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MEIOSIS AND SEXUAL REPRODUCTION

MEIOSIS AND SEXUAL REPRODUCTION. MEIOSIS: FORMATION OF HAPLOID CELLS. SOME ORGANISMS REPRODUCE BY JOINING GAMETES TO FORM THE FIRST CELL OF A NEW INDIVIDUAL GAMETES ARE HAPLOID THEY CONTAIN ONE SET OF CHROMOSOMES.

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MEIOSIS AND SEXUAL REPRODUCTION

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  1. MEIOSIS AND SEXUAL REPRODUCTION

  2. MEIOSIS: FORMATION OF HAPLOID CELLS • SOME ORGANISMS REPRODUCE BY JOINING GAMETES TO FORM THE FIRST CELL OF A NEW INDIVIDUAL • GAMETES ARE HAPLOID THEY CONTAIN ONE SET OF CHROMOSOMES

  3. MEIOSIS IS A FORM OF CELL DIVISION THAT HALVES THE NUMBER OF CHROMOSOMES WHEN FORMING SPECIALIZED REPRODUCTIVE CELLS SUCH AS GAMETES OR SPORES.

  4. BEFORE MEIOSIS BEGINS THE DNA IN THE ORIGINAL CELL IS REPLICATED. THEREFORE, MEIOSIS STARTS OUT WITH HOMOLOGOUS CHROMOSOMES. *** PREFIX HOMO MEANS THE SAME

  5. STEP 1: PROPHASE 1 THE CHROMOSOMES CONDENSE AND THE NUCLEAR ENVELOPE BREAKS DOWN. HOMOLOGOUS CHROMOSOMES PAIR ALONG THEIR LENGTH.

  6. CROSSING OVER OCCURS WHEN PORTIONS OF A CHROMATID ON ONE HOMOLGOUS CHROMOSOMES ARE BROKEN AND EXCHANGED WITH THE CORRESPONDING CHROMATID PORTIONS OF THE OTHER HOMOLOGOUS CHROMOSOMES.

  7. Step 2: Metaphase 1 The pairs of homologous chromosomes are moved by the spindle to the equator of the cell and remain together.

  8. Step 3: Anaphase 1 The homologous chromosomes separate and each pair are pulled to opposite poles of the cell. Each chromosomes is still composed of 2chromatids. The geneticmaterial has been recombined.

  9. Step 4: Telophase 1 Individual chromosomes gather at each of the poles, the cytoplasm divides (Cytokinesis) forming 2 new cells containing1 chromosome from each pair of homologous chromosomes.

  10. Step 5: Prophase II A new spindle forms around the chromosomes

  11. Step 6: Metaphase II The chromosomes line up along the equator and are attached at the centromeres to the spindle fibers.

  12. Step 7: Anaphase II The centromeres divide and the chromatids (now called chromosomes) move to opposite poles of the cell.

  13. Step 8: Telophase II A nuclear envelope forms around each set of chromosomes. The cells undergo Cytokinesis. Results: 4 haploid cells.

  14. Meiosis and Genetic Variation Meiosis is an important process that allows for the rapid generation of new genetic combinations.

  15. Mechanisms contributing to Genetic Variations 1. Independent assortment: each of the 23 pairs of chromosomes segregates (separately) independently 2. Crossing over: DNA exchange adds even more recombinations 3. Random Fertilization: random joining of two gametes

  16. Importance of Genetic Variation Meiosis and the joining of gametes are essential to evolution. The pace of evolution is sped up by genetic recombination The combination of genes from two organisms results in a 3rd type not identical to either parent. Natural Selection does not always favor genetic change. This means it may favor existing combinations of genes slowing the pace of evolution.

  17. Meiosis and Gamete Formation • The fundamental events of meiosis occur in all sexually reproducing organisms, but may vary in timing and structures associated with gamete formation. • Meiosis is the primary event in the formation of gametes-gametogenesis.

  18. Meiosis in Males • The process by which sperm are produced in male animals is called spermatogenesis and occurs in the testes. 4 cells change in form, develop a tail to become male gametes called sperm.

