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Meiosis Notes
A. What is Meiosis?
I. Purpose: to create sex cells called
gametes.
a. Female gamete= egg (ovum)
b. Male gamete= sperm
II. How are gametes different from normal cells?
a. Mitosis creates more somatic cells that contain 46
chromosomes.
i. 23 pairs of homologous chromosomes
1. homologous: 1 from mom and 1 from dad, both
contain the same genes.
ii. Mitosis creates cells that are Diploid (2n)
1. Diploid = 2 of each chromosome
b. Meiosis creates gametes that contain 23 chromosomes
i. Gametes are Haploid
1. Haploid= only contain 1 of each chromosome.
B. How do we become a Diploid Zygote?
a. Egg + Sperm = Zygote
i. Fertilization = process of forming a zygote
C. Phases of Meiosis
a. Meiosis produces 4 genetically DIFFERENT daughter cells.
b. Interphase = same as mitosis
c. MEIOSIS I
i. Prophase I:
- pairing of homologous
chromosomes occurs to form
tetrads. This is called “synapsis”
- nuclear envelope breaks down
- crossing over occurs
ii. Metaphase I:
- Spindle fibers attach to the
centromere of the chromosomes
- Homologous chromosomes line up
at middle of cell.
iii. Anaphase I:
- Homologous chromosomes
separate and move to opposite
poles
iv. Telophase I
- The spindles break down.
- Chromosomes uncoil and
form two nuclei.
- The cell divides.
d. MEIOSIS II (Similar to the steps of mitosis accept with only 23
chromosomes instead of 46)
i. Prophase II
- Spindle apparatus forms
- Chromosomes condense
ii. Metaphase II
- Haploid number of
chromosomes line up at
middle of cell.
iii. Anaphase II
- The sister chromatids are
pulled apart at the
centromere by spindle
fibers
- Move toward the opposite
poles of the cell.
iv. Telophase II
- The chromosomes reach
the poles, and the nuclear
membrane and nuclei
reform.
v. Cytokinesis
- Cytokinesis results in
four haploid cells
- Each with n number of
chromosomes.