Control of Cell Division
• 1. Checkpoints
a. Cell growth checkpoint (GI)
b. DNA synthesis Checkpoint (G2)
c. Mitosis checkpoint (leave mitosis)
2. Uncontrollable cell division is known as
cancer.
Meiosis Notes
The Purpose of Meiosis
• To cut # of chromosomes in half to create
reproductive cells called gametes.
• gametes are eggs and sperm =23 chromosomes
each(haploid)
• Meiosis occurs only in testes and ovaries of
humans
• sperm fertilizes the egg it is called a zygote and
the zygote has 46 chromosomes(diploid)
Like a
Deck of cards
Hearts: Girl gamete
Clubs: Boy gamete
Cell Division Preparation
• Go through Cell Cycle
– Have duplicate set of chromosomes from S
phase
Before they enter Meiosis they are diploid (2n)
Two phases of Meiosis
• Meiosis I
• Meiosis II
Meiosis I
• Prophase I
–
–
–
–
–
–
DNa Coils into chromosomes
Spindle fibers appear from centrosomes
Nucleus breaks down
Synapsis- homologous chromosomes line up
Tetrad ( pairs of homologous chromosomes)
Tetrads Meet, Hug and then exchange Emails
• Match up Genes
• Chromosomes twist around each other- Chiasmata
• Swap Genes –Crossing Over
Metaphase I
• Tetrads Line up in the middle to get
picked by spindle fibers (polar and
kinetechore)
Anaphase I
• Homologous Chromosomes are randomly
moved to the poles
• Separation of Maternal and Paternal
Chromosomes leads to Independent
Assortment (genetic recombination)
Telophase I
• Chromosomes are on opposite sides of
cell and divides
• After Cytokinesis Cells are Haploid!
Meiosis II
• Sometimes the new cells from meiosis I
have nuclear membranes sometimes the
don’t
• (Sometimes you feel like a nuc sometimes
you don’t!)
Meisosis II –here we go again!
• Prophase II –Spindle fibers move
chromosomes
• Metaphase II- chromosomes in the middle
• Anaphase II- Chromatids separate
towards poles
• Telophase II- nucleus forms, cytokinesis
• End with 4 haploid cells
• Humans
– Spermatogenesis makes spermatids
– Oogenesis makes eggs
• Only 1 out of the four eggs made become fully
developed
• Others degenerate because lack of cytoplasm
Asexual Reproduction
• Does not use Mitosis
• Examples
– Binary Fission in Bacteria
– Budding in plants and other organisms
– Identical to parent
Sexual Reproduction
• Offspring are different from parent
• Genes combined in new ways from
Meiosis
• Evolutionary Advantage because adapt to
new conditions rapidly
• The End of Chapter Eight
• Yayyyyyy!!!!