Understanding Meiosis and Sexual Reproduction Process

1. MeiosisOverview: sexual reproduction requires special cells made by meiosis. • Asexual reproduction occurs when a single parent produces identical offspring. • Sexual reproduction produces offspring from the fusion of two reproductive cells (gametes: eggs and sperms) in a process called fertilization. • Some species (particularly among plants) can use both methods of reproduction.

2. Chromosome number-Ploidy levels Diploid: two sets of chromosomes • Humans- 46 Haploid : one set of chromosomes; eggs and sperms Polyploidy: more than two complete sets of chromosomes

3. Sexual Reproduction and Meiosis DIPLOID CELLS have two copies of each chromosome ADULT FEMALE • 23 chromosome pairs (46 chromosomes total) ADULT MALE • 23 chromosome pairs (46 chromosomes total) • Meiosis • Meiosis enables organisms to produce haploid gametes. • It produces gametes that differ from one another with respect to the combinations of alleles they carry and brings about genetic variability MEIOSIS HAPLOID CELLS have one copy of each chromosome EGG (female gamete) • 23 chromosomes total SPERM (male gamete) • 23 chromosomes total FERTILIZATION DIPLOID CELLS have two copies of each chromosome FERTILIZED EGG • 23 chromosome pairs (46 chromosomes total) MITOSIS Fertilization = fusion of haploid egg and haploid sperm to form the single cell called zygote OFFSPRING • 23 chromosome pairs (46 chromosomes total)

4. “Reduction Division” 1 diploid (2n) cell  4 haploid (1n) cells involves 2 nuclear divisions (meiosis I & meiosis II) occurs in gonads (reproductive organs) What is meiosis?

5. Life Cycle of an Animal http://www.biosci.uga.edu/almanac/bio_103/notes/apr_4.html.

6. Meiosis II Meiosis I Meiosis: consists of meiosis I and meiosis II • Two nuclear divisions (that result in 4 daughter cells) 46

7. Sperm and egg are produced by meiosis: the details, step by step. Maternal chromosome Paternal chromosome HOMOLOGUES The maternal and paternal copies of a chromosome • Meiosis only takes place in the gonads. • Meiosis begins with a cell containing 46 chromosomes: • Maternal and paternal copy of each chromosome (homologues) Replication SISTER CHROMATIDS The two identical copies of a chromosome created during replication CENTROMERE The point at which two sister chromatids are held together

8. Homologous pair of chromosomes or homologues Centromere the maternal and paternal copies of a chromosome Sister chromatids

9. Meiosis reduces the genome by half. INTERPHASE Each chromosome in a homologous pair replicates to form two sister chromatids. (Chromosomes shown condensed here for diagrammatic purposes.) Diploid parent cell Homologues Maternal chromosome Paternal chromosome Sister chromatids MEIOSIS I In the first division of meiosis, the homologous pairs separate. MEIOSIS II In the second division of meiosis, the sister chromatids separate. Haploid daughter cells (gametes) The products of meiosis are four haploid cells, each containing just one copy of each chromosome, rather than a homologous pair.

10. 19 39 78 156 0 Skin cells from a dog have a TOTAL of 78 chromosomes. How many TOTAL chromosomes do each sperm cell from the dog have?

11. Meiosis in detail • Meiosis I • Prophase I • Metaphase I • Anaphase I • Telophase I --Cytokinesis happens here • Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II • Cytokinesis happens again

12. Meiosis I: Reduction Division 1. Prophase I • replicated chromosomes condense • spindle fibers forms • nuclear membrane breaks down • Homologous chromosomes pair • crossing over occurs

13. Crossing over and meiosis are important sources of variation. • Crossing over (genetic recombination) occurs when homologous chromosomes swap genetic information. • Chiasmata are the points along the homologues where the genetic information was exchanged. Each of these 4 chromatids gets packaged into a haploid gamete (sperm or egg cell). HOMOLOGOUS CHROMOSOMES Maternal copy Paternal copy Crossing over between the sister chromatids of the homologous chromosomes Homologous chromosomes after the exchange of genetic information Chromatids with recombined DNA Sister chromatids Crossing over doesn’t create new alleles but it does create new combinations of alleles on a chromatid.

