Two Types of Diploid Cells In Sexual Organisms Somatic GERM • Body Cells • Diploid • Divide by mitosis to make exact copies • In every tissue except reproductive tissue • “Germinating” (reproductive) cells • Diploid • (Most) Divide by meiosis to make gametes (sex cells) (sperm & ova) • In testes & ovaries
What is an “n”? Haploid (n) is the number of chromosomes in a gamete. Diploid (2n) – number of chromosomes in a somatic cell. You have one copy from the mother, and one copy from the Father.
In Meiosis: ONEdiploid, germ cell divides to become FOUR (4) daughter cells • (*circle*) … diploid / haploid • (*circle*) … somatic / gamete
In order to go from diploid to haploid, Meiosis Involves Two Divisions Meiosis I Meiosis II • Splits homologous pairs of chromosomes • Splits sister chromatidsapart
Time To Name The Phases • PROPHASE I • METAPHASE I • ANAPHASE I • TELOPHASE I • PROPHASE II • METAPHASE II • ANAPHASE II • TELOPHASE II
Make 2 Tables TABLE I
Make 2 Tables TABLE II
PROPHASE I • Nuclear envelope disintegrates • Centrioles migrate, produce spindle fibers • Homologous chromosomes pair up • Homologous pairsCROSS OVER!
PROPHASE I CROSSING OVER Occurs when homologous chromosomes exchange segments of genes
METAPHASE I • Homologous pairs of chromosomes line up along metaphase plate
ANAPHASE I • Homologous pairs pulled apart to opposite poles
TELOPHASE I & CYTOKINESIS • Chromosomes (no longer homologous pairs!) prepare for meiosis II • Cytoplasm begins to split into first 2 (of 4) daughter cells
TELOPHASE I & CYTOKINESIS NOTE: At the end of meiosis I, daughter cells are already Haploid! (because homologous pairs were separated)
2 haploid daughter cells then go through MEIOSIS II In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!
PROPHASE II • Centrioles migrate, produce spindle fibers Just like in mitosis!
METAPHASE II • Chromosomes line up along metaphase plate Just like in mitosis!
ANAPHASE II • Sister chromatids pulled apart to opposite poles Just like in mitosis!
TELOPHASE II & CYTOKINESIS • Nuclear envelopes reform around 4 daughter cells • Cytoplasm splits to separate the 4 daughter cells • Each of the 4 daughter cells is a new gamete withgenetic information different from either parent!
Is This Meiosis I or Meiosis II? Why?
Meiosis II Meiosis I Check – Point: At what point do the cells become haploid?
200 to 400 million sperm per ejaculate! But what happens to the other 3 eggs? The largest ovum absorbs the others! Only one wins per cycle, usually
Finally, Fertilization • Male and female gametes unite • Fusion of two haploid nuclei produces diploid nucleus • Egg + Sperm = Zygote(new life in one diploid cell)
Meiosis and Sexual Reproduction Guarantee Variation and Diversity! In Three Ways • Independent Assortment • Homologous Chromosomes separate randomly • Crossing Over • Homologous pairs exchange gene segments • Random Fertilization • Which sperm meets which egg? (game of chance)