MeiosisReduction Division

Post Falls High SchoolScience Department

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 called diploid– c. Sex cell is haploid--23

chromosomes in egg or sperm

B. Importance of Sex Cells

• 2. Sexual repro starts with sex cell & ends with fertilization– a. Zygote is formed in fertilization--in

humans it is diploid or 2n with 46 chromosomes

Meiosis occurs in reproductive organs only; not in somatic (body)

cellsMeiosis is called “reduction

division” because the chromosome number is reduced to half the

normal number for that particular species; from “diploid” (46 in

humans) to “haploid” (23) in the gametes (sex cells)

See Chapter 10 (Honors) or 11 (General Biology) to

discover the value, importance, and advantage organisms gain by meiosis and sexual reproduction

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 but no duplication of chromatids occurs

• 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; cytokinesis occurs

– each nucleus has half the # of chromosomes as the original (haploid)

– now there are 4 sex cells (daughter cells)

So, all meiosis occurs identically, right?

Actually, it is different in the male and female

processes!

Check out the differences between spermatogenesis in males and oogenesis in

females

Spermatogenesis

Click below for an animation. Read the intro, view the

animation, read the conclusion, and take the quiz.

Meiosis

Why does the meiotic process produce so many more spermatozoa than ova?

For a challenge, respond to the questions on the following site:

Some Questions for You