CELLULAR ASPECTS OF REPRODUCTION AND MEIOSIS Asexual reproduction vegetative propagation clonal propagation only mitosis not meiosis each new generation is identical to parent only 1 parent common in simple eukaryotes but also possible in many plants Potatoes bananas strawberries apples oranges all fruits more or less cloning vs inbreeding eg corn results from self pollination produce monocultures new virus bacterium can wipe out all of them Cloning is just recently possible somatic cell nuclear transfer Sexual reproduction gametes from 2 parents fuse offspring have different genomes than either parent results in genetic diversity n of different chromosomes humans 23 ploidy of copies of each chromosome somatic diploid 2 copies 2n gametes haploid 1 copy n some plants polyploidy potatoes have 6 copies hexaploid karyotype display of chromosomes in human somatic cells 2 sex chromosomes XX or XY and 22 pairs of autosomes not X or Y homolog member of a pair of chromosomes homologous pairs 2 pairs of homologs sister chromatids In production of gametes special kind of cell division occurs to reduce ploidy to haploid meiosis only occurs in ovaries and testes Fertilization fusion of gametes syngamy forms a zygote first single diploid cell formed by fusion of gametes Why meiosis Meiosis divisions if sex cells fused as diploids next generation would be tetraploid then octaploid etc chromosome must be 2n for survival 1 diploid parent cell 4 haploid daughters reductional division produces cells with only one copy of each replicated chromosome equational mitosis like sister chromatids separate Phases interphase prophase I pairing up of homologous pairs synaptememal complex forms in synapsis chromosomes condense spindles form nuclear envelope disappears homologous pairs find each other crossover points chiasmata each pair of chromosomes has at least one tetrads bivalents paired chromosomes metaphase I anaphase I synapsed homologous pairs of chromosomes line up on metaphase plate independent assortment each pair lines up independently of the other not in collaboration sister chromatids DO NOT separate homologous pairs move to opposite poles of each other telophase I cytokinesis reduction division diploid haploid Meiosis II separation of sister chromatids similar to mitosis only one copy of each chromosome Advantage of meiosis genetic diversity of each generation crossing over at chiasmata creates new combinations of genes on chromosomes independent assortment gametes have many possible combinations of paternal and maternal chromosomes Rules for making gametes every gamete must have 1 copy of each chromosome 23 a gamete can have either paternal or maternal chromosomes with equal probability final distribution depends on how they line up along the metaphase plate in meiosis I How many combinations are possible in gametes 2 possible outcomes for each chromosome this 2 is produced 23 times n 2n of outcomes in humans 223 8 million outcomes o this doesn t even count variation from crossing over o EVERY HUMAN IS UNIQUE