1. KEY CONCEPT Gametes have half the number of chromosomes that body cells have.

2. You have body cells and gametes.
Body cells are also called somatic cells.
Gametes are sex cells: egg and sperm.
Gametes have DNA that can be passed to offspring.
body cells
sex cells (sperm)
sex cells (egg)

3. Your cells have autosomes and sex chromosomes.
Your body cells have 23 pairs of chromosomes.
Chromosome pairs #1-22 are autosomes.
Sex chromosomes, X and Y, determine gender in mammals.
Homologous pairs of chromosomes have the same structure.
Homologous chromosomes are similar but not identical.
For each homologous pair, one chromosome comes from each parent.
homologous chromosomes
sister
chromatids

4. Haploid (n) cells have one copy of every chromosome.
Gametes are haploid
The production of gametes is called gametogenesis
Genesis = formation

5. Body cells are diploid; gametes are haploid.
Fertilization occurs when gametes fuse during sexual reproduction
Diploid (2n) cells have two copies of every chromosome.
Somatic cells are diploid.
Half the chromosomes come from each parent.

6. Check for understanding
Which type of cell is haploid?
a. Skin cell
b. Sperm cell
Somatic cell
Liver cell

7. Check for understanding
The fusion of gametes is called
Fertilization.
Reassortment.
Meiosis.
Gametogenesis.

8. Meiosis makes haploid cells from diploid cells.
Meiosis occurs in reproductive germ cells (testes & ovaries)
Meiosis produces gametes with half the # of chromosomes as the parent cell.
Unlike the daughter cells of Mitosis, the daughter cells and parent cells are NOT IDENTICAL

9. Cells go through two rounds (Meiosis I & Meiosis II) of division in meiosis.
Meiosis I and meiosis II each have 4 phases, similar to those in mitosis
Meiosis I
Meiosis I occurs after DNA has been replicated.
Pairs of homologous chromosomes separate in four phases (just like in Mitosis-same names of stages)

10. Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I

11. Meiosis II divides sister chromatids* in four phases.
DNA is not replicated between meiosis I and meiosis II.
*remember a sister chromatid = copy of same DNA

12. Meiosis differs from mitosis in significant ways.
Meiosis has two cell divisions while mitosis has one.
In mitosis, homologous chromosomes never pair up.
Meiosis results in haploid cells; mitosis results in diploid cells.