1. Meiosis Nestor T. Hilvano, M.D., M.P.H. (Images Copyright Discover Biology, 5 th ed., Singh-Cundy and Cain, Textbook, 2012.)

2. Learning Objectives 1.Distinguish between autosomes and sex chromosomes; somatic cells and gametes; diploid cells and haploid cells. 2.Define and describe the events of meiosis. 3.Describe key differences between mitosis and meiosis. 4.Define karyotyping and give its importance. 5.Describe the causes and symptoms of Down Syndrome

3. Inheritance of Genes ___ is the scientific study of heredity and variation ___ is the transmission of traits from one generation to the next ___ are the units of heredity, and are made up of segments of DNA a. Heredity b. Genes c. Genetics d. Genomes Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into chromosomes One set of chromosomes is inherited from each parent

4. Terminologies Somatic cells – body cells; in human have 23 pairs of chromosomes (diploid chromosomes) (2n) (2 sets) Homologous chromosomes – pair (2 sets) of chromosomes that carry genes.

5. Gametes – have 23 chromosomes (haploid) (1n) (1 set); 22 autosomes +X or 22 autosomes + Y The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome (XY) The 22 pairs of chromosomes that do not determine sex are called autosomes Sexual life cycles involve the alternation of haploid and diploid stages The fusion of haploid gametes in the process of fertilization results in the formation of a diploid (2n) zygote (fertilized egg= new child)

6. http://ghr.nlm.nih.gov/handbook/illustrations/normalkaryotype

7. Fig. 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSISFERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

8. Meiosis Reduces the chromosome number from diploid to haploid. But divide twice to form 4 daughter cells (haploid = gametes). Cross-over= homologous chromosomes swap to increase genetic diversity Consists of: Meiosis I – homologous chromosomes separate, results in 2 haploid daughter cells with replicated chromosomes Meiosis II – sister chromatids separate, results in 4 haploid chromosomes

9. Figure 8.12B A pair of homologous chromosomes in a diploid parent cell A pair of duplicated homologous chromosomes Sister chromatids 1 2 3 I NTERPHASE M EIOSIS I M EIOSIS II

10. Figure 8.11 Pair of homologous chromosomes Locus Centromere Sister chromatids One duplicated chromosome

11. Meiosis I Prophase I – chromatin coils up; homologous pairing (tetrad; with 4 chromatids); crossing over; nucleoli disappear; nuclear envelop breaks; spindle forming Metaphase I – chromosomes tetrads aligned on the metaphase plate Anaphase I – migration of chromosomes toward the 2 poles; sister chromatids remain attached Telophase I and cytokinesis – 2 haploid chromosome; cleavage furrow

12. Meiosis II Prophase II – spindle forms and moves chromosomes toward the middle of cell. Metaphase II – chromosomes are aligned on the metaphase plate Anaphase II – sister chromatids separate Telophase II and cytokinesis – nuclei form; cytokinesis occur; 4 haploid daughter cells (single chromosome)

14. Comparison between mitosis and meiosis Mitosis – results to 2 daughter cells, each with the same chromosomes as the parent cell. Meiosis – results to 4 daughter cells, each with half the number of chromosomes as the parent cell.

15. Figure 8.14 Prophase Metaphase Duplicated chromosome (two sister chromatids) M ITOSIS Parent cell (before chromosome duplication) Chromosome duplication Chromosome duplication Site of crossing over 2n  4 Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Tetrad formed by synapsis of homologous chromosomes Metaphase I Prophase I M EIOSIS I Anaphase Telophase Sister chromatids separate during anaphase 2n2n 2n2n Daughter cells of mitosis No further chromosomal duplication; sister chromatids separate during anaphase II nnnn Daughter cells of meiosis II Daughter cells of meiosis I Haploid n  2 Anaphase I Telophase I Homologous chromosomes separate during anaphase I ; sister chromatids remain together M EIOSIS II

16. Genetic variations could be due to: Random orientation of homologous chromosome pairs at metaphase of meiosis I Crossing over creates recombinant chromosomes

19. Karyotype Ordered display of chromosomes in which the : - genetic sex of an individual can be determined. - abnormalities in chromosomal structure and number can be detected.

20. Figure 8.19A_1 Trisomy 21 Trisomy 21 – Down syndrome Results from non- disjunction in meiosis Increase risk with maternal age Down syndrome

21. Figure 8.20B_s3 Normal meiosis I M EIOSIS I M EIOSIS II Nondisjunction Abnormal gametesNormal gametes n  1n  1 nn

22. Homework 1. Define – meiosis, haploid chromosome, diploid chromosome, gametes, somatic cell, autosomes, sex chromosomes, Down syndrome, karyotype, genetics, heredity, genes 2. Describe the events that characterize each phase of meiosis (I & II) 3. Differentiate mitosis and meiosis as to # of cell division, # of daughter cells, # of chromosomes in daughter cells, genetic relationship of daughter cells to parent cell, and functions performed in the human body. 4. Compare sexual and asexual reproduction.