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Meiosis and Sexual Reproduction Chapter 7. 7.1 Meiosis In some organisms haploid gametes join to form a diploid zygote In some organisms haploid gametes.

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Presentation on theme: "Meiosis and Sexual Reproduction Chapter 7. 7.1 Meiosis In some organisms haploid gametes join to form a diploid zygote In some organisms haploid gametes."— Presentation transcript:

1 Meiosis and Sexual Reproduction Chapter 7

2 7.1 Meiosis In some organisms haploid gametes join to form a diploid zygote In some organisms haploid gametes join to form a diploid zygote If gametes werent haploid imagine how the number of chromosomes would increase of time If gametes werent haploid imagine how the number of chromosomes would increase of time Meiosis is the form of cell division that makes diploid cells haploid when forming gametes Meiosis is the form of cell division that makes diploid cells haploid when forming gametes There are 2 divisions in Meiosis – Meiosis I and Meiosis II There are 2 divisions in Meiosis – Meiosis I and Meiosis II Before Meiosis begins DNA is replicated so cell has homologous chromosomes Before Meiosis begins DNA is replicated so cell has homologous chromosomes

3 Prophase I Chromosomes condense Chromosomes condense Nuclear envelope breaks down Nuclear envelope breaks down Homologous chromosomes pair up Homologous chromosomes pair up Crossing over occurs Crossing over occurs Chromatids exchange Chromatids exchange

4 Metaphase I Homologous pairs move to equator by spindle Homologous pairs move to equator by spindle

5 Anaphase I Homologous pairs separate and pulled towards poles Homologous pairs separate and pulled towards poles Chromatids do not separate; still connected by centromeres Chromatids do not separate; still connected by centromeres

6 Telophase I Individual chromosomes gather at poles Individual chromosomes gather at poles Most organisms now have cytokinesis Most organisms now have cytokinesis 2 new cells formed 2 new cells formed Chromosomes do not replicate again Chromosomes do not replicate again

7 Prophase II New spindle forms New spindle forms

8 Metaphase II Chromosomes line up along equator and are still attached at centromeres Chromosomes line up along equator and are still attached at centromeres

9 Anaphase II Centromeres divide Centromeres divide Chromatids separate; Now called chromosomes Chromatids separate; Now called chromosomes

10 Telophase II Nuclear envelope forms around chromosome sets Nuclear envelope forms around chromosome sets

11 Meiosis in Action

12 Meiosis and Genetic Variation Three key ways meiosis creates new genetic variations Three key ways meiosis creates new genetic variations Independent Assortment Independent Assortment Crossing Over Crossing Over Random Fertilization Random Fertilization

13 Independent Assortment Most organisms have more than 1 chromosome Most organisms have more than 1 chromosome In humans the gamete gets 1 chromosome from the 23 pairs In humans the gamete gets 1 chromosome from the 23 pairs Which of the pair you get is chance Which of the pair you get is chance This random distribution is called Independent Assortment This random distribution is called Independent Assortment Mathematically 2 23 = over 8.3 million combinations Mathematically 2 23 = over 8.3 million combinations

14 Crossing Over Portion of 1 homologous chromosome is broken off and exchanged with the corresponding portion of its homologous partner Portion of 1 homologous chromosome is broken off and exchanged with the corresponding portion of its homologous partner

15 Random Fertilization What egg and sperm meet is also completely random What egg and sperm meet is also completely random Mathematically that means that the possible combinations of the sperm (about 8.3 million) can be multiplied by the combinations for the egg (about 8.3 million) giving 70 trillion possible combinations Mathematically that means that the possible combinations of the sperm (about 8.3 million) can be multiplied by the combinations for the egg (about 8.3 million) giving 70 trillion possible combinations

16 Meiosis generates variation in a species very quickly Meiosis generates variation in a species very quickly Pace of evolution is sped up due to variation Pace of evolution is sped up due to variation A + B = C, a new individual not identical to its parents A + B = C, a new individual not identical to its parents

17 Gametogenesis – Formation of Gametes Spermatogenesis – sperm production occurs in testes Spermatogenesis – sperm production occurs in testes Diploid cell increases in size and becomes immature germ cell Diploid cell increases in size and becomes immature germ cell Meiosis I = 2 immature sperm cells Meiosis I = 2 immature sperm cells Meiosis II = 4 haploid cells Meiosis II = 4 haploid cells Develop tails and become sperm Develop tails and become sperm

18 Oogenesis – egg production occurs in ovaries Oogenesis – egg production occurs in ovaries Diploid cell increases and becomes germ cell Diploid cell increases and becomes germ cell Meiosis I = after cytokinesis cytoplasm is not equally divided Meiosis I = after cytokinesis cytoplasm is not equally divided Smaller one called polar body Smaller one called polar body Meiosis II = larger cell divides again, again cytoplasm not equally divided Meiosis II = larger cell divides again, again cytoplasm not equally divided Forms another polar body Forms another polar body Polar bodies will die Polar bodies will die Ovum (pl. = ova) is larger cell Ovum (pl. = ova) is larger cell More cytoplasm means more stored energy More cytoplasm means more stored energy

