1. 1 Mitosis and Meiosis Traits are controlled by genes Each individual contains two copies of a gene. Gene segregation Each individual has thousands of genes Independent assortment What is the biological basis for Mendel’s laws? greenxyellow yellow x yellow yellow: yellow:yellow:green

2. 2 Genes reside on Chromosomes Genes reside on chromosomes, understanding the behavior and inheritance patterns of individual genes requires an understanding of the behavior of inheritance patterns of chromosomes. The processes of mitosis and meiosis describe the two basic patterns of chromosome behavior in higher eukaryotes Mitosis: a form of cell division that produces two daughter cells of identical genotypes Meiosis: a form of cell division in a diploid cell that produces four haploid cells Meiosis only occurs in a small specialized set of cells known as the germ cells.

3. 3 Development Meiosis2N ---->4N---->N+N+N+N Mitosis2N ---->4N----> 2N+2N 2N | 4N | N | 2N | 4N | 2N

4. 4 The Mitotic cell cycle The mitotic cycle alternates between the replication of each chromosome (S phase) and the segregation of the replicated chromosomes to two daughter nuclei (M phase). The intervals between these phases are known as gap phases and this divides the cell cycle into four phases M, G1, S and G2. Interphase consists of G1, S, and G2.

5. 5 Chromosome number Smallest number: The female of a subspecies of the ant, Myrmecia pilosula, has one pair of chromosomes per cell. Its male has only one chromosome in each cell. Largest number: In the fern family of plants, the species Ophioglossum reticulatum has about 630 pairs of chromosomes, or 1260 chromosomes per cell. SpeciesHaploid number (n) Human23 Monkey21 Mouse20 Frog13 Fruit fly4 C. Elegans6 Corn10 S. Cerevisiae16 S. Pombe3 Indian muntjack3 n=2 Homologous Chromosomes (99.99% similar) Haploids are 1N Diploids are 2N Tetraploids are 4N

6. 6 Mitosis Mitosis is the period in which the chromosomes condense align along the metaphase plate and migrate to opposite poles. In part because this is the most visibly dramatic stage in the cell cycle much research has focused on these mitotic events. Net result: The creation of two daughter cells with identical chromosome complements.

7. 7 Replication of DNA Each DNA mol is a chromatid The two chromatids attached to the centromere are called sister chromatids Homologous Chromosomes 99.99% identical Mitosis n=2 2N n=2 4N

8. 8 Mitosis Sister chromatids line up at the metaphase plate. Sister chromatids separate to opposite poles n=2 4N n=2 2N

9. 9 Mitosis in haploid and diploid Replication of DNA A a AA a a n=1 2N n=1 4N A a A a n=1 2N Replication of DNA A AA n=1 1N n=1 2N A A n=1 1N

10. 10 Cell cycle and cancer Currently the cell cycle/mitosis is an intensively investigated area of research. This is primarily due to the fact that: 1.The structural and regulatory components governing the cell cycle are conserved throughout the phyla. That is, the same proteins are used in yeast, flies and humans. 2. A number of the mutations that produce cancer in humans disrupt the genes involved in regulating cells during the cell cycle. Example: Cancers result from uncontrolled and inappropriate division of cells Cells actually contain a set of genes whose job it is to prevent cells from dividing inappropriately ( these genes are known as tumor suppressor genes, anti-oncogenes, and more poetically as the "guardians of the cell"). One gene known as p53 ensures that the chromosomes have replicated properly before allowing the cells to proceed into mitosis. Recently it has been found that lesions in this gene are one of the most common in all human cancers.

11. 11 Chromosomes Basic terms and key features of the chromosome: Telomere: Centromere: Sister chromatids: Homologue-. Metaphase plate: Haploid (N)- Diploid (2N):

12. 12 Karyotype Description of length, number, morphology. Karyotype analysis is extremely important in medicine. Alternations in karyotypes are linked to birth defects and many human cancers. Metacentric- Acrocentric- Telocentric-

13. 13 Meiosis Meiosis: While the mitotic cycle is designed to produce two cells with the identical genotype, the meiotic cycle is designed to produce four cells each with half of the chromosome complement. (Increase in ploidy leads to increase in size of cells.) Meiosis allows the cell to maintain constant ploidy (following mating) and at the same time to shuffle the genetic deck (in the progeny) In meiosis: Diploid cells undergo one round of chromosome replication followed by two divisions thereby reducing ploidy and producing four haploid cells. The two divisions are referred to as Meiosis I and Meiosis II. N 2N-----> 4N----->N N

14. 14 Meiosis Meiosis I: Interphase I: chromosomes replicate Prophase I: chromosomes condense members of a chromosome pair (homologues) physically associate with one another and lie side by side on the metaphase plate. This process is known as synapsis. The paired chromosome physically overlap forming structures known as chiasma. Metaphase I: the paired homologous chromosomes, known as bivalents,move to the center of the cell and line up along the metaphase plate. Anaphase I: in a process known as disjunction, the members of a homologous pair migrate to opposite poles. This effectively reduces the total number of chromosomes by half and is therefore called a reductional division.

15. 15 Meiosis Telophase I: if this stage were equivalent to telophase of mitosis, the nuclear envelope would reform and DNA synthesis would take place. This does not occur and the anaphase meiotic products proceed directly into Prophase II of meiosis Net result: Four haploid meiotic products Meiosis II is analogous to mitosis; chromosomes, rather than homologous pairs align along the metaphase plate and the chromatids separate

16. 16 MeiosisI Chromosomes replicate Homologous Chromosomes pair locate on metaphase plate at random This is Mendels random assortment OR

17. 17 Random assortment anaphaseI. Centromeres do not separate The two sister chromatids go to the same pole OR Reductional division

18. 18 MetaphaseIIa Cell division without intervening replication!! Similar to mitotic metaphase 25% The reduced number of chromosomes in each of the two cells align on the metaphase plate (no pairing of homologous occurs), divide to produce four haploid cells.

19. 19 MetaphaseIIb Cell division without intervening replication!! Similar to mitotic metaphase 25% The reduced number of chromosomes in each of the two cells align on the metaphase plate (no pairing of homologous occurs), divide to produce four haploid cells.

20. 20 Meiosis A a Bb AA a a bb BB AAa a bb BB AA BB a a bb A B A B a b a b

21. 21 With 23 human chromosomes, there is a possible 2 23 = 8.4 x 10 6 distinct gametes. Little Alberts 1 st edition 9-36 © Garland Publishing 1st mechanism for genetic diversity: independent assortment of chromosomes

22. 22 Gene Shuffling Unlike mitosis, the meiotic products are not genetically identical. There are two reasons for this 1.The arrangement of paired homologous on the plate at Metaphase I is random. This random arrangement is the mechanism behind Mendel's principle of independent assortment 2.The paired homologues physically recombine (or crossover with one another).

23. 23 Crossing over There are two ways of generating variation: Random assortment of chromosomes (shuffling of maternal and paternal chromosomes) Recombination between homologous chromosomes (crossing-over) in metaphase I Homologous chromosomes pair in metaphaseI At least one crossover occurs per homologous pair

24. 24 Crossing over is the result of a physical exchange between homologous chromosomes Cytological studies in maize by Creighton and McClintock (1931) were the first to demonstrate that recombination is the result of a physical exchange between homologous chromosomes On chromosome 9 in corn there were two markers: Endosperm composition: Seed color:

25. 25 Crossing over is the result of a physical exchange between homologous chromosomes Cytological studies in maize by Creighton and McClintock (1931