1. If parents have two copies of each chromosome, how do they pass on only one to their offspring?
MEIOSIS is the process of cell division that decreases the number of chromosomes to make sperm or egg cells
Original cell
Sperm or egg cells

2. The purposes of Meiosis:
Split the total number of chromosomes in half so that only ONE chromosome from each pair is inherited from each parent.
Produce variation in offspring by randomly sorting the chromosomes.

3. Meiosis Diploid (2n) (n = # of pairs of chromosomes) Haploid (n)
Original cell
Diploid (2n)
(n = # of pairs of chromosomes)
Sperm or egg cells (GAMETES)
Haploid (n)
(single chromosomes)

4. Fertilization Gametes: Zygote: (haploid sex cells) (diploid embryo)
Sperm
Egg
Gametes:
(haploid sex cells)
Zygote:
(diploid embryo)
Fertilized egg

5. The Cell Cycle – “life” of the cell
4 stages in cell life: G1 – Growth, making proteins, doing work S – Synthesis – making a copy of DNA G2 – Growth (more) getting ready for cell division M – Mitosis or Meiosis – cell division

6. Mitosis Produces two identical daughter cells
Each daughter cell has the same kind and number of chromosomes as the original parent cell
Mitosis takes place AFTER DNA replication

7. Interphase (Resting stage)
Chromosomes cannot be seen
Chromosomes duplicate and double in number
How do the chromosomes duplicate?

8. nuclear membrane disappearing
Prophase
nuclear membrane disappearing
chromatids
centromere
Chromosomes become visible
Each chromosome consists of two identical chromatids
homologous chromosomes

9. Metaphase Centrosome (made of 2 centrioles)
Chromosomes line up at the equator (centre of the cell)
Spindles are formed to attach to the centromere of each chromosome

10. Anaphase Sister chromatids separate as individual chromosomes
They move apart towards the opposite poles

11. nuclear membrane forming
Telophase
nuclear membrane forming
Chromosomes gradually disappear
Nuclear membrane is formed around each set of chromosomes

12. Can you identify the different stages of mitosis from the diagram below?
anaphase
interphase
telophase
metaphase
prophase

13. Meiosis
Same steps as Mitosis: Prophase, Metaphase, Anaphase, Telophase
Meiosis happens TWICE!!!
Metaphase I, Anaphase II, etc.

14. Some Vocab
Homologous Chromosomes: chromosomes that contain the same genes, but may have different alleles (one is from mom, one is from dad)
Sister Chromatids: identical copies of a single chromosome connected by a centromere

15. Meiosis 1

16. Close-up of Metaphase I
Crossing over: segments of DNA are exchanged between homologous chromosomes

17. Law of Independent Assortment
It is random where the chromosomes line up! Mom can be either side, Dad either side.

18. Meiosis II

19. Close up of Metaphase II

20. Summary of Meiosis
Segregation of alleles: one chromosome in each of the 4 cells: only one copy of the allele
Creates 4 haploid daughter cells
Meiosis Video

21. Comparison of Mitosis and Meiosis

22. Mitosis vs. Meiosis Mitosis Meiosis Purpose
To produce new cells for 1) growth 2) replacement of old or damaged cells
To produce new gametes for sexual reproduction
Start with
1 Diploid somatic cell (i.e., skin cells, liver cells, muscle cells, lung cells, etc.)
1 Diploid germ cell
End with
2 identical Diploid somatic cells
4 haploid gametes
Number of cell divisions that occur 1 2

23. Demonstration of mitosis and meiosis

24. Mendel’s Laws related to Meiosis
Law of Segregation
Law of Independent Assortment

25. Pop bead modeling of meiosis
Follow directions from teacher
Use pop beads to model each step in animation with your teacher
Draw each step on your worksheet
Meiosis step 1
Meiosis step 2
Meiosis step 3