BIO 101 1st Edition Lecture 17 Outline of Last Lecture I Cell division a Genetic material II Cell Cycle a Interphase b Mitosis i Prophase ii Prometaphase iii Metaphase iv Anaphase v Telophase c Cytokinesis III Cell Division Controls a Checkpoints Outline of Current Lecture I Reproduction a Asexual b Sexual i Meiosis 1 Meiosis I 2 Meiosis II ii Fertilization II Sources of Genetic Variation a Independent Assortment of Chromosomes b Crossing Over c Random Fertilization Chapter 13 Meiosis and Sexual Life Cycles 2 kinds of reproduction in living things 1 Asexual Reproduction single individual is sole parent and passes all of its genetic information to offspring Occurs basically by cell division process we just covered in Chapter 12 Offspring are GENETICALLY IDENTICAL to the parent and to all other offspring of that parent 2 Sexual Reproduction 2 parents give rise to offspring with a unique combination of genes Sexually produced offspring differ genetically from their parents and from other offspring Sexual Reproduction includes 2 Major Events 1 Meiosis 2 Fertilization To understand sexual reproduction we need to look more closely at chromosomes Every organism has a characteristic number of chromosomes BIO 101 1st Edition All the cells of the organism have this characteristic number of chromosomes EXCEPT for the SEX CELLS GAMETES EGGS AND SPERM The gametes egg and sperm have exactly that number Number of chromosomes in body cells somatic cells is called the DIPLOID number 2N number Number of chromosomes in sex cells gametes is called the HAPLOID number 1N number Examples 2N in body cells 1N in gametes Mosquito 6 3 Corn 20 10 Dogs 78 39 Humans 46 23 Thus there are twice as many chromosomes in DIPLOID 2N body cells as in HAPLOID 1N gamete cells That is because chromosomes in DIPLOID CELLS come in PAIRS each chromosome has a PARTNER The pairs of chromosomes in 2N cells are called HOMOLOGS or HOMOLOGOUS PAIRS o The chromosomes in a pair are similar to one another in SIZE SHAPE and GENETIC INFORMATION o One comes from the MOTHER maternal homolog o The other comes from the FATHER paternal homolog Gametes have as many chromosomes as body cells But not just any half Gametes have ONE of each homologous PAIR Meiosis a special type of cell division that produces haploid 1N cells starting from special DIPLOID cells called GERMLINE CELLs In humans meiosis occurs in ovaries of female and in testes of male Sexual Life Cycle in Humans Fertilization fusion of haploid gametes to form a diploid zygote Meiosis a special kind of cell division to make gametes Many steps in common with cell division mitosis Uses much of same cell machinery Occurs in 2 stages MEIOSIS I separates homologous pairs MEIOSIS II separates sister chromatids Meiosis I first the chromosomes replicates just as they do prior to mitosis Prophase replicated homologs pair up cross over occurs between homologs Metaphase replicated homologs line up in pairs at metaphase plate remember that in mitosis the chromosomes line up single file NOT in pairs Anaphase replicated homologs pulled to opposite ends of cell Telophase nucleus divides cytokinesis occurs BIO 101 1st Edition At the end of Meiosis I there are 2 cells each with one of each homologous pair of chromosomes it is a replicated chromosome Meiosis II each of these 2 cells from meiosis I divides again this time it is like mitosis separates sister chromatids Ex human germline all Overview Cell Division vs Gamete Formation Cell Division Mitosis Gamete Formation Meiosis Starting cell may be either 1N or 2N Only 2N cells special germ line cells 1 division 2 cells 2 divisions 4 cells Daughter cells have SAME number of Daughter cells have the number of chromosomes as the parent cell chromosomes as the parent ONE of each homologous pair Daughter cells are genetically identical Daughter cells have NEW genetic combinations Sources of Genetic Variation Main advantage of sexual reproduction is that it provides GENETIC VARIATION each individual is unique Genetic variation is important because it is the raw material for natural selection not all individuals are the same only the best suited survive and reproduce Meiosis and fertilization are the primary sources of genetic variation due to sorting and recombining of chromosomes Three major sources of variation 1 Independent Assortment of Chromosomes BIO 101 1st Edition Chromosomes come in homologous pairs paternal and maternal In Meiosis I metaphase these homologs line up at the metaphase plate IN PAIRS Each pair aligns RANDOMLY paternal homolog on side maternal on the other Each pair orients independently of the other pairs i e they don t all line up the same it is a RANDOM PROCESS INDEPENDENT ASSORTMENT Therefore at the end of Meiosis I when the cell divides to give 2 cells each will have ONLY ONE of each homologous PAIR of chromosomes and there are all possible combinations of the PATERNAL and MATERNAL homologs The number of possible combinations of paternal and maternal homologs in the gametes depends on the number of chromosome pairs Number of possible combinations 2 N where N haploid number Examples N possible combinations of homologs in gametes 1 2 1 2 2 2 2 4 3 2 3 2x2x2 8 4 2 4 2x2x2x2 16 23 2 23 8x10 6 Example germline cell with 2N 4 chromosomes Why is independent assortment of maternal and paternal homologs important Because maternal and paternal homologs may carry different alleles at many corresponding loci Allele alternative form of gene BIO 101 1st Edition Ex gene locus caries code for protein involved in NOSE SIZE Alleles big nose or cute little nose 2 Crossing Over During prophase of Meiosis I the homologous pairs of chromosomes come into gene for gene contact SYNAPSIS During synapsis the paternal and maternal chromosomes often exchange pieces of DNA CROSSING OVER Results in immense genetic scrambling Each homolog is a random mix of paternal and maternal genes 3 Random Fertilization Human egg female gamete is 1 of 8 million possible combinations of paternal and maternal homologs Will be fertilized by a sperm male gamete that is also 1 of 8 million possible combinations of homologs Resulting ZYGOTE new little baby is 1 of 64 trillion DIPLOID combinations