II. Sexual Reproduction & Meiosis
By Addy Lundberg
John Swett High School
Crockett, CA
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
pairs– a. Pairs form because chromosomes are alike– b. Cell with 2 of every kind of chromosome is
diploid– c. Sex cell is haploid--23 in egg or sperm
B. Importance of Sex Cells
• 2. Sexual repro starts with cex 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: double stranded c’somes and spindle fibers appear; nuclear membrane and nucleolus fade
• 3. Metaphase I: chromosome pairs (chromatids) line up– spindle fibers attach to centromeres and
centrioles
• 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)
Diploid (2n)
Haploid (n)