Lecture 11 13 1 13 2 Genetic consequences of sexual and asexual reproduction Asexual reproduction essentially creates clones by mitosis is genetically identical to parent Sexual Reproduction unique combos of genes inherited by parents allows genetic variation by meiosis Chromosomes exist as homologous pairs in diploids meaning diploids are pairs of homologous chromosomes one chromosome is maternal one paternal Meiosis produces haploid daughter cells from diploid parents because it is counterbalancing the doubling that occurs in fertilization Meiosis occurs in germ cells ovaries and testes as a result each sperm and egg is haploid n 23 but fertilization restores the diploid condition by combining the two haploid sets of chromosomes to make a full 46 this maintains a constant number of chromosomes in every generation Sexual life cycles alternate meiosis and fertilization meiosis divides into haploid then meiosis restores diploidy through fertilization Sexual life cycles vary in the timing of meiosis and fertilization at which multicellular organism is produced by mitosis 3 MAIN TYPES OF LIFE CYCLES BELOW All create genetic variation among offspring Haploid n Diploid 2n n gametes n mitosis n n n n n n meiosis Fertilization meiosis fertilization 2N 2N 2n Plants differ from animals because they have diploid multicellular stage and haploid multicellular stage animals only have diploid produce spores and then gametes 2n mitosis Plants some algae mitosis Animals gamete mitosis n n n mitosis n n meiosis fertilization zygote 2n Most fungus and some protists Gametes fuse and form diploid zygote meiosis occurs w o formation of diploid offspring developing instead produces haploid cells that give rise to either unicellular descendants or haploid multicellular adult Vocabulary Haploid only one chromosome from each homologous pair necessary for sexual life cycle egg and sperm only contain number of chromosomes each offspring will inherit For humans haploid number n is 23 Diploid pairs of homolougs homologous chromosomes Formula 2n n representing haploid number in humans the diploid number is 46 Homologs vs sister chromatids o Homologous chromosomes genetically similar but not necessarily identical different versions of genes alleles inherited from each parent o Sister chromatids genetically identical same DNA copied Gonad in animals male testes female ovary organs that produce gametes Gamete sperm eggs Zygote fertilized egg Meiosis cell division that reduces the number of sets of chromosomes from two diploid to one haploid in gametes counterbalancing the doubling that occurs in fertilization Fertilization union of gametes and the fusion of their nuclei Sexual Reproduction two parents give rise to offspring that have unique combos of genes inherited from two parents vary genetically between parents and siblings creates genetic variation meiosis is the precondition for sexual reproduction Asexual reproduction single individual is sole parent all genes are passed down without fusion of gametes mitosis Clone group of genetically identical individuals formed by asexual reproduction Sporophyte Asexual Multicellular diploid stage of plants and some algae produces spores from which gametophytes arise Spore haploid cells produced by meiosis in the sporophyte Gametophyte Offspring of sporophyte multicellular haploid cell give rise to gametes by mitosis which then produce the zygote from which more sporophytes arise Lecture 12 13 3 13 4 Meiosis has two stages meiosis 1 and meiosis 2 End product of meiosis 2 4 haploid cells Meiosis 1 homologous chromosomes are separated Meiosis 2 sister chromatids are separated Meiosis 1 stages prophase crossing over and anaphase separation of homologs are crucial o Prophase 1 o Metaphase 1 duplicated homologous chromosomes pair connect along chromosomes line up by their homologous pairs on lengths by zipper like protein structure synapsis and cross over chiasma point where crossover occurs metaphase plate sister chromatids remain attached at centromere chromosome still consists of two sister chromatids joined at centromere each pair of homologous chromosomes separate but two haploid cells form each o Telophase 1 and cytokinesis o Anaphase 1 o Prophase 2 Chromosomes composed of two chromatids condense Meiosis 2 stages and spindle form o Metaphase 2 Chromosomes are positioned at metaphase plate similar to mitosis two sister chromatids are no longer identical because of crossing over in meiosis 1 o Anaphase 2 sister chromatids separate o Telophase 2 and Cytokinesis haploid daughter cells form containing unduplicated chromosomes each genetically distinct from eachother and parent cell o Pairing of homologs need to be a pair to share genes from both o Crossing over exchange of corresponding segments of DNA allows Prophase 1 importance parents to offspring for genetic diversity Genetic Variability in o Pairing of homologs occurs at metaphase 1 can orient with either maternal or paternal Formula for calculating the number of possibilities is 2 n n haploid number in case of humans 2 23 8 4 million which is the breakage and rejoining of DNA of nonsister chromatids in a homologous pair result in recombinant chromosomes which contain DNA from two different parents can have more than one area of crossing over on each chromosome pair o Crossing Over Mitosis vs Meiosis Property DNA Replication Number of divisions Mitosis Occurs during interphase before mitosis begins One including prophase prometaphase metaphase anaphase and telophase Synapsis of homologous chromosomes Doesn t occur Number of daughter cells and genetic composition Two each diploid 2n and genetically identical to parent cell Role in animal body Enables multicellular adult to arise from zygote produces cells for growth repair and in some species asexual reproduction Meiosis Occurs during interphase before meiosis 1 begins Two each including prophase metaphase anaphase and telophase Occurs during prophase 1 along with crossing over between nonsister chromatids resulting chiasmata hold pairs together due to sister chromatid cohesion Four each haploid n containing half as many chromosomes as parent cell genetically different from parent cell and each other Produces gametes reduces number of chromosome sets by half and introduces genetic variability among the gametes Vocabulary Synapsis joining of chromosomes in a homologous pair zipper like occurs in prophase 1 results in crossing over Crossing Over genetic rearrangement
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