1. Stages of Meiosis

2. Interphase the cell replicates its chromosomes.
The Phases of Meiosis
Interphase
Interphase
the cell replicates its chromosomes.
After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

3. Prophase I The chromosomes coil up and a spindle forms.
The Phases of Meiosis
Prophase I
Prophase I
The chromosomes coil up and a spindle forms.
As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

4. Prophase I The Phases of Meiosis
The chromatids in a tetrad pair tightly
In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

5. The Phases of Meiosis
Metaphase I
Metaphase I
During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber.
The spindle fibers pull the tetrads (homologous pairs) into the middle

6. Prophase I + Metaphase 1 The Phases of Meiosis
Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.
Genetic recombination results from crossing over during prophase I of meiosis
This increases variation further

7. Anaphase I The Phases of Meiosis
begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell.
This critical step ensures that each new cell will receive only one chromosome from each homologous pair.
*Random assortment*

8. The Phases of Meiosis Telophase I
Nucleus reforms, chromosomes may or may not unwind
The spindle is broken down and the cytoplasm divides to yield two new cells
Cells are now HAPLOID- half the amount of genetic information

9. Meiosis 1 overview 1st division of meiosis 4 chromosomes diploid 2n
Divide 1
Line Up in
tetrads
Copy DNA during interphase
prophase 1
metaphase 1
4 chromosomes
diploid 2n
telophase 1
2 chromosomes
haploid 1n

10. The Phases of Meiosis Meiosis II
Prophase II
Meiosis II
The second division in meiosis is simply a mitotic division of the products of meiosis I.
During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.
Nucleus dissolves

11. The Phases of Meiosis Metaphase II
The chromosomes, still made up of sister chromatids, are pulled to the center of each cell and line up randomly at the equator.

12. The Phases of Meiosis Anaphase II
the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles of each cell.

13. The Phases of Meiosis Telophase II
nuclei reform, the spindles break down, and the cytoplasm divides.
At the end of meiosis II, four haploid cells have been formed from one diploid cell.
These haploid cells will become gametes, transmitting the genes they contain to offspring.

14. Meiosis 2 overview 2nd division of meiosis gametes looks like mitosis
telophase 2
telophase 1
Line Up 2
2nd division of meiosis
looks like mitosis
metaphase 2
2 chromosomes
haploid 1n
gametes

15. The Phases of Meiosis
At the end of meiosis II, four haploid cells have been formed from one diploid cell.
These haploid cells will become gametes, transmitting the genes they contain to offspring.

16. Gamete Formation
Formation of Gametes (egg and sperm- sexual reproductive cells)
Meiosis in males occurs in the testes and produces 4 sperm cells. The process is called spermatogenesis.
Meiosis in females occurs in the ovaries and produces 1 egg and 3 polar bodies, which disintegrate. The process is called oogenesis.
Meiosis allows for crossover and genetic recombination (gene mixing), which creates the variation between parents and offspring.

18. Meiosis = reduction division
special cell division in sexually reproducing organisms
reduce number of chromosomes
2n  1n
diploid  haploid
half
makes gametes
sperm, eggs

19. Meiosis to make gametes
Meiosis & mitosis
Meiosis to make gametes
sperm & egg
Mitosis to make copies of cells
growth
repair
development

20. Comparison of Mitosis & Meiosis