Meiosis – Chapter 6

1. Meiosis – Chapter 6 Dr. Bill Stafford 2017

2. How to PassThere is no secret to doing well in school.To pass any class just do the four following things: 1. Show up every day. 2. Pay attention. 3. Always do your work. 4. Behave. RETURN HOME

3. Meiosis – Chapter 6 • Mendel determined his laws and explanation of genetics without the benefit of the technology to see a cell and its DNA. • He knew nothing about how gametes are formed (meiosis) which is how genetics is explained. • He formed all his correct conclusions about genetics based on the results of his pea experiments.

4. Meiosis _ Chapter 6 Genes are a small segment of DNA found on a chromosome and it holds the recipe or code for making a protein Each chromosome contains up to 1,000 genes

5. Meiosis – Chapter 6 • All body (somatic) cells of animals and most plants have all their chromosomes in pairs – two of each chromosome – one from mama and one from daddy • A cell with two of each kind of chromosome is called a diploid – the organism gets one from its mother and one from its father • A diploid cell is said to be 2n which explains Mendel’s conclusion

6. Meiosis – Chapter 6 • Gametes are the reproductive cells of organisms • Female gametes are known as eggs or ovules • Male gametes are known as sperm or pollen • Fusion of a sperm and egg (fertilization) produces a diploid zygote • Gametes have only one of each kind of chromosome and are referred to as haploid or n • The only haploid cells are the gametes • They are produced by meiosis

7. Meiosis – Chapter 6 • Different kinds of chromosomes • Autosomes – chromosomes 1-22 that are not directly related to the sex of the organism • Sex chromosomes – 23rd pair – XX in female and XY in male – determines the sex of the organism • X is larger than Y and contains more genes • A Y chromosome makes the individual a male

8. Meiosis – Chapter 6 • Homologous chromosomes – the two chromosomes of each pair in a diploid cell • One comes from the mother’s egg and the other comes from the father’s sperm • Each homologous chromosome has the genes for the same trait in the same location – could be a different allele

9. Meiosis – Chapter 6 • Homologous chromosome (cont.) • If the genes are the same allele for a particular trait on both homologous chromosomes, the organism is homozygous for that trait • If the genes are different alleles for a particular trait on each homologous chromosome, the organism is heterozygous for that trait

10. Meiosis – Chapter 6 The Big Picture Meiosis is a way to produce haploid gametes from diploid parents This allows fertilization from two diploid parents the capability to produce a diploid offspring The process of meiosis produces genetic recombination which results in a change and variety in the offspring. Genetic variability is the raw material necessary for evolution.

11. Meiosis – Note quiz What is a gene and where is it found? Which cells in our body are diploid? What are gametes? Which cells are haploid and how are they produced?

12. Meiosis – Note Quiz 5. What are homologous chromosomes? Where does each one of the pair come from? 6. What is the purpose of meiosis?

13. Meiosis – Chapter 6 • Why meiosis? • In sexual reproduction, two gametes fuse (combine) together to form a zygote which is diploid • The zygote grows and develops into a complete individual by the process of mitosis • The only way for two cells to combine and fuse to form one diploid cell is for the two gametes to be haploid (n) • Meiosis is the cell process that creates haploid gametes to participate in sexual fertilization

14. Meiosis – Chapter 6 • The process of meiosis: • Meiosis is much like mitosis in the use of a spindle and separation of the nucleus and cytoplasm • However, what happens to the chromosomes in meiosis is much different, especially during meiosis I • Meiosis II is like mitosis

15. Meiosis – Chapter 6 • Process of meiosis (cont.) • Interphase – chromosomes replicated during S-phase – sister chromatids • Prophase I – • Chromatin condenses to form visible chromosomes • Homologous chromosomes come together to form a tetrad (four) – two homologous chromosomes, each made up of two sister chromatids • Crossing over occurs – nonsister homologous chromosomes exchange genetic material • Happens 2-3 times for each homologous pair

