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)