BIOL 1st Edition Lecture 15 Outline of Last Lecture I Cell Division II Cytokinesis III Control of Cell Division IV Important Cell Cycle Control Points V Cell Control in Cancer Cells Outline of Current Lecture I Types of Reproduction II Sexual Reproduction III Meiosis IV Mitosis vs Meiosis V Sources of Genetic Variation Current Lecture Chapter 13 Meiosis and Sexual Life Cycles I Two Types of Reproduction a Asexual Reproduction gives genetically identical offspring i Single individual is the sole parent ii Passes all genetic information to offspring iii Occurs basically by cell division iv All offspring of the offspring is also genetically identical b Sexual Reproduction genetically unique offspring i Requires 2 parent cells ii Differ from parents iii Offspring differ from grandparent and parent cells II Sexual Reproduction a Includes two major events i Meiosis ii Fertilization b Requires understanding of chromosomes i Each organism has a characteristic number of chromosomes ii All cells have this number of chromosomes EXCEPT for sex cells aka gametes aka eggs and sperm cells iii Gametes have the characteristic number of chromosomes iv 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 cells vii Diploid cells come in pairs each chromosome has a partner 1 Pairs of chromosomes are called homologs aka homologous pairs 2 Chromosomes in a pair are similar in size shape and genetic information 3 One comes from the mother maternal homolog 4 One comes from the father paternal homolog viii Gametes have as many chromosomes as body cells 1 ONE of each homologous pair c Meiosis i 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 males iii Sexual life cycle in humans 1 2N germline cell meiosis egg IN sperm 1N fertilization 2N zygote cell division embryo fetus person iv Fertilization fusion of haploid gametes to form a diploid zygote III Meiosis a A special kind of cell division to make gametes b Many steps in common with mitosis c Uses much of the same cell machinery d Occurs in 2 stages i Meiosis I ii Meiosis II e Meiosis I i 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 plate iv Anaphase replicated homologs pulled to opposite ends of the cell v Telophase nucleus divides cytokinesis occurs vi At the end of Meiosis I 2 cells each with one of each homologous pair of chromosomes replicated chromosomes f Meiosis II i 2 cells from Meiosis I divides again ii Uses process like Mitosis iii Separates sister chromatids iv At the end of Meiosis II 4 cells each with one set of 23 chromosomes g 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 V Mitosis vs Meiosis a Mitosis cell division b 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 germline cells 1 division gives 2 cells 2 divisions gives 4 cells Daughter cells have the same number of chromosomes as the parent cell Daughter cells have half the number of chromosomes as the parent one of each homologous pair Daughter cells are genetically identical Daughter cells have new genetic combinations Sources of Genetic Variation a Main advantage of sexual reproduction is that it gives genetic variation each individual is unique b 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 chromosomes d Three major sources of variation i Independent assortment of chromosomes 1 Chromosomes come in homologous pairs one from the mother and one from the father 2 In Meiosis I metaphase these homologs line up at the metaphase plate in pairs 3 Each pair aligns randomly paternal on one side maternal on the other 4 Each pair orients randomly of the others at the end of Meiosis I there are all possible combinations of the paternal and maternal homologs 5 The number of combinations depends on the number of chromosome pairs 6 Number of possible combinations 2N where N the haploid number 7 Example Germline cell with 2N 4 chromosomes 22 4 possible combinations a Pair 1 paternal with pair 1 maternal b Pair 1 paternal with pair 2 maternal c Pair 2 paternal with pair 1 maternal d Pair 2 paternal with pair 2 maternal 8 Important because maternal and paternal homologs may carry different alleles at many corresponding loci alleles alternative form of a gene ii Crossing over 1 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 scrambling 4 Each homolog is a random mix of paternal and maternal genes 5 Paternal pair 1 maternal pair 1 crossing over paternal 1 with piece of maternal 1 maternal 1 with a piece of paternal 1 iii Random fertilization 1 Human egg female gamete is 1 of 8 million possible combinations of paternal and maternal homologs 2 Will be fertilized by a sperm male gamete that is also 1 of 8 million possible combinations of homologs 3 Resulting zygote new baby is 1 in 64 trillion diploid combinations