Transcript Oh! MEIOSIS
Oh! MEIOSIS • Cell division in which one DIPLOID CELL produces four HAPLOID CELLS called sex cells or gametes • Meiosis is reduction division; the cells produced contain half the number of chromosomes as before meiosis • Meiosis occurs in two steps. During meiosis I, the chromosome pairs are divided between the two cells produced. During meiosis II, the two sister chromatids are segregated into each daughter cell Stages Of Meiosis! Genes and Chromosomes • Organisms have tens of thousands of genes! • A thousand or more are arranged on each chromosome • Chromosome number of a species is not related to the complexity or the organism Chromosomes in different Organisms: 1400 1200 1000 Human Garden Pea Dog Chimpanzee Apple Indian Fern 800 600 400 200 0 1st Qtr Diploid Time: • A cell with 2 of each kind of chromosome • 2n • Organisms have 2 factors (or alleles) for each trait • One allele is located on each of the paired chromosomes Happy Haploids: • A cell with one of each kind of chromosome • n • Organisms produce gametes that contain one of each kind of chromosome • Parent organisms give one factor (or allele) for each trait to each of their offspring • Together, the 2 chromosomes of each pair in a diploid cell help determine what the individual looks like • These paired chromosomes are called H.C. • Have genes for the same traits arranged in the same order • However, because there are different possible alleles for the same gene, the 2 chromosomes in a homologous pair are not identical to each other Homologous Chromosomes Genes on a chromosome Interphase • Remember that in Interphase the cell carries out its usual metabolic activities and replicates its chromosomes • Just hanging out doing its cell thing Prophase 1 • Chromosomes coil up and a spindle forms • Each pair of homologous chromosomes comes together to form a four-part structure called a tetrad Tetrads Unite! • Consists of 2 homologous chromosomes, each made up of 2 sister chromatids • The chromatids in a tetrad pair tightly • In fact, they pair so tightly that non-sister chromatids from homologous chromosomes sometimes actually exchange genetic material in a process known as Crossing Over Crossing Over • Results in new combinations of alleles on a chromosome • This is why there are so many unique folks on this planet! Metaphase 1 • Tetrads line up in the midline of the spindle • Unique to meiosis • Homologous line up together in pairs Anaphase • Homologous chromosomes separate and move to opposite ends of the cell • Centromeres don’t split like in mitosis • Ensures that each new cell will receive only one chromosome from each homologous pair Telophase 1 • Spindle is broken down and the chromosomes uncoil • Cytoplasm divides to form 2 new cells (each cell only has half the genetic information of the original cell because it has only one chromosome from each homologous pair • However, another cell division is needed because each chromosome is still doubled, containing 2 identical sister chromatids OH Meiosis II! • Meiosis 2 is similar to mitosis. However, there is no "S" phase (Chromosomes don’t replicate • The chromatids of each chromosome are no longer identical because of recombination • Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid Prophase II • Spindle forms in each of the 2 new cells • Chromosomes are still made up of sister chromatids Metaphase II • Chromosomes line up at the equator or middle Anaphase II • Centromeres of each chromosome splits • This allows the sister chromatids to separate and move to opposite poles Telophase II • • • • Nuclei re-form Spindles break down Cytoplasm divides (These are the same events as in mitosis) Meiosis is Finally Over! • Four haploid cells have been formed from the original diploid cell • Each haploid cell contains one chromosome from each homologous pair • These haploid cells will become gametes, transmitting the genes they contain to offspring! Difference Between Mitosis and Meiosis!