1. Vocabulary: Homologous Diploid Haploid Meiosis Tetrad Crossing-over
Key Concept:
What happens during the process of meiosis?
How is meiosis different than mitosis?
11-4 Meiosis

2. Vocabulary
Homologous pair—chromosome from mom and corresponding chromosome from dad
Diploid—Cells with two sets of chromosomes
Code for same genes (ex: hair color)
Haploid—Cells with one set
sex cells only (gametes)
These will join with sex cells from the opposite sex (ex: egg joins with sperm)

4. Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm.
Meiosis appears much more complicated than mitosis…
it is really just two divisions in sequence
each one of which has strong similarities to mitosis.
Meiosis

5. INTERPHASE Interphase in meiosis is identical to interphase in mitosis
There is no way, by simply observing the cell, to determine what type of division the cell will undergo when it does divide.
Meiotic division will only occur in cells associated with male or female sex organs.

6. Chromosomes
Genetic information is passed on from one generation to the next through chromosomes
Every organism has a specific number of chromosomes
ex #1: carrots = 18 chromosomes;
ex #2: humans = 46  23 came from MOM & 23 came from DAD
Chromosomes are not visible in most cells except in cell division

7. Why are Chromosomes only visible During Cell Division?
centromere
chromatin
DNA

8. MEIOSIS I
Meiosis I, the first of the two divisions, is often called reduction division
WHY???
it is here that the chromosome number is reduced from 2N (diploid) to 1N (haploid).
Ex: Humans: diploid number = 46 (haploid = 23)

9. PROPHASE I Prophase I is similar to prophase in mitosis
involving the appearance of the chromosomes
the development of the spindle apparatus
breakdown of the nuclear envelope

10. Prophase I—Important Events
Homologous chromosomes pair up to form a tetrad
The sister chromatids press together at points along their length
It is during this alignment that chromatid arms may overlap and temporarily fuse resulting in crossing over
Result: chomosomes (consisting of 2 sister chromatids) are no longer just from “mom” or “dad”—the chromosomes are a mixture!!!

11. Vocabulary Tetrad—each chromosome pairs with homologous chromosome
4 sister chromatids!
Crossing over– when chromosomes are in tetrads, they may exchange parts
This happens in Prophase I
Result–increased variety in offspring (a good thing)

12. Result: INCREASED genetic variety!!!

13. METAPHASE I
Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis.

14. METAPHASE I
In Mitosis, all the chromosomes line up on the metaphase plate in no particular order.
In Metaphase I (of Meiosis), the chromosome pairs are aligned on either side of the metaphase plate.

15. ANAPHASE I During Anaphase I the spindle fibers contract
the homologous pairs are pulled away from each other
toward each pole of the cell.

16. TELOPHASE I Cleavage is followed by cytokinesis, but…
the nuclear membrane (envelope) usually is not reformed and the chromosomes do not disappear.
At the end of Telophase I
each daughter cell has a single set of chromosomes
half the total number in the original cell where the chromosomes were present in pairs.

17. It was 4, now it will be 2 in each cell!!
While the original cell was diploid (2n), the daughter cells are now haploid (1n).
This is why Meiosis I is often called reduction division.
Diploid = 4
Haploid = 2
It was 4, now it will be 2 in each cell!!

18. MEIOSIS II Can You Guess?
Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I.
There is no Interphase between Meiosis I and Meiosis II and the latter begins with…
Can You Guess?

19. PROPHASE II A new set of spindle fibers forms
the chromosomes begin to move toward the equator of the cell.

20. METAPHASE II
All the chromosomes in the two cells align with the metaphase plate.

21. ANAPHASE II The centromeres split The spindle fibers shorten
drawing the chromosomes toward each pole of the cell.

22. TELOPHASE II Cleavage is followed by cytokinesis
The nuclear membrane (envelope) reforms
The chromosomes begin to fade, replaced by the granular chromatin characteristic of interphase.
Total of four daughter cells, each with half the total number of chromosomes as the original cell.

23. Female life cycles in “higher” organisms
Male structures
all four cells will eventually develop into typical sperm cells.
Female life cycles in “higher” organisms
three of the cells will typically abort
leaving a single cell to develop into an egg cell
usually much larger than a typical sperm cell.

24. Gametes

25. NOVA video