  19. Meiosis in Females • The process by which gametes are produced in female animals is called oogenesis and occurs in the ovaries. The cytoplasm divides unequally with one cell getting nearly all of the cytoplasm and will ultimately give rise to an egg cell or an ovum with a large storehouse of nutrients to nourish young after fertilization. The other cell is small and called a polar body; its offspring will not survive.

  20. Sexual and Asexual Reproduction • Reproduction, the process of producing offspring, can be asexual or sexual • AsexualReproduction a single parent passes copies of all its genetic genes to each of its offspring, a clone; no fusion of haploid cells such as gametes. • Example: prokaryotes

  21. In contrast, in sexual reproduction two parents each form reproductive cells that have half the number of chromosomes. Therefore both parents contribute genetic material to the offspring, however the offspring are not exactly like either parent they have similar traits.

  22. Types of Asexual Reproduction • Fission – separation of a parent into two or more individuals of about equal size Examples: amoebas • Multicellular eukaryotes undergo fragmentation, a type of reproduction in which the body breaks into several pieces; some are all later develop into complete adults when missing parts are regrown. • Budding, is where new individuals split off from existing ones; the buds may break from the parent and become an independent organism, or it may remain attached to the parent. Example: hydra

  23. Genetic Diversity Asexual Reproduction is the simplest and most primitive method of reproduction. In a stable environment, many offspring can be produced in a short period of time without using energy to produce gametes or to find a mate. A disadvantage is the changing environment because a population of organisms may not be able to adapt to a new environment

  24. Sexual Reproduction provides a powerful means of quickly making different combinations of genes among individuals. Evolution of Sexual Reproduction Only diploid cells can repair certain kinds of chromosomes damage, such as breaks in both strands of DNA. The process of meiosis and the pairing of homologous chromosomes may have allowed early protistan cells to repair damaged DNA.

  25. Sexual Life Cycles in Eukaryotes • The entire life span in the life of an organism from one generation to the next is called a life cycle. • Eukaryotes that undergo sexual reproduction can have one of three types of sexual life cycles: Haploid, Diploid or alternation of generation

  26. Facts for the Haploid Life Cycle • Simplest of sexual life cycles • Haploid cells occupy the major portions of meiosis immediately after it is formed; creating new haploid cells. • Fusion – gametes fuse to produce a zygote

  27. Facts for the Haploid Life Cycle 4. Damaged is repaired during meiosis 5. Two homologous chromosomes line up side by side in preparation for crossing over 6. Found in many protists, some fungi and algae

  28. Facts for the Diploid Life Cycle • Adult individuals are diploid • Inherit chromosomes from each parent • A reproductive cell undergoes meiosis to produce gametes. • Sperm + egg = zygote (fertilization)

  29. Facts for the Diploid Life Cycle 5. A single diploid cell eventually gives rise to all of the cells of the adult. 6. The diploid individual that develops from the zygote occupies the major portion of the diploid life cycle.

  30. Facts on Alternation of Generations • Plants, algae, and some protists have a life cycle that regularly alternates between a haploid phase and a diploid phase. • In plants, the diploid phase in the life cycle that produces spores is called sporophyte.

  31. Facts on Alternation of Generations 3. A spore is a haploid reproductive cell produced by meiosis that is capable of developing into an adult without fusing with another cell. 4. In the life cycle of a plant, the gametophyte is the haploid phase that produces gametes by mitosis.

  32. Facts on Alternation of Generations 5. In moss, haploid spores develop in a capsule at the tip of the sporophyte “stalk” and when the lid of the capsule pops off, the spores scatter. 6. The male gametophytes release sperm which swim through a film of moisture to the egg in the female gametophyte.

  33. SUMMARY ALL THREE INVOLVE AN ALTERNATION OF HAPLIOD AND DIPLOID PHASES THE THREE TYPES OF SEXUAL LIFE CYCLES DIFFER FROM EACH OTHER ONLY IN WHICH PHASES BECOME MULTICELLULAR

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