1. Meiosis vs. Mitosis
Mitosis: period of nuclear division in which two daughter cells are formed, each identical to the parent cell

2. Meiosis vs. Mitosis
Meiosis: type of division in which four cells are formed, each with half the number of chromosomes as the parent cell

3. Terms to Know Body cells (diploid, 2n)
contain two of each kind of chromosome.
chromosomes are paired, one set from each parent
Paired chromosomes are called homologous chromosomes
Human body cell: 46 chromosomes/23 pairs

4. Terms to Know Gamete (haploid, n) Zygote Also called a “sex cell”
Contains one of each kind of chromosome (or half the number of a body cells)
Human sex cell (sperm or egg): 23 chromosomes
Zygote
the result of the fertilization of an egg cell with a sperm cell forming a diploid cell

5. Meiosis
fertilization is the process in which two haploid cells come together to form a unique diploid
This is the premise of sexual reproduction

6. Meiosis
Interphase: stage in which chromosomes duplicate themselves (same as mitosis)
Meiosis is then separated into two division stages
Meiosis I and Meiosis II

7. Phases of Meiosis

8. Meiosis I Prophase I Chromosomes coil and spindle forms
Each pair of homologous chromosomes and their sister chromatids come together to form a four-part tetrad joined together by a centromere

9. Meiosis I
Prophase I
During this phase, an exchange of genetic material may occur between two non-sister chromatids (crossing over)
Crossing over allows for more variation in sexual reproduction

10. Meiosis I Metaphase I Chromosomes become attached to spindle
Tetrads are pulled to the mid-line of the cell

11. Meiosis I
Anaphase I
Homologous chromosomes, along with their sister chromatids, separate

12. Meiosis I
Telophase I
Spindle breaks down
Chromosomes uncoil
Cytoplasm divides to form two new haploid cells
* Each cell has half the genetic information as the original parent cell, BUT the chromosomes are still doubled so another division must take place

13. Interkinesis
Period of rest that may, or may not, occur between Meiosis I and Meiosis II

14. Meiosis II
Prophase II
A spindle forms and the chromosomes in each cell attach
Sister chromatids are pulled to the center of the cell

15. Meiosis II Metaphase II
Sister chromatids line up on the mid-line of the cells

16. Meiosis II
Anaphase II
The centromere breaks allowing the sister chromatids to migrate to opposite ends the cells

17. Meiosis II Telophase II Nuclei reform Spindles break down
Cytoplasm splits

18. End of Meiosis
Meiosis produces FOUR haploid cells, each containing one chromosome from each homologous pair

19. Meiosis
genetic recombination: the reassortment of chromosomes and the genetic information they carry, providing a major source of genetic variation

20. How much variation? “n”: The number of homologous pairs in a cell
Human body cell contains 46 chromosomes (23 homologous pairs)
2n=46
Human sperm cell contains 23 chromosomes (one representative from each homologous pair) n=23
Human egg cell (same as sperm)
n=23

21. WOW!!!
Possible different kinds of egg or sperm cells created by a person = 2²³ or 8.4 million
Fertilization of one egg by one sperm results in
2²³ x 2²³ or 70 trillion possible zygote combinations
* This does not take into consideration possible crossovers that occur during prophase I

22. End Result---We are ALL truly unique

23. Does meiosis ever go wrong?
YES!
Nondisjunction occurs
when chromosomes do not
divide evenly during an
anaphase stage of meiosis
Result: a sex cell with an
extra chromosome, or
missing a chromosome

24. What’s the big deal?
In humans, zygotes that form from gametes with an abnormal number of chromosomes will often spontaneously abort.
Several disorders found in humans are a result of nondisjuntion
Down Syndrome (Trisomy 21)
Turner Syndrome (Monosomy X)
Klinefelter’s Syndrome (XXY)
Edwards Syndrome (Trisomy 18)

25. Are extra chromosomes EVER a good thing?
YES!
Polyploidy: A state in which a cell has more than two sets of chromosomes
Many plants (wheat, bananas, apples, strawberries, carnations) and some animals (goldfish, salamanders)
Plants, in particular, benefit greatly from extra chromosomal information