1. Chromosomes and Meiosis

2. Recall, DNA on each chromatid is Sectioned into many segments called Genes: govern the expression of a particular trait

3. Diversity of chromosomes among organisms The number and length of chromosomes differ among organisms There is no relationship between size of an organism and number of chromosomes

4. Numbers of Pairs of Chromosomes in Some Plant and Animal Species Common nameSpecies# chrom. Pairs MosquitoCulex pipiens3 HouseflyMusca domestica6 ToadBufo americanus11 RiceOryza sativa12 FrogRana pipiens13 AlligatorAlligator mississippienis16 Rhesus monkeyMacaca mulatta21 WheatTriticum aestivum21 HumanHomo sapiens23 PotatoSolanum tuberosum24 DonkeyEquus asinus31 HorseEquus caballus32 DogCanis famliaris39 CarpCyprinus carpio52

5. Most animal species contain two types of cells: 1. Somatic/Body Cells –Contain two sets of chromosomes written as 2n (  called diploid cells) 2. Gamete/Sex Cells –Contain one set of chromosomes written as n (  called haploid cells)

6. (I.e. Humans) –Somatic Cells = 2n (46 chromosomes) –Sperm/egg Cells = n (23 Chromosomes)

7. Humans have 46 chromosomes or 23 pairs of chromosomes [ 2(23) ] 22 pairs = autosome chromosomes –not involved in sex determination 1 pair = sex chromosomes –responsible for sex determination

8. Why do we need meiosis? If all cells including sex cells/gametes were diploid (2n) then…. 2n (male) 2n (female) 4n (offspring) ?

9. Meiosis: Occurs in reproductive organs Produces gametes (haploid cells) to maintain the diploid number in offspring Involves 2 sequences of phases –each similar to mitosis –Called meiosis I and meiosis II

10. Meiosis I: DNA replication –(Same as mitosis)

11. Homologous chromosomes –Identical in size and appearance –Corresponding genes may differ in form One chromosome comes from the female gamete and one from the male gamete

12. Homologous chromosomes pair to form homologous pairs –All 4 chromatids are called a Tetrad

13. One spindle from one pole attaches to one chromosome One spindle from the other pole attaches to the other Tetrads are pulled to the equator of the cell –Not in single file

14. Homologous chromosomes separate and move to opposite poles of the cell –Centromere does not split – holds sister chromatids together

15. The chromosomes gather into nuclei, and the original cell divides

16. Brief interphase: –No DNA replication

17. Results of Meiosis I: Reduction division –Chromosome number is reduced by half 2n  n Each daugher cell going into Meiosis II is haploid (n)

18. Meiosis II (same as mitosis) (see notes from last class)

19. Same as mitosis

22. RESULT FROM MEIOSIS II: 4 Haploid daughter cells Containing single unreplicated chromosomes Gamete cells (n)

23. P.162

24. Recap: 2n2(23) = 46 chromosomes Meiosis I n (23) = 23 chromosomes (each with 2 sister chromatids) Meiosis II n(23) = 23 single chromosomes