2. Meiosis

3.  Human body cells have 46 chromosomes Meiosis (sexual reproduction) - General Overview Sexual Reproduction and Genetics  Each parent contributes 23 chromosomes  Takes place in sex cells (gametes): egg and sperm  Each gamete has 23 chromosomes, or HALF the normal number.  One cell becomes 4 NON-IDENTICAL cells

4. What is a chromosome? the nucleus...... is the control center...... which has the genetic material...... which is DNA (deoxyribonucleic acid) * lots of DNA when it is bunched up is called a chromosome

5. Chromosome CHROMOSOMES = LOTS OF DNA Chromosomes are only visible during cell division

6. Chromosomes and Chromosome Number Meiosis – Homologous Chromosomes Sexual Reproduction and Genetics  Same length  Same centromere position  Carry genes that control the same inherited traits  One is from mother, one is from father = a matching set

7. Meiosis I  The sexual life cycle in animals involves meiosis. Sexual Reproduction and Genetics  Meiosis produces gametes (male and female). Meiosis  When gametes combine in fertilization, the number of chromosomes is restored.

8. Meiosis I Sexual Reproduction and Genetics Meiosis  Interphase  Chromosomes replicate.  Chromatin condenses. Interphase

9. Meiosis I Sexual Reproduction and Genetics Meiosis  Prophase I  Pairing of homologous chromosomes occurs.  Each chromosome consists of two chromatids.  The nuclear membrane breaks down.  Spindles form. Prophase I

10. Meiosis I Sexual Reproduction and Genetics Meiosis  Prophase I  Crossing over produces exchange of genetic information.  Crossing over—chromosome DNA gets swapped

11. Meiosis I Sexual Reproduction and Genetics Meiosis  Metaphase I  Chromosome centromeres attach to spindle fibers.  Homologous chromosomes line up at the equator. Metaphase I

12. Meiosis I Sexual Reproduction and Genetics Meiosis  Anaphase I Anaphase I  Homologous chromosomes separate and move to opposite poles of the cell.

13. Meiosis I Sexual Reproduction and Genetics Meiosis  Telophase I  The spindles break down.  Chromosomes uncoil and form two nuclei.  The cell starts to divide – no cytokinesis! Telophase I

14. Meiosis II  Prophase II Sexual Reproduction and Genetics Meiosis  A second set of phases begins as the spindle apparatus forms and the chromosomes condense. Prophase II

15. Meiosis II  Metaphase II Sexual Reproduction and Genetics Meiosis  The chromosomes line up at the equator. Metaphase II

16. Meiosis II Sexual Reproduction and Genetics Meiosis  Anaphase II Anaphase II  The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell.

17. Sexual Reproduction and Genetics Meiosis Meiosis II  Telophase II  The chromosomes reach the poles, and the nuclear membrane and nuclei reform. Telophase II

18. Meiosis II Sexual Reproduction and Genetics  Cytokinesis results in four cells, each with half the number of chromosomes we started with. Meiosis Cytokinesis

19. The Importance of Meiosis  Meiosis consists of two sets of divisions Sexual Reproduction and Genetics  Produces four daughter cells that are not identical Meiosis  Results in genetic variation

20. Sexual Reproduction and Genetics Meiosis Provides Variation  Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.  Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine. Meiosis

21. Mitosis vs Meiosis