16. Meiosis • Prior to division, amount of DNA doubles

17. Meiosis – Chapter 6 • Process of meiosis (cont.) • Metaphase I – The tetrads of homologous chromosomes with their sister chromatids are lined up in the middle of the cell by the spindle fibers – in mitosis the homologous chromosomes line up independently of each other during metaphase • Anaphase I – homologous chromosomes separate and go to opposite ends of the cell – sister chromatids are still attached at their centromeres

18. Meiosis – Chapter 6 • Process of meiosis (cont.) • Telophase I – happens as it did in mitosis except now theer is only one of each homologous chromosome still attached to their exact copy at the centromere – sister chromatid • Another cell division (meiosis II) is required to separate the sister chromatids • A short interphase occurs between meiosis I and meiosis II but the DNA is not copied • Some cells divide between meiosis I and meiosis II, and some don’t

19. Figure 13.8a Telophase I andCytokinesis Anaphase I Metaphase I Prophase I Centrosome(with centriole pair) Sister chromatidsremain attached Chiasmata Sisterchromatids Centromere(with kinetochore) Spindle Metaphaseplate Cleavagefurrow Homologouschromosomesseparate Fragmentsof nuclearenvelope Homologouschromosomes Microtubuleattached tokinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosomestill consists of two sister chromatids. Chromosomes line upby homologous pairs. Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.

20. Meiosis – Chapter 6 • Process of meiosis (cont.) • Meiosis II – identical to mitosis – sister chromatids are lined up and separated • Result of meiosis is four new cells that are haploid (n) and different genetically from the parent cell and each other • The resulting cells of meiosis are the gametes (sperm and egg) that will participate in sexual fertilization

21. Figure 13.8b Telophase II andCytokinesis Metaphase II Prophase II Anaphase II During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes. Haploid daughtercells forming Sister chromatidsseparate

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29. Meiosis – Chapter 6 • Genetic variation – meiosis unlike mitosis results in cells that have different DNA than each other or the parent cells • The chromosomes are shuffled randomly and result in offspring different from the parents or siblings.

30. Meiosis – Chapter 6 • Genetic recombination – results in different DNA than you started with. • The total number of possible gametes (sperm or eggs) produced is 2 to the power of the number of different types of chromosomes an organism has. In humans 223 different gametes can be produced which is more than 8 million different possible eggs or sperm.

31. Meiosis – Chapter 6 • Genetic recombination (cont.) • Then the sperm and eggs combine at random to give 8 million x 8 million which = 70 trillion different possible genetic (DNA) combinations in the offspring from one pair of parents. • In addition, crossing over in prophase I increases the number of possible variations in the offspring’s DNA • All of this genetic recombination leads to genetic variation in the offspring

32. Meiosis – Chapter 6 • Genetic recombination (cont.) • This variation in the offspring of sexual reproduction serves as the raw material for evolution. • This variation allows for options to choose from if the environment of an organism changes. The organism will select for those variations that give it the best chance to survive in the new environment.

33. Meiosis – Chapter 6 • Mistakes in meiosis • Nondisjunction – the failure of homologous chromosomes to separate properly during meiosis I – results in too many chromosomes in one gamete and not enough in another gamete • Trisomy – “tri means three” - a condition where the offspring has three of one of the chromosomes – Downs syndrome kids have three of chromosome number 21 • Monosomy – “mono means one” – a condition where the offspring has only one of a certain chromosome – usually results in the death of the baby

34. Meiosis – Chapter 6 • Mistakes in meiosis (cont.) • Triploidy – “tri means three” – a condition where a whole set of homologous chromosomes fails to separate in meiosis – results in a gamete with a full pair of all homologous chromosomes – when it fuses with a normal gamete in fertilization, the offspring will have three sets of chromosomes • Polyploidy – any condition with more than two sets of homologous chromosomes in the baby – could be triploidy or tetraploidy

35. Meiosis – Chapter 6 • Mistakes in meiosis (cont.) • Sometimes polyploidy is good • Wheat and strawberries are tetraploid (4n) • Apples are triploid (3n) • Plant engineers use chemicals to cause nondisjunction as these polyploidy plants are better at producing fruit or food.