Meiosis: (Fig. 7-1)
Chp. 7 (pp. 140-157)
• Production of sex cells
• 1/2 the number of chromosomes

Link!-video
Gametesreproductive cells

Stem Cells
• A cell that can differentiate to become a specific type of cell
• Stem Cells become cells that can do mitosis or meiosis

Definitions for Meiosis
• Homologous Chromosomes-
– Chromosomes that are similar in size, shape and content
• Crossing Over-
– Portions of one chromatid are shared with a chromatid on another (Homologous
Chromosome)
- Occurs during meiosis

Definitions for Meiosis
• Haploid- containing one set of chromosomes
– 23 for humans (sex cells)
• Diploid- containing two sets of chromosomes
• Zygote- fertilized egg
– 46 chromosomes in humans
– 78 chromosomes in dogs
– Some plants have thousands

Meiosis contributes to Diversity
• Asexual Reproduction- a single parent passes copies of all of its genes to each of its offspring.
(Clones- not diverse)
– Ex: Binary Fission
• MORE GENETIC VARIATION = MORE DIVERSITY
• Sexual Reproduction- two parents, each donating a haploid reproductive cell
– IMPORTANT for increasing diversity

More diversity added by:
• Mutations- a change in the nucleotide sequence of a gene.
– substitution of bases (A replaced by C)
– adding bases (A added to gene)
– deleting bases (A removed from gene)

Prophase 1
• The chromosomes become visible
• Nuclear envelope breaks down

Metaphase 1:
• Pairs of homologous chromosomes move to the equator of the cell

Anaphase 1:
• Homologous chromosomes move to opposite poles of cell

Telophase 1:
• Chromosomes gather at the poles of the cell
• Cytoplasm divides

Prophase 2:
• A new spindle forms around the chromosomes

Metaphase 2:
• Chromosomes line up at the equator

Anaphase 2
 Centromeres divide.
 Chromatids are now called chromosomes
 Chromosomes move to opposite poles of the cell.

Telophase 2:
• Cells begin split
• Cell membrane forms between cells
• Four gamettes are formed

Comparison of Mitosis and Meiosis

46 Chromosomes (Diploid)-23 pairs

Trisomy 21 or Downs Syndrome
Extra Chromosome