19 7.2 Sexual Reproduction Asexual Reproduction Asexual Reproduction Single parent passes on copies to offspring called clone Single parent passes on copies to offspring called clone Identical to parent Identical to parent Prokaryotes use binary fission Prokaryotes use binary fission

20 Many eukaryotes use asexual reproduction Many eukaryotes use asexual reproduction Fission – parent into 2 equal sized offspring Fission – parent into 2 equal sized offspring Fragmentation Fragmentation Body into several pieces Body into several pieces Missing parts grow back Missing parts grow back Budding – new split off from old; may stay attached Budding – new split off from old; may stay attached

21 Genetic Diversity Genetic Diversity Asexual reproduction is simplest method Asexual reproduction is simplest method In stable environment can produce many offspring in short time In stable environment can produce many offspring in short time Dont need to waste energy making gametes or find a partner Dont need to waste energy making gametes or find a partner DNA varies very little DNA varies very little In changing environment may not be able to adapt In changing environment may not be able to adapt

22 Sexual Reproduction Sexual Reproduction 2 parents form reproductive cells with half the number of chromosomes 2 parents form reproductive cells with half the number of chromosomes Diploid Mother makes haploid gamete Diploid Mother makes haploid gamete Diploid Father makes haploid gamete Diploid Father makes haploid gamete Haploid gametes make diploid offspring different from parents Haploid gametes make diploid offspring different from parents

23 Evolution of Sexual Reproduction May have come as a way for protists to repair DNA May have come as a way for protists to repair DNA Only diploid cells can repair Only diploid cells can repair Protists form diploid cells under stress Protists form diploid cells under stress Many enzymes that repair DNA are involved in meiosis Many enzymes that repair DNA are involved in meiosis

24 Sexual Life Cycle in Eukaryotes Life Cycle – span from 1 generation to next Life Cycle – span from 1 generation to next All sexually reproductive organisms have a diploid-haploid-diploid lifecycle All sexually reproductive organisms have a diploid-haploid-diploid lifecycle Only 3 Types of Sexual Life Cycles Only 3 Types of Sexual Life Cycles Only difference is which phase becomes multicellular Only difference is which phase becomes multicellular

25 Haploid – simplest and most common Zygote is the only diploid cell and it undergoes meiosis immediately Zygote is the only diploid cell and it undergoes meiosis immediately Haploid cells in multicellular organism produce gametes through mitosis Haploid cells in multicellular organism produce gametes through mitosis Fusion – gametes fuse to form zygote and meiosis follows Fusion – gametes fuse to form zygote and meiosis follows Meiosis allows for damage repair Meiosis allows for damage repair 2 homologous chromosomes are lined up, special repair enzymes remove damaged sections of DNA and fill in gaps 2 homologous chromosomes are lined up, special repair enzymes remove damaged sections of DNA and fill in gaps Protists, Fungi, Algae Protists, Fungi, Algae

26 Haploid Life Cycle

27 Diploid Adults are diploid, inherit from 2 parents Adults are diploid, inherit from 2 parents Diploid cell then meiosis then haploid gamete Diploid cell then meiosis then haploid gamete Gametes join during fertilization forming diploid zygote Gametes join during fertilization forming diploid zygote Zygote divides by mitosis Zygote divides by mitosis Gametes are the only haploid cells Gametes are the only haploid cells

28 Diploid Life Cycle

29 Parthenogenesis New individual forms from unfertilized egg New individual forms from unfertilized egg Hypothesized that mothers chromosomes are copied instead of getting fathers chromosomes Hypothesized that mothers chromosomes are copied instead of getting fathers chromosomes Produces clones Produces clones Honeybees provide drones this way (haploid) Honeybees provide drones this way (haploid) Whiptail lizards are all females Whiptail lizards are all females Doesnt seem possible in mammals Doesnt seem possible in mammals Only mammal clones are twins from split egg Only mammal clones are twins from split egg

30 Alternation of Generations Regularly alternates between haploid and diploid Regularly alternates between haploid and diploid Plants – diploid phase called sporophyte produces spores Plants – diploid phase called sporophyte produces spores Spore forming cells produce haploid spore that can form adult without another cell Spore forming cells produce haploid spore that can form adult without another cell Gametophyte is haploid phase Gametophyte is haploid phase Produces gametes by mitosis Produces gametes by mitosis Gametes fuse and produce diploid phase Gametes fuse and produce diploid phase Diploid – Haploid – Diploid – Haploid – etc… Diploid – Haploid – Diploid – Haploid – etc… Moss – haploid spores develop at top of sporophyte stalk Moss – haploid spores develop at top of sporophyte stalk

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