2. Reminders, Review and Something New Chromosomes = condensed chromatin (DNA), 46 in Humans, 23 pairsChromosomes = condensed chromatin (DNA), 46 in Humans, 23 pairs Made up of 2 sister chromatids held together in the center by the centromereMade up of 2 sister chromatids held together in the center by the centromere Genes- sections of a chromosome responsible for one trait (eye color, height, etc)Genes- sections of a chromosome responsible for one trait (eye color, height, etc)

5. What is MEIOSIS? A type of cell division that results in 4 daughter cells with half as many chromosomes as the parent cells!

6. Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

7. only gametes or sex cells gametes: females-eggs and males-sperm What cells go through meiosis?

8. Why do we need meiosis? It is necessary to reduce the number of chromosomes in half in gametes (sex cells)!It is necessary to reduce the number of chromosomes in half in gametes (sex cells)! Why 1/2 the chromosomes in gametes? the male and female sex cells need to provide ½ the chromosomes each – so the offspring has 1/2 the genes from the mother and 1/2 the father!the male and female sex cells need to provide ½ the chromosomes each – so the offspring has 1/2 the genes from the mother and 1/2 the father!

9. Rules of Meiosis Chromosomes are in homologous pairs. Chromosomes carry genes. The gene forms on a pair of chromosomes may be identical Brown eyes Tall Brown eyes Blue eyes Tall Short or different.

10. Gametes (sex cells) are haploid (n) Body cells are diploid 2(n) n = number of chromosomes from each parent

11. Chromosomes Humans: 46 (23 pairs) Mosquito: 6 (3 pairs) Toad: 22 (11 pairs) Potato: 48 (24 pairs) Dog: 78 (39 pairs)

12. What are Homologous chromosomes? are paired chromosomes with genes for the same traits arranged in the same order

13. Bellringer Thu, 12/04 1.What are gametes? 2.What is a gene? 3.Name the 2 processes that meiosis does to scramble genes and create variety in the gametes. USE YOUR NOTES FROM YESTERDAY!!! REMINDERS: HW #14 due Fri, Progress Reports, LAB FEE anytime, Notebook check tomorrow!!!

14. Stages of Meiosis

16. Meiosis I Prophase I: homologous pairs (and copies) synapse or pair up. Crossing over occurs! Metaphase I: chromosomes align at the middle at the equator of the cell

17. Anaphase I: homologous chromosomes (each with 2 chromatids) move to opposite ends Telophase I: 2 nuclei form, original cell divides

18. Meiosis II No INTERPHASE or DNA copied between I and II Prophase II: chromosomes condense again and start to move toward the equator Metaphase II chromosomes line up in the middle

19. Anaphase II: chromatids separate to opposite poles Telophase II: two nuclei form - and cells divide Cytokinesis happens as in mitosis! 4 New cells with half the amount of chromosomes formed!

20. Meiosis – PMAT I and II Parent cell 4 gametes 1 st division 2 nd division

21. Results after meiosis… Sex cells (gametes) that are haploid (n) - half as many chromosomes as parent cell (2n)

22. In females meiosis results in one large egg cell and 3 polar bodies * meiosis produces only a few hundred mature eggs over a lifetime, as the baby girl develops in the womb

23. In males meiosis results in 4 sex cells * meiosis produces roughly 250,000,000 sperm per day all their adult life

24. “Putting It All Together” - Fertilization

25. A fertilized egg, called a zygote, has a diploid set of chromosomes. After fertilization, the zygote rapidly divides by mitosis and becomes an embryo.

26. 2 Key Differences Between Mitosis and Meiosis 1. Look at the Way Chromosomes Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

27. 2. Crossing-Over Chromosomes exchange genetic material between homologous pairs!

29. Chromosomes 1 to 22 are autosomes Chromosome #23 sex chromosome

30. Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

32. Boy or Girl? The Y Chromosome “Decides”

33. Meiosis – division error Chromosome pair

34. Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

35. 21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?