiosis I – homologous chromosomes pair and separate
Interphase – chromosomes replicate
Prophase I – chromosomes condense and each chromosomes pairs with its homologue, forming a tetrad (tetrads – 4 chromosomes). Once contact is made at any point, crossing over (exchanging genetic information) can occur = VERY important for genetic diversity
Metaphase I – paired homologues line up in middle of cell
Anaphase I – paired homologues separate to opposite ends of the cell (tetrad separates)
Telophase I – two genetically different daughter cells form

8. A Closer Look at Crossing Over
Section 11-4
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9. A Closer Look at Crossing Over
Section 11-4
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10. A Closer Look at Crossing Over
Section 11-4
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11. Stages in Meiosis – Meiosis II
Meiosis II – chromatids of each homologue separate – no replication occurs, which allows for reduction in number of chromosomes
Prophase II – cell prepares for Metaphase II (spindles form, nuclear envelope disappears)
Metaphase II – chromatid pairs line up in middle of cell
Anaphase II – chromatids separate into individual chromosomes
Telophase II – 4 haploid cells form, each genetically different from one another and from the parent cell

12. 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.
Go to Section:

13. 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.
Go to Section:

14. 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.
Go to Section:

15. 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.
Go to Section:

16. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
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.
Go to Section:

17. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
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.
Go to Section:

18. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
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.
Go to Section:

19. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
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.
Go to Section:

20. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
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.
Go to Section:

21. Meiosis

22. Meiosis

23. Gamete Formation In males – haploid gametes = sperm
In females – haploid gametes = egg

24. Because of Meiosis… We can get a great deal of genetic diversity
Random Fertilization – 1 egg and 1 sperm each with such different genetic combinations
Crossing Over – Prophase I, different combinations of parts of chromosomes
Independent Assortment – what “side of cell” each chromosome is on – Mendel’s 2nd Law

25. Independent Assortment

26. Meiosis vs. Mitosis
Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells
Mitosis allows for multicellular organisms to grow
Meiosis produces gametes with many different genetic possibilities