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Sexual Reproduction

Sexual Reproduction. SBI3U0. Sexual Reproduction. Multicellular organisms mostly do not reproduce via mitosis Most multicellular organisms reproduce sexually, meaning that the cells of offspring are created by the joining of two specialized gamete cells

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Sexual Reproduction

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  1. Sexual Reproduction SBI3U0

  2. Sexual Reproduction • Multicellular organisms mostly do not reproduce via mitosis • Most multicellular organisms reproduce sexually, meaning that the cells of offspring are created by the joining of two specialized gamete cells • One gamete is often referred to as female, or an egg • The other gamete is referred to as male, or a sperm cell • When the gametes combine to make a cell it is called fertilization

  3. Sexual Reproduction • There are several ways that organisms reproduce sexually • Mammals and birds have two distinct sexes, male and female. The sperm fertilize the eggs (meaning the cells combine) internally • Earthworms are hermaphrodites, meaning they have both sperm and eggs within each worm and can self-fertilize • Certain fish have distinct male/female members, but they can change back and forth. Females lay their egg cells in the water and the males spray sperm over them • Giant clams are all males when young and turn into females when they are older. Fertilization occurs externally

  4. Sexual Reproduction • These organisms may seem to reproduce very differently but their method of gamete production is essentially the same • These organisms all create their sperm and eggs through Meiosis • Meiosis is a method of cell division somewhat similar to mitosis, but with some very important differences

  5. Gametes • Gametes are the cells produced by mitosis • Gametes have half the number of chromosomes that regular cells do • Human cells have 46 chromosomes • Human eggs and sperm have 23 chromosomes • We call the number of chromosomes in a regular cell the diploid number, while the number of chromosomes in a gamete is the haploid number • Why do gametes have half the chromosomes of regular cells?

  6. Chromosomes • All organisms that reproduce sexually have two different copies of each chromosome • For example: the chromosome that contains the gene for ear shape • We have two different chromosomes that contain this gene • Each one has a different version of the gene • Each chromosome came from a different parent • Thus in humans we have 23 different types of chromosomes, and for each we have two copies • Resulting in a total diploid number of 46

  7. Chromosomes • There are two versions of each chromosome • These two versions are NOT the same, but they code for the same traits • We call the two versions of the same chromosome homologous chromosomes • Note the same shapes and lengths

  8. Homologous Chromosomes • Homologous chromosomes contain the same genes • But each chromosome may have different forms of the gene • These are referred to as alleles • Eg: There are several alleles for eye colour • Brown, blue, green • One chromosome is paternal (from the father) and one is maternal (from the mother)

  9. Gametes • Gametes have half the number of chromosomes (the haploid number) so that each gamete has 1 copy of each chromosome • When the sperm and egg combine to form a zygote, the resulting zygote cell will have the diploid number of chromosomes • Making it a regular cell 23 Sperm 46 Zygote 23 Egg

  10. Reproduction • Once the gametes combine to form the zygote, the zygote cell is free to divide (via mitosis) to eventually form an embryo, then a fetus, then finally a baby • Since the zygote divides by mitosis, all of the resulting cells in the embryo, fetus, and baby contain the diploid number of chromosomes (46 in humans)

  11. Meiosis • Let’s look at the steps of meiosis and how it differs from mitosis • Meiosis occurs over two distinct divisions • We refer to them as meiosis I and meiosis II • Meiosis I has some features that are distinctly different from mitosis • To prepare for meiosis I, all of the same steps occur as before mitosis (during G1, S, and G2) • The cell enlarges, creating more cytoplasm and organelles • The DNA duplicates

  12. Prophase I • Meiosis starts during prophase I • The DNA coils itself around proteins forming thick chromosomes • Centrioles move to the poles of the cell, while spindle fibres form • The nuclear membrane and nucleolus dissolve

  13. Prophase I • One big difference is that the homologous pairs of chromosomes are grouped into tetrads • The tetrads contain two copies of each homologous chromosome • A total of 4 chromatids per tetrad • The tetrads form during prophase I in a process called synapsis • The homologous chromosomes merge to form tetrads

  14. Crossing Over • The other main difference is that the maternal and paternal chromosomes can exchange genes • This process is called crossing over • Two adjacent chromatids connect at a chiasma • At the chiasma the chromatids exchange whole sections of DNA • Notice that the 4 chromatids are now distinctly different Chiasma

  15. Crossing Over • Multiple crossovers can occur • Only 1 is shown • Crossovers can also occur between sister chromatids • These crossovers mix up the genes • This is extremely important for creating genetic diversity

  16. Metaphase I • Metaphase I is similar to metaphase in mitosis • The main difference is that the tetrads line up along the equator • As opposed to all of the chromosomes lining up separately • Notice that in each tetrad one pair of sister chromatids is attached to one centriole, while the other pair is attached to the other centriole

