Sexual Reproduction & Meiosis
A. Sexual Reproduction
• 1. Asexual reproduction needs only one
parent
• 2. Sexual reproduction needs two sex cells
– a. Sperm--male sex cell
– b. Egg--female sex cell
B. Importance of Sex Cells
• 1. In somatic cells, chromosomes are 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 reproduction 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: double stranded
chromosomes and spindle fibers appear;
nuclear membrane and nucleolus fade
• 3. Metaphase I: pairs of chromosomes line
up
– spindle fibers attach to centromeres and
centrioles
• 4. Anaphase I: pair of chromosomes
separate
• 5. Telophase I: cytoplasm divides and 2
cells form
• 6. Prophase II: chromatids and spindle
fibers reappear
• 7. Metaphase II: chromosomes 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)
Another Way Meiosis Makes Lots of Different Sex Cells –
Crossing-Over
Crossing-over multiplies the already huge number of different gamete types
produced by independent assortment.
One Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –
Independent Assortment
Independent assortment produces 2n distinct gametes,
where n = the number of unique chromosomes.
In humans, n = 23 and 223 ≈ 8,000,0000.
That’s a lot of diversity by this mechanism alone.