Meiosis Overview Notes

Both meiosis and mitosis are preceded by replication of chromosomes Meiosis 1 2 homologous pairs separate into 2 daughter cells Each daughter ahs 1 set of chromosomes 2n to n Known as reduction division Meiosis 2 Sisters separate into 2 daughter cells n to n Results in 4 haploid cells Meiosis 1 and Reduction Diploid cells produce 4 haploid daughter cells that become eggs or sperm by gametogenesis Fertilization results in a zygote that has restored number of chromosomes Independent assortment of chromosomes Early Prophase 1 Homologs pairs come together through protein binding to form tetrads Bivalent (2 homologs) Chromatids of the homologs are non-sister chromatids Nuclear envelope beings to break down Synapsis crossing occurs between non-sister chromatids Late Prophase 1 Homologs begin to separate Remain attached at points called chiasmata (this is where crossing over occurs) Crossing over occurs between non-sister chromatids Produces chromosomes with a combination of maternal/paternal alleles Metaphase 1 Paired homologs line up on plate in tetrads Alignment is random (this provides variation) NOT SISTER CHROMATIDS Homologous chromosomes Anaphase 1 Paired homologs separate and migrate to opposite ends of the cell Again, NOT SISTER CHROMATIDS Telophase 1 Movement of homologous chromosomes continues until there is a haploid set at each plate Chromosomes are in random assortment as a result of crossing over and random distribution of mom/dad genes After telophase 1, the cell will undergo cytokinesis Meiosis 1 Overview Results in daughter cells with only 1 chromosome of each homologous pair Haploid but still contain replicated chromosomes Random assortment of chromosomes due to crossing over and random distribution of maternal/paternal homologs to daughter cells NO CHROMOSOME REPLACTION OCCURS BETWEEN MEIOSIS 1 AND 2 Meiosis 2 Similar to mitosis (n n) Sister chromatids separate Prophase 2 Spindle apparatus forms Spindle fibers attach to kinetochores (one to each sister chromatid) Metaphase 2 Replicated chromosomes line up at metaphase plate Each chromosome is attached by spindle fibers to both spindle poles Anaphase 2 Sister chromatids separate via centromeres Resulting daughter chromatids move to opposite sides of the cell Telophase 2 Chromosomes arrive at opposite sides of the cell Nuclear envelope forms around each haploid set of chromosomes After telophase 2 the cells undergo cytokinesis resulting in 4 haploid daughter cells In meiosis, homologous chromosomes are separated during Anaphase I. After telophase I of meiosis, the chromosomal makeup of each daughter cell is haploid, and the chromosomes are each composed of two chromatids. Synapsis and Crossing Over As chromosomes condense, sister chromatids stay joined together along their kinetochore’s entire length by cohesions Homologs pair by synapsis and are held together by proteins called the synaptonemal complex Breaks are made in the DNA and crossover occurs between corresponding segment of non-sister chromatids Synaptonemal complex disassembles and homologs are held together only as chiasmata Crossing Over New combinations of alleles on the same chromosome that didn’t exist before Increases genetic variability of gametes produced in meiosis Produced by random assortment Mitosis Promotes Genetic Variation Independent assortment Crossing over during Meiosis 1 Fertilization makes it even more unique and restores the chromosome number Asexual no variation, based on meiosis, does not involve gametes (clones) Sexual each offspring is different (shuffling of alleles) Independent assortment Random separation of homolog chromosomes results in variation Genetics recombination creates genetic diversity How to calculate number of unique gametes: 2^n when n=haploid chromosome number How many different combinations of maternal and paternal chromosomes can be packaged in gametes made by an organism with a diploid number of 8 (2n=8)? 16 When Thing Go Wrong in Meiosis Non-disjunction (homologs fail to separate) Meiosis starts normally with bivalent lining up at center Non-disjunction occurs with one set of homologs Sister chromatids separate normally Aneuploidy results in too many or too few n+1 = trisomy n-1 = monosomy Why Do Mistakes Occur? Age Egg development (oogenesis) Primary oocytes enter meiosis 1 during embryonic development Oocytes arrest in prophase 1 till sexual maturity is reached Don’t complete meiosis till ovulation Why Does Meiosis Exist? Sexual reproduction is uncommon, but offspring may lack deleterious alleles Asexual reproduction is more efficient and produces more offspring, but a damaged gene will be inherited Offspring genetically identical to parents are less likely to survive if environment changes Plants produce more seeds when they reproduce asexually than sexually. Yet most plants reproduce sexually in nature. What is the probable explanation for the prevalence of sexual reproduction? Sexual reproduction…Mixes up alleles contributing to variation in species The egg of a fly has 4 individual chromosomes. How many chromosomes are in a somatic cell of a fly? 8 – somatic cells have two times the number of chromosomes than gametes, multiply 4 by 2 to get the diploid number Activity Feature: Meiosis, Mitosis Number of cell divisions: 2, 1 Number of chromosomes in daughter cells compared to parent: ½, Same number Synapsis of homologs: Yes (prophase 1), No Spindle fiber attachment: Yes, Yes Number of crossing over events: 1 or more times per pair of homologous chromosomes, None Make-up of chromosomes in daughter cells: Different, Same DNA content amount of daughter cells compared with parent: ¼, ½ Role in organism life cycle: Gamete production, Somatic cell growth/repair