Meiosis Why do we look different from our parents? Where do we begin? What type of cells does mitosis take place in? What type of cells does Meiosis take place in? What is a chromosome? http://learn.genetics.utah.edu/content/begin /tour/index.htmlhttp://learn.genetics.utah.edu/content/begin /tour/index.html Meiosis A Source of Distinction Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). In meiosis, one diploid cell produces four haploid cells. 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. Meiosis a cell division forming gametes Goal: reduce genetic material by half. Why? from momfrom dadchild meiosis reduces genetic content too much! Karyotype Replication of chromosomes Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere What phase is this called?Interphase Meiosis: cell division in two parts Homologs separate Sister chromatids separate Result: one copy of each chromosome in a gamete. Diploid Meiosis I (reduction division) Meiosis II (equational division) Haploid A replicated chromosome Homologs separate in meiosis I and therefore different alleles separate. homologs same genes maybe different alleles sister chromatids same genes same alleles Gene X Meiosis I : the reduction division Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Nucleus Spindle fibers Nuclear envelope Prophase I Early prophase Homologs pair-up. Crossing over occurs. Late prophase Chromosomes condense Chromosomes form a tetrad. Spindle fibers form. Nuclear membrane fragments. Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Creates chromosomes with new combinations of alleles for genes A to F. a b c d e f A B C D E F A B C D E F a b c d e f Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Letters denote genes Case denotes alleles Creates chromosomes with new combinations of alleles for genes A to F. A B C D E F a b c d e f c d e f A B a b C D E F Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Letters denote genes Case denotes alleles Creates chromosomes with new combinations of alleles for genes A to F. A B C D E F a b c d e f c d e f A B a b C D E F Metaphase I 1.) Tetrads line-up in middle of cell 2.) Spindle fibers attach to centromere region of each homologous chromosome pair Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain Attached at their centromeres. Telophase I Nuclei reform. Spindle fibers break down. Cytokinesis divides cell into two. Meiosis II : the equational division Prophase IIMetaphase IIAnaphase IITelophase II Four nonidentical haploid daughter cells Prophase II Nucleus breaks down. Centrioles move to the poles Spindle fibers form. Metaphase II Chromosomes align along equator of cell. Spindle fibers attach to the centromeres Anaphase II Sister chromatids separate and move to opposite poles. Telophase II Chromosomes unwind. Nuclei reform. Spindle fibers break down. Cytokinesis occurs. Results of meiosis Gametes Four haploid cells One copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome Oscar Winning Meiosis Click on image for link to animation More Meiosis hill.com/sites/ /student_view0/chapter3/animation__ how_meiosis_works.htmlhttp://highered.mcgraw- hill.com/sites/ /student_view0/chapter3/animation__ how_meiosis_works.htmlations/content/meiosis.htmlhttp://www.sumanasinc.com/webcontent/anim ations/content/meiosis.htmlorials/meiosis/page3.htmlhttp://www.biology.arizona.edu/CELL_BIO/tut orials/meiosis/page3.html Meiosis A Source of Distinction Meiosis does two things - 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing- over. Genetic diversity is important for the evolution of populations and species. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. Thats a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 = 6,000, begin/tour/index.htmlmations/content/independentassortment.html Another Way Meiosis Makes Lots of Different Sex Cells Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment. Between Independent Assortment and Crossing-Over, No Two Gametes Are Identical. The Mechanics of Meiosis Following a Single Chromosome Pair Crossing over Mitosis vs. Meiosis This step is the key difference Independent assortment occurs at this step The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis MitosisMeiosis Number of divisions 12 Number of daughter cells 24 Genetically identical? YesNo Chromosome #Same as parentHalf of parent WhereSomatic cellsGermline cells WhenThroughout lifeAt sexual maturity RoleGrowth and repairSexual reproduction Gametogenesis formation of gametes 1) Spermatogenesis 2) Oogenesis Spermatogenesis: sperm formation Oogenesis Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid) Polar bodies die Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Mature egg A X A X Oogenesis: ovum formation One of four meiotic products becomes an ovum. The three remaining meiotic products die. Oogenesis versus Spermatogenisis Fertilization Fertilization is the joining of SPERM (n) and EGG or OVUM (n) to form a Zygote (2n). Chromosomal Mutations Mutations Deletion Occurs when part of a chromosome is left out Most are lethal Mutations Insertion Segment of a chromosome is removed and inserted into another one Mutations Duplication Segment of DNA is copied twice Mutations Inversion Occurs when part of a chromosome breaks off and is reinserted backwards Mutations Translocation Occurs when segments of DNA on 2 chromosomes are rearranged