1. Making gametes for reproduction
MEIOSIS
Making gametes for reproduction
Diploid – two sets of chromosomes (humans = 46 chromosomes)
Haploid – one set of chromosomes (humans = 23 chromosomes)
Homologous chromosomes = chromosomes that are similar in size, shape and genetic content (one of a pair)
we get one homologue from each parent
Briefly review mitosis stages (I Promised My Aunt Teresa Cookies) using the teacher-directed coloring sheet
Ask: If a human sperm and egg each had 46 chromosomes, like other human body cells, how many chromosomes would a fertilized egg have? 92
Why might increasing the number of chromosomes in a human cell cause problems? Answers vary, may include that it would change the genetic makeup of humans
Meiosis is the process of cell division that results in gametes (sperm and egg) that have half of the number of chromosomes so that the joining of sperm and egg results in a zygote with the correct number of chromosomes

2. Meiosis
Type of cell division where ONE body cell produces FOUR gametes
Each gamete* contains half the number of chromosomes as a body cell
(body cell: 46gamete: 23)
*GAMETE- Male and female sex cells; sperm and egg
Any organism that reproduces sexually (male and female produce gametes that join to produce new individual) must undergo meiosis to produce those gametes

3. Why does Meiosis happen?
To reduce the chromosome number in the gametes to half of the original.
When sperm and egg join to form a zygote, there will be the correct number of chromosomes
23 (egg) + 23 (sperm) = 46 (zygote)
Meiosis has TWO divisions of the nucleus (meiosis I and meiosis II)

4. Meiosis Video Germ cells undergo meiosis (not all cells)
Haploid – chromosomes not paired (humans – 23)

5. Phase I of Meiosis 0) Interphase Chromosomes invisible; cell growing
Chromosomes replicate (DNA Synthesis)
Centrioles replicate (other organelles, too)
Interphase starts same as mitosis

6. Phase I of Meiosis Prophase I
Centrioles migrate to the poles / spindle fibers form
Chromosomes coil (now visible)
Nuclear Membrane disappears
Chromosomes (sister chromatids) pair up with another Chromosome to form a TETRAD
First three steps same as mitosis
Difference – Homologous chromosomes pair up with each other to form a Tetrad
Crossing over can occur were portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome
Demo crossing over with the pop beads on the board

7. Tetrad- consists of two HOMOLOGOUS chromosomes, each made up of two sister chromatids
Homologous chromosomes- paired chromosomes with genes for the same traits arranged in the same order
Crossing over occurs within the tetrads
Homologous chromosomes – one of the pair comes from father and other of the pair comes from the mother

8. Sometimes crossing over occurs – exchange of genetic material between non-sister chromatids to create new allele combination
* Allele – alternate forms of a gene for each variation of a trait
ex. Alleles for flower color – red, white, pink

9. Phase I of Meiosis 2) Metaphase I
Spindle fibers attach to the centromere of each chromosome (tetrads)
Spindle fibers pull tetrads to middle of the cell
(In mitosis each pair of sister chromatids line up alone, but in meiosis they line up in tetrads)
Lining up along the middle, but as tetrads rather than individual chromosomes
Homologous chromosomes remain together in meiosis, while in mitosis they line up independent of each other

10. Phase I of Meiosis 3) Anaphase I Spindle fibers shorten
Chromosomes (each with 2 sister chromatids) separate and move to opposite ends
(Each new cell will get only ONE chromosome from each homologous pair or tetrad)
Homologous chromosomes are pulled apart, but the centromere remains intact
each daughter cell will have a single chromosome, but the genetic material is still duplicated (two chromatids attached at a centromere)
No pattern to which of the two chromosomes ends up in each cell / random distribution of homologous chromosomes
(independent assortment)

11. Phase I of Meiosis 4) Telophase I Spindle fibers disappear
Chromosomes uncoilchromatin
Cytoplasm divides (Cytokinesis)
(Each cell has HALF of the genetic information, but chromosomes still made up of sister chromatids)
Process same as in mitosis, but result is different due to previous phase differences

12. Prophase I
Metaphase I
Telophase I
Anaphase I

13. Phase II of Meiosis 0) Interphase Cell grows a bit
Centrioles replicate, but chromosomes DO NOT replicate
In some cases, interphase between meiosis I and meiosis II does not exist
NO replication of chromosomes at this point

14. Phase II of Meiosis 5) Prophase II Centrioles move apart
Spindle fibers form and attach to chromosomes
Meiosis II is same process as mitosis, except that there are only single chromosomes (instead of pairs of chromosomes) involved

15. Phase II of Meiosis 6) Metaphase II
Spindle fibers pull the chromosomes to the middle of the cell (equator)

16. Phase II of Meiosis 7) Anaphase II Spindle fibers shorten
Chromosomes split (sister chromatids are pulled apart) and move to opposite poles

17. Phase II of Meiosis 8) Telophase II Nuclear membrane reforms
Spindle fibers disappear
Chromosomes uncoil (chromatin)
CYTOKINESIS occurs
End Result: 4 Haploid cells
(each cell has 23 chromosomes- not 46)

18. Prophase II
Metaphase II
Anaphase II
Telophase II

19. Importance of Meiosis Meiosis is important because
1. allows sexual reproduction to occur while maintaining the same number of chromosomes in the offspring
2. DNA of parents is resorted and recombined to produce unique individuals

20. Meiosis Video Quiz
As a results of meiosis, sex cells are formed that have ________ the number of chromosomes of body cells. Half
T/F Pairs of identical chromosomes are also called homologous chromosomes. True
T/F Normal body cells are diploid because they possess one of each different chromosome. False, two
T/F During meiosis, the DNA is replicated twice. False, once
T/F In plant and animal cells, meiosis occurs only in germ cells. True

21. References
Mitosis and Meiosis. Ancient Lights Discovery Education. 28 November
Meiosis pictures from