1. (a)
(c)
(b)
(d)

2. Making Copies and Making Babies
Mitosis & Meiosis
Making Copies and Making Babies

3. Meiosis
What would happen if sexually reproducing organisms didn’t reduce their chromosome number prior to conception?
Parent with 1 chromosome would have offspring with 2 chromosomes
Next generation would have 4 chromosomes, next would have 8, then 16, then 32, 64, 128, 256, 512…

4. Meiosis
Gamete cells (sperm & egg) have only half the chromosomes of their parent
Somatic cells (not gametes) are diploid
2 copies of each chromosome (one from each parent)
called “homologous” chromosomes

5. Homologous Chromosomes
have genes for the same traits in the same location
The version of those genes may be different (e.g. red hair from mom and brown from dad) but still determine the same trait (hair colour)

6. Homologous Chromosomes
centromere
sister chromatids

7. Homologous Chromosomes
position of a gene on a chromosome is called its locus (plural = loci)
chromosomes of a homologous pair carry genes for the same trait at the same locus

8. Genes & Alleles
genes for a specific trait on each homologous chromosome might not be the same
different forms of the same gene are called alleles

9. Diploid vs. Haploid
2 copies of each chromosome (one from mom and one from dad)
Only 1 copy of each chromosome i.e. gametes
Meiosis creates haploid gametes

10. Stages of Meiosis similar to mitosis but some significant differences
broken down into Meiosis I and Meiosis II
In Meiosis I, cells become haploid
In Meiosis II the sister chromatids separate into haploid daughter cells

11. Before Meiosis I
During interphase the cell replicates its genetic information (just like in mitosis)
This is referred to as the pre-meiotic S phase
replicated chromosomes are joined by a centromere to which the spindle fibers attach

12. Meiosis I Meiosis I is broken down into the following stages
Prophase I
Metaphase I
Anaphase I
Telophase I & Cytokinesis

13. Meiosis II Meiosis II is broken down into the following stages
Prophase II
Metaphase II
Anaphase II
Telophase II & Cytokinesis

14. Prophase I Similarity to mitosis:
the chromosomes condense & the nuclear membrane dissolves

15. Prophase I Different from mitosis: homologous chromosomes pair up
known as “synapsis”
forms a tetrad
Non-sister chromatids exchange pieces of themselves in a process called “crossing-over” or genetic recombination
allows for genetic variation to exist in a species

16. Crossing Over

17. Metaphase I homologous chromosomes are attached
this pair moves together to the metaphase plate at the equator of the cell

18. Anaphase I
homologous chromosomes are each pulled by their centromere to opposite poles of the cell
sister chromatids remain attached

19. Telophase I & Cytokinesis
Homologs are now separate
Each pole is now haploid
Each chromosome still has 2 chromatids
Cytokinesis occurs simultaneously
result is two haploid daughter cells, each with half the number of chromosomes of the parent cell

20. Interphase? Some cells may have no interphase after meiosis I
Some cells may have a short one
Some cells may have a very long period of dormancy before meiosis II occurs
In any case, there is NO replication of DNA! (No S-phase)

21. Prophase II spindle begins to appear again
Spindle fibers attach to chromosomes at their centromere and begin to move to the metaphase plate

22. Metaphase II
chromosomes line up on the metaphase plate or equator (Just like mitosis!)

23. Anaphase II Centromeres of sister chromatids finally separate
sister chromatids of each pair are now independent chromosomes
sister chromatids move toward opposite poles

24. Telophase II & Cytokinesis
Nuclei begin to form at opposite poles of the cells
Chromosomes unwind and elongate
Cytokinesis occurs simultaneously
End result of the entire process is 4 daughter cells with half (haploid) number of chromosomes

27. Meiosis Animation