Cellular Division Mitosis Meiosis

Chromosomes Humans have 23 pairs of chromosomes 1 set inherited from mother 1 set inherited from father

Chromosome DNA is called chromatin when in cell during normal functioning of cell – cannot discern anything except nucleolus DNA replicates When cell divides, DNA condenses into tight “X” shape, one side the original and the other side copied DNA

Mitosis – Somatic Line Mitosis

Mitosis Stages - Interphase Cell spends most of its life in interphase (G1) Signaled to replicate DNA (S phase)replicate Cell resumes normal functions with replicated DNA (G2)

Mitosis Stages - Prophase DNA condenses Spindles form (they organize and guide DNA as it separates to each side of the cell Nucleus begins to dissolve Spindle fibers attach DNA

Mitosis Stages - Metaphase DNA lines up along equator of cell

Mitosis Stages - Anaphase Spindle fibers attached to DNA shorten and DNA moves to one side of the cell Notice that arms of the chromosome separate

Mitosis Stages - Telophase Cell begins to pinch in along the equator of the cell Nucleus begins to form Mitosis

Mitosis Stages – Interphase – 2 cells Cell spends most of its life in interphase (G1) Signaled to replicate DNA (S phase) Cell resumes normal functions with replicated DNA (G2)

Meiosis – Germ Line Sexual Reproduction In humans – 23 pairs of chromosomes, or a total of 46 chromosomes  1 chromosome from mother  1 chromosome from father Meiosis

Meiosis Stages - Interphase Cell spends most of its life in interphase (G1) Signaled to replicate DNA (S phase) Cell resumes normal functions with replicated DNA (G2)

Meiosis Stages – Prophase I DNA condenses Replicated DNA from adjoining chromosomes crossover Spindles form (they organize and guide DNA as it separates to each side of the cell Nucleus begins to dissolve Spindle fibers attach DNA

Meiosis Stages - Metaphase I Replicated chromosomes of DNA lines up along equator of cell

Meiosis Stages - Anaphase I Spindle fibers attached to chromosome DNA shorten and one of the pairs of chromosome of DNA moves to one side of the cell – or the other Chromosome number in humans – 23 – no longer in pairs

Meiosis Stages – Telophase I Cell begins to pinch in along the equator of the cell Division not yet complete NOTE – cells now only have half the chromosome number

Meiosis Stages - Prophase II Spindle fibers form and attach to DNA

Meiosis Stages - Metaphase II DNA lines up along equator of cell

Meiosis Stages - Anaphase II Spindle fibers attached to DNA shorten and DNA moves to one side of the cell Notice that arms of the chromosome separate

Meiosis Stages - Telophase II Cells begins to pinch in along the equator of the cell Nucleus begins to form

Meiosis Stages – Interphase – 4 cells Cell spends most of its life in interphase (G1) Signaled to replicate DNA (S phase) Cell resumes normal functions with replicated DNA (G2)

Fertilization (Humans) Egg with 23 chromosomes Sperm with 23 chromosomes Fertilization – zygote with 23 PAIR of chromosomes! TA DA! (Sea Urchins) TA DA!

Questions? BJ Shaw Panochthus frenzelianus American Museum of Natural History 2004