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Meiosis Meiosis occurs in sexual reproduction when a diploid germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg). Meiosis Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right! Meiosis: cell division in two parts Sister chromatids separate Chromatids separate Meiosis I (reduction division) Meiosis II (equational division) Diploid 2n Haploid n Haploid n Result: one copy of each chromosome in a gamete. Meiosis I : the reduction division Spindle fibers Nucleus Nuclear envelope Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) 1st Period begins http://highered.mcgrawhill.com/sites/0072495855/student_view0/ chapter28/animation__how_meiosis_work s.html Prophase I Early prophase Late prophase Chromatids pair. Crossing over occurs. Chromosomes condense. Spindle forms. Nuclear envelope fragments. Metaphase I Chromatid homologous pairs align along the equator of the cell. This is random and results in Genetic variation Homologous – a chromosome with the same gene sequence as another, one is paternal and the other is maternal Metaphase I Homologous chromosomes cross over and genes may be exchanged. Crossing over results in genetic variation. Anaphase I Chromosomes separate and move to opposite poles. Sister chromatids remain Attached at their centromeres. Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. Meiosis II Gene X Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Meiosis II : the equational division Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four Non-identical haploid daughter cells Prophase II Nuclear envelope fragments. Spindle forms. Metaphase II Chromosomes align along equator of cell. Anaphase II Sister chromatids separate and move to opposite poles. Telophase II Nuclear envelope assembles. Chromosomes unravel. Spindle disappears. Cytokinesis divides cell into two. Results of meiosis • Four haploid cells • One copy of each chromosome “Putting It All Together” - Fertilization What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction. http://highered.mcgrawhill.com/sites/0072495855/student_v iew0/chapter28/animation__unique_ features_of_meiosis.html Nondisjunction – When chromosomes do not separate during meiosis. Chromosomal Mutation Extra or less chromosomeshttp://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::5 50::400::/sites/dl/free/0078695104/383925/ Chapter11_NGS_VisualizingNondisjunction _10_10_06.swf::Visualizing%20Nondisjunc tion 1st Period Begins Here Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Yes No Same as parent Half of parent Where Somatic cells Sex cells When Throughout life At sexual maturity Growth and repair Sexual reproduction Genetically identical? Chromosome # Role MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter Reduction division results in haploid (n) cells cells 2 Factors What causes genetic variations? Why are no to individuals alike? 2 Factors What causes genetic variations? Why are no to individuals alike? Animation: Crossing over, independent assortment. What causes genetic diversity? 1. CROSSING OVER. –TETRADS What causes genetic diversity? 1. CROSSING OVER. –TETRADS 2. RANDOM ASSORTMENT OF CHROMOSOMES. - MEIOSIS 3. GENE MUTATION 4. NONDISJUNCTION - FAILURE OF CHROMOSOMES TO SEPARATE - - - 2N +1, 2N - 1 5. FERTILIZATION – WHICH EGG AND SPERM JOIN FORM ZYGOTE? What causes genetic diversity? 2. RANDOM ASSORTMENT OF CHROMOSOMES. - MEIOSIS What causes genetic diversity? 3. GENE MUTATION What causes genetic diversity? 4. NONDISJUNCTION - FAILURE OF CHROMOSOMES TO SEPARATE - - - 2N +1, 2N - 1 What causes genetic diversity? 5. FERTILIZATION – WHICH EGG AND SPERM JOIN FORM ZYGOTE? What is the difference between a gene mutation and a chromosomal mutation? Gene mutations are usually with the nitrogen base, chromosomal mutations involve hundred’s of genes. What is non-disjunction? Failure of chromosomes to separate during meiosis. Trisomy 21 Down’s Syndrome When does crossing – over occur? Prophase I of Meiosis Is it meiosis or mitosis Power Point – Game- White Boards – Get a 2 sheets of paper – Write the word Mitosis (Write Big) on one and Meiosis on the other.