1. Meiosis

2. Diploid (2n)  Haploid (n)
Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n)  Haploid (n)
If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die.

3. Meiosis Meiosis is Two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes.

4. Meiosis:
As in mitosis, if a cell wants to make a duplicate of itself, it first must copy its DNA (part of a chromosome).
Results in reproductive cells (sperm, eggs, etc).
Part of each parent is carried to the four new cells.
Meiosis has evolved to solve a problem.
The problem is this: some organisms have sex with other organisms – that is, they combine their genes together (creating genetic diversity).
Why would this be a problem?

5. Mitosis: Meiosis:
Explain the randomness in meiosis
End of first class
Each resulting cell still has chromosomes from mom & dad

6. MITOSIS MEIOSIS Diploid Diploid 1 somatic cell 2n 2n 2 2n 2n 3 2n 2n 4
gamete
precursor
somatic
cell 2n 2n
duplication 2 2n 2n 3 2n 2n 4 2n 2n
Figure: 10-01
Title:
Meiosis compared to mitosis.
Caption:
1. Both mitosis and meiosis begin with diploid cells, meaning cells that contain paired sets of chromosomes. The two members of each pair are homologous, meaning the same in shape and function. Two sets of homologous chromosomes are shown in both the mitosis and meiosis figures.The larger chromosome pairs in each cell represents one homologous pair, while the smaller chromosome pairs represent the other homologous pair. One member of each homologous pair (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other member of the pair (in blue) comes from the father of this person. 2. In both mitosis and meiosis, the chromosomes duplicate. Each chromosome is now composed of two sister chromatids.
3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, meanwhile, homologous chromosomes -- not sister chromatids -- line up on opposite sides of the metaphase plate. 4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 5 In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, in the first of two cell divisions, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on oppositesides of the plate. 6. The sister chromatids of each chromosome then separate. 7. The cells divide again, yielding four haploid cells.
division
diploid
haploid 5 2n 2n 1n 1n 6
division 7 1n 1n 1n 1n

7. Vocabulary Diploid Haploid Germ cell Somatic cell Interphase Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

9. Meiosis Interphase
Meiosis is preceded by interphase. The chromosomes have not yet condensed.

10. Meiosis Interphase
The chromosomes have replicated, and the chromatin begins to condense.

11. Meiosis Prophase I
The chromosomes are completely condensed. In meiosis (unlike mitosis), the homologous chromosomes pair with one another

12. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

13. During Prophase I “Crossing Over” occurs.
Crossing Over is one of the Two major occurrences of Meiosis
(The other is Non-disjunction)
During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over.

14. Crossing Over creates variation (diversity) in the offspring’s traits.
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

15. Meiosis Metaphase I
The nuclear membrane dissolves and the homologous chromosomes attach to the spindle fibers. They are preparing to go to opposite poles.

16. Meiosis Anaphase I The chromosomes move to opposite ends of the cell.

17. Meiosis Telophase I & Cytokinesis
The cell begins to divide into two daughter cells. It is important to understand that each daughter cell can get any combination of maternal and paternal chromosomes.

18. Meiosis Prophase II The cell has divided into two daughter cells.

19. Meiosis Metaphase II
As in Meiosis I, the chromosomes line up on the spindle fibers.

20. Meiosis Anaphase II
The two cells each begin to divide. As in Meiosis I, the chromosomes move to opposite ends of each cell.

21. Telophase II & Cytokinesis
With the formation of four cells, meiosis is over. Each of these prospective germ cells carries half the number of chromosomes of somatic cells.

22. Sexual vs. Asexual Reproduction

23. Compare the results of uniform or diverse offspring from sexual or asexual reproduction
Uniform offspring
Diverse offspring
Sexual reproduction
Asexual reproduction

24. Sexual Reproduction
A type of reproduction in which the genetic materials from two different cells combine, producing an offspring
The cells that combine are called sex cells
Female – egg
Male – sperm
Fertilization: an egg cell and a sperm cell join together
A new cell is formed and is called a zygote

25. Advantages: Sexual Reproduction
Diverse offspring: genetic variation among offspring
Half of the DNA comes from mom
Half of the DNA comes from dad
Due to genetic variation, individuals within a population have slight differences
Plants – resist diseases
Traits can develop to resist harsh environments that allows an organism survive

26. Advantages: Sexual Reproduction
Selective Breeding
Used to develop many types of plants and animals that have desirable traits
Agriculture/Farming: better plants, larger animals
Desirable pets

27. Disadvantages: Sexual Reproduction
Time and Energy
Organisms have to grow and develop until they are old enough to produce sex cells
Search and find a mate
Searching can expose individuals to predators, diseases, or harsh environmental conditions
Fertilization cannot take place during pregnancy, which can last as long as 2 years for some mammals.