14. The image represents ___ • A pair of sister chromatids • A pair of unduplicated chromosome • Duplicated homologous chromosomes • All of the above

15. Metaphase I • pairedhomologous chromosomes line up along cell equator

16. Anaphase I • homologous chromosomes separate & move to opposite poles

17. Telophase I • spindle fibers breaks down • chromosomes become long and thin • nuclear membranes reform Cytokinesis occurs between meiosis I & II

18. Meiosis II (equational division) • Prophase II • chromosomes condense • spindle fibers forms • nuclear membrane breaks down • Metaphase II • chromosomes line up along equator of cell • Fibers attach to chromatids

19. Meiosis II (equational division) • Anaphase II • sister chromatids (now called chromosomes) separate & move toward opposite poles • Telophase II • spindle fibers break down • chromosomes de-condense • nuclear membranes reform Cytokinesis divides two cells into four nonidentical cells.

20. Meiosis Division 1: Homologues Separate (Stages 1−3) Piece of maternal chromatid now on paternal chromatid following crossing over Spindle fiber Homologues Replicated chromosome Centromere Crossing over among the sister chromatids generates variation. Random assortment of the maternal and paternal sister chromatids at the metaphase plate generates variation. Spindle PROPHASE I • Replicated chromosomes condense. • Spindle is formed. • Homologous pairs of sister chromatids come together and cross over. • Nuclear membrane disintegrates. ANAPHASE I • Homologues separate and are pulled to opposite poles. Sister chromatids going to each side are a mix of maternal and paternal genetic material. 1 2 3 METAPHASE I • Homologues move toward the center of the cell (at this point, referred to as the metaphase plate) and line up. Nuclear membrane INTERPHASE • Chromosomes (uncondensed in this phase) replicate in preparation for meiosis.

21. Meiosis II: Separating the Sister Chromatids Daughter cell 1 Daughter cell 1 Daughter cell 2 Sister chromatids Daughter cell 3 Daughter cell 2 4 TELOPHASE I AND CYTOKINESIS • Sister chromatids arrive at the cell poles, and the nuclear membrane reassembles around them. • The cell pinches into two daughter cells. • Chromosomes may unwind slightly. Daughter cell 4 TELOPHASE II AND CYTOKINESIS • The nuclear membrane reassembles around the chromosomes. • The two daughter cells pinch into four haploid daughter cells. 5 6 7 8 ANAPHASE II • Sister chromatids are pulled apart by the spindle fibers toward opposite cell poles. PROPHASE II • Chromosomes in daughter cells condense. • Spindle forms. METAPHASE II • Sister chromatid pairs line up at the center of the cell. There is a brief interphase prior to prophase II. Chromosomes are not replicated again at this stage. http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

22. 0 Which of the following is most like mitosis? • Meiosis I • Meiosis II

23. Male and female gametes are produced in slightly different ways. Diploid female cell MEIOSIS I (Telophase II) REPLICATION MEIOSIS I (Telophase I) Polar body Polar body Genetic material is divided evenly, but nearly all of the cytoplasm goes to just one of the cells. Functional female gamete (haploid cell) As in the first division, one cell gets nearly all of the cytoplasm. The net result is one large egg and smaller cells (called polar bodies) that degrade almost immediately. Females produce the larger gamete. Males produce the smaller, more motile gamete.

24. Sources of genetic variation There are multiple reasons why offspring are genetically different from their parents and from one another. ALLELES COME FROM TWO PARENTS Each parent donates his or her own set of genetic material. CROSSING OVER Crossing over during meiosis produces a mixture of maternal and paternal genetic material on each chromatid. REASSORTMENT OF HOMOLOGUES The homologues and sister chromatids distributed to each daughter cell during meiosis are a random mix of maternal and paternal genetic material.

25. Random alignment of homologous chromosomes (reassortment of homologs)

26. Random alignment of chromosomes contributes to the genetic diversity of gametes Possibility2 Possibility1 Possibility3 Possibility4

27. Crossing over A. creates new alleles (genes). B. creates new combinations of alleles. C. creates new chromosomes. D. recombines alleles from non-homologous chromosomes.

28. Meiosis Summary • Occurs in _____ cells • Produces ___________ daughter cells • Gametes (sperm and egg) are made by meiosis • Increases genetic variability