Lecture 15 Outline of Last Lecture I. Cell DivisionII. CytokinesisIII. Control of Cell DivisionIV. Important Cell Cycle Control PointsV. Cell Control in Cancer CellsOutline of Current Lecture I. Types of ReproductionII. Sexual ReproductionIII. MeiosisIV. Mitosis vs. MeiosisV. Sources of Genetic VariationCurrent LectureChapter 13 – Meiosis and Sexual Life CyclesI. Two Types of Reproductiona. Asexual Reproduction  gives genetically identical offspringi. Single individual is the sole parentii. Passes all genetic information to offspringiii. Occurs basically by cell divisioniv. All offspring of the offspring is also genetically identicalb. Sexual Reproduction  genetically unique offspringi. Requires 2 parent cellsii. Differ from parentsiii. Offspring differ from grandparent and parent cellsII. Sexual Reproductiona. Includes two major eventsi. Meiosisii. Fertilization BIOL 1st Editionb. Requires understanding of chromosomes:i. Each organism has a characteristic number of chromosomesii. All cells have this number of chromosomes EXCEPT for sex cells (aka gametes, aka eggs and sperm cells)iii. Gametes have ½ the characteristic number of chromosomesiv. Number of chromosomes in body cells (somatic cells) is called the diploid number (aka 2N number)v. Number of chromosomes in sex cells (gametes) is called the haploid number (aka 1N number)vi. Twice as many chromosomes in diploid (2N) body cells than haploid (1N) gamete cellsvii. Diploid cells come in pairs  each chromosome has a partner1. Pairs of chromosomes are called homologs (aka homologous pairs)2. Chromosomes in a pair are similar in size, shape, and genetic information3. One comes from the mother (maternal homolog) 4. One comes from the father (paternal homolog)viii. Gametes have ½ as many chromosomes as body cells1. ONE of each homologous pairc. Meiosisi. A special type of cell division that produces haploid (1N) cells starting from special diploid cells called “germline cells”ii. In humans, this occurs in the ovaries of women and the testes of malesiii. Sexual life cycle in humans:1. 2N germline cell  meiosis  egg (IN), sperm (1N)  fertilization  2N zygote  cell division  embryo  fetus  personiv. Fertilization – fusion of haploid gametes to form a diploid zygoteIII. Meiosisa. A special kind of cell division to make gametesb. Many steps in common with mitosisc. Uses much of the same cell machineryd. Occurs in 2 stages:i. Meiosis Iii. Meiosis IIe. Meiosis Ii. The chromosomes replicate (just as they do prior to mitosis)ii. Prophase – replicated homologs pair up (cross over occurs between homologs)iii. Metaphase – replicated homologs lime up in pairs at metaphase plateiv. Anaphase – replicated homologs pulled to opposite ends of the cellv. Telophase – nucleus divides, cytokinesis occursvi. At the end of Meiosis I  2 cells, each with one of each homologous pair of chromosomes (replicated chromosomes)f. Meiosis IIi. 2 cells from Meiosis I divides againii. Uses process like Mitosisiii. Separates sister chromatidsiv. At the end of Meiosis II  4 cells, each with one set of 23 chromosomesg. Human germline i. Germline cell (2N, 23 pairs of chromosomes  Meiosis I  2 cells, each with 1 set of 23 replicated chromosomes  Meiosis II  4 cells, each with one set of 23 chromosomes (1N gametes)IV. Mitosis vs. Meiosisa. Mitosis = cell divisionb. Meiosis = gamete formation c.Mitosis (cell division) Meiosis (gamete formation)Starting cell may be either 1N or 2N Starting cells must be 2N cells or special germ-line cells1 division gives 2 cells 2 divisions gives 4 cellsDaughter cells have the same number ofchromosomes as the parent cellDaughter cells have half the number ofchromosomes as the parent (one of eachhomologous pair)Daughter cells are genetically identical Daughter cells have new genetic combinationsV. Sources of Genetic Variationa. Main advantage of sexual reproduction is that it gives genetic variation  each individual is uniqueb. Genetic variation is important because it is the raw material for natural selection (not all individuals are the same – only those best suited survive and reproduce)c. Meiosis and fertilization are the primary sources of genetic variation due to the sorting and recombining of chromosomesd. Three major sources of variation:i. Independent assortment of chromosomes1. Chromosomes come in homologous pairs, one from the mother and one from the father2. In Meiosis I metaphase, these homologs line up at the metaphase plate in pairs3. Each pair aligns randomly, paternal on one side, maternal on the other4. Each pair orients randomly of the others  at the end of Meiosis I, there are all possible combinations of the paternal and maternal homologs5. The number of combinations depends on the number of chromosome pairs6. Number of possible combinations = 2N, where N = the haploid number7. Example: Germline cell with 2N = 4 chromosomes; 22 = 4 possible combinationsa. Pair #1 paternal with pair #1 maternalb. Pair #1 paternal with pair #2 maternalc. Pair #2 paternal with pair #1 maternald. Pair #2 paternal with pair #2 maternal8. Important because maternal and paternal homologs may carry different alleles atmany corresponding loci (alleles = alternative form of a gene)ii. Crossing over1. During Prophase of Meiosis I, the homologous pairs of chromosomes come into gene for gene contact (gene contact = synapsis)2. During synapsis, the paternal and maternal chromosomes often exchange pieces of DNA (aka crossing over)3. Results in immense genetic scrambling4. Each homolog is a random mix of paternal and maternal genes5. Paternal pair #1 + maternal pair #1  crossing over  paternal #1 with piece of maternal #1 + maternal #1 with a piece of paternal #1iii. Random fertilization1. Human egg (female gamete) is 1 of 8 million possible combinations of paternal and maternal homologs2. Will be fertilized by a sperm (male gamete) that is also 1 of 8 million possible combinations of homologs3. Resulting zygote (new baby) is 1 in 64 trillion diploid