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Sexual Reproduction & Meiosis
A. Sexual Reproduction
• 1. Asexual repro. needs only one parent
• 2. Sexual repro. needs two sex cells
– a. Sperm--male sex cell
– b. Egg--female sex cell
B. Importance of Sex Cells
• 1. In body cells, chromosomes found in
– a. Pairs form because chromosomes are alike
– b. Cell with 2 of every kind of chromosome is
– c. Sex cell is haploid--23 in egg or sperm
B. Importance of Sex Cells
• 2. Sexual repro starts with sex cell & ends
with fertilization
– a. Zygote is formed--in human now is diploid
or 2n with 46 chromosomes
C. Meiosis--Sex Cell Formation
• 1. In meiosis, there are 2 divisions of the
nucleus: meiosis I & meiosis II
• 2. Prophase I: The tetrad--double stranded
chromosomes and spindle fibers appear;
nuclear membrane and nucleolus fade,
crossing over occurs during prophase I
• 3. Metaphase I: chromosome pairs
(chromatids) line up
– spindle fibers attach to centromeres and
• 4. Anaphase I: chromotids separate from
matching pair
• 5. Telophase I: cytoplasm divides and 2
cells form
• 6. Prophase II: chromatids and spindle
fibers reappear
• 7. Metaphase II: chromatids line up in the
center of the cell
– spindle fibers attach to centromere & centriole
• 8. Anaphase II: centromere divides
– chromosomes split and move to opposite poles
• 9. Telophase II: spindle fibers disappear
– nuclear membrane forms around chromosomes
at each end of cell
– each nucleus has half the # of chromosomes as
the original (haploid)
– now there are 4 sex cells (daughter cells)
Meiosis Animation
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