Unit 5: Mendelian Genetics

Content Outline: MeiosisProcess (5.4)

  1. Human Life Cycle is Diploid Majority.

A.Somaticcells– These make up most of our body. (“Soma” means “body”.)

1.These cells possess 46 chromosomes inside them. (They are 2n –diploid.)

2.Karyotypes will display all 46 chromosomes. A karyotype is basically pictures of the chromosomes. (“Kary” refers to “nucleus”.)

3.Homologous (“same”) Chromosomes can be seen. (These are called Autosomes.) 44 = 22pairsexist in all human cells. If female, the two sex chromosomes are the same … two X chromosomes.

  1. Heterologous (“different”) Chromosomescanbe seen in males. These may be the 2sex chromosomes. In males, there is one X and one Y chromosome. Female (XX); Male (XY).
  1. Germcells (A.K.A. gametes) – These are your sex cells. (“Germ” means “beginning”.)
  2. They are n – haploid. (Egg-come from females; sperm- comes from males.)
  3. Fertilization, which is the fusion of egg and sperm together, must occur to be able to reproduce sexually.
  4. This fusion between egg and spermproduces a single diploid cell called a zygote.
  5. The zygote, through repeated mitosis, produces the new organism.
  1. Meiosis - means “The process of Gamete Formation”.
  2. Meiosis occurs in the sex organs of the organism. Sex organs are called Gonads.
  3. This process has 1 DNA replication followed by 2 cell divisions; therefore, the result can be4 haploid cells.
  4. Remember, that the S phase, starts with a diploid cell and thendoubles the amount of DNA. In humans,when all 46 chromosomes are replicated the parent cell has 46 original and 46 replicated chromosomes, twice the DNA of a non-dividing cell. “Chromosome” becomes “chromatid” to represent the fact it was replicated.
  5. Meiosis I- This division is the separation of 46 homologouspairs. This takes the cell to 46 chromosomes, some are original, some are replicated, but most are now different due to crossover.
  6. Meiosis II- This division is the separation of sister chromatids. In humans, 46  23 chromosomes.
  7. In this process, Males produce 4 haploid sperm with each having 23 chromosomes.
  8. In this process, Females produce 1 haploid egg with 23 chromosomes. The other three cells degrade.
  9. Stages in the process of Meiosis:
  10. These stages are very similar to the stages of Mitosis.
  11. Three major differences from Mitosisare present to increase variation: (Remember, Mitosis is normal cell division. It basically makes clones of the adult. No variation exists.)
  12. Crossover(“genetic swapping”) occurs in Prophase I. (Creates variation.)
  13. Chromosomepairs independently sort as they line up in Metaphase I (Creates variation.)

c. Sister Chromatids separate in Anaphase II. (Creates Variation.)

  1. Crossover (“genetic swapping”) between homologous chromatids.
  2. This occurs to create variation from the parent’s genome. (They are then called Recombinant Chromatids.)
  3. Synapsis – Chromatids are in a state of being intertwined together. (“Syn” means “together”; “sis” means “process of”.)
  4. Tetrad - Four chromatids twisted together. (“Tetra” means “four”… Like the game Tetris has four different shapes.)
  5. Chiasmata – Where the chromatidsphysically overlap making an “x”. (“Chi” is the Greek letter for “X”.)
  1. Major differences between Mitosis and Meiosis:
  2. The number of divisions. (Mitosis has 1; Meiosis has 2.)
  3. The final products of each process. (Mitosis – “cloned” [identical] daughter cells; Meiosis – variable haploid gametes.)
  4. Crossover, in Prophase I of Meiosis, creates variation. (No crossover in Mitosis.)
  1. Evolution
  2. The genetic variation within individuals, produced during meiosis (both prophase – crossing over, and metaphase – segregation and independent assortment), along with random fertilization (the recombination of two haploid genomes) between individuals emerges as genotypic and phenotypic variety within a population. Natural selection seizes upon this variety, and drives the adaptation of a population over time.