  17. Metaphase I • The other important point about metaphase I is independent assortment • Independent assortment means that the tetrads arrange themselves in random order • Some have the parental chromosome facing the top, while others have the maternal chromosome facing the top • This order is entirely random • This is another way that maternal and paternal genes become mixed up in the gametes

  18. Anaphase I • In anaphase I, the homologous chromosomes are separated from one another • The microtubules pull the tetrads apart, pulling homologues to opposite sides of the cell • Thus, each side is receiving 23 chromosomes (in humans), each containing two chromatids • Note that due to crossing over, the chromatids are not identical anymore

  19. Telophase I • In telophase I, the homologues chromosomes are now at opposite ends of the cell • Nuclear membranes form around the different set of chromosomes • Cytokinesis occurs and separates the cell into two distinct daughter cells • These daughter cells are not identical to one another, and each has the haploid number of chromosomes (23 in humans)

  20. Telophase I to Meiosis II • In telophase I, the DNA unravels to become chromatin once again • After cytokinesis is completed the daughter cells often enter a rest stage called interphase II • No DNA replication occurs during interphase II • The second round of cell division (called meiosis II) occurs after this, resulting in a separation of the sister chromatids • Meiosis II is functionally very similar to mitosis

  21. Prophase II • Both daughter cells undergo meiosis II • Prophase II contains the same steps as mitotic prophase • The nuclear membrane and nucleolus dissolve • Spindle fibre forms and attaches to the kinetocores of the chromosomes • The centrioles move to the poles of the cell

  22. Metaphase II • In metaphase II, the chromosomes line up along the equator of the cell • Notice that the sister chromatids are not identical • Due to cross overs during prophase I • Also notice that it is entirely random which pole of the cell gets the altered chromatid • It is random for all 23 chromosomes • This again adds to the mixing of genes

  23. Anaphase II • In anaphase II, the chromatids are separated and pulled to opposite poles of the cell • In this manner, each pole of the cell gets 23 chromosomes • These chromosomes do NOT have duplicates

  24. Telophase II • During telophase II, the nuclear membrane forms around the different sets of DNA • Since each daughter cell from meiosis I undergoes meiosis II, we get four distinct daughter cells • The chromosomes unspool becoming chromatin • Cytokinesis occurs, separating the daughter cells

  25. Results of Meiosis • We now have four distinctly different daughter cells • Each cell contains the haploid number of chromosomes (23 in humans) • The maternal and paternal genes have been mixed up and randomly distributed due to • Crossing over • Independent assortment • These cells are the gametes (sperm and eggs), and each one contains all 23 chromosomes, but they are not identical

  26. Purpose of Meiosis • One of the main advantages of meiosis is that is mixes up the genes • This gives the offspring new combinations of genes, which may confer unique advantages to the offspring that its parents did not have • This is very important to evolution • How much variation can this make?

  27. Independent Assortment • Independent assortment • Since the paternal and maternal chromosomes are sorted randomly, there are several different combinations of genes • There are 23 homologous chromosomes, and each pair of chromosomes contains 2 different chromosomes • This gives us 223 = 8,388,608 possible combinations of chromosomes in the gamete cells • Since the gametes must merge to form a zygote, each zygote has 223 x 223 =70,368,744,177,664 possible combinations of chromosomes

  28. Mitosis Vs. Meiosis • Mitosis • 1 division • Diploid daughter cells • 2 daughter cells • Daughter cells identical to parent cell Meiosis • 2 divisions • Haploid daughter cells • 4 daughter cells • Daughter cells NOT identical to parent cell

  29. Spermatogenesis

  30. First polar body may divide (haploid) a X Polar bodies die a a X X a X X A Mitosis Meiosis I Meiosis II (if fertilization occurs) Oogonium (diploid) A Primary oocyte (diploid) X Ovum (egg) X A Mature egg Secondary oocyte (haploid) A X Second polar body (haploid) Oogenesis

  31. Comparison • The key differences between spermatogenesis and oogenesis are • The primary spermatocyte divides to create four functional gametes • The primary oocyte only creates one functional ovum • The other gametes are called polar bodies and they die off at the end of meiosis • Oogenesis stops at metaphase II and waits until fertilization to complete division

  32. Comparison

  33. Comparison

  34. Asexual Vs. Sexual • Asexual Reproduction • Mitosis • Very fast • Advantageous if organism is well adapted to its environment • Disadvantageous if the environment changes since none of the organisms will be well adapted to the new env. Sexual Reproduction • Meiosis • Slow due to requirement of fertilization • Advantageous when the environment changes since there is likely some individuals adapted to the new environment • Only a few individuals will be optimally adapted to any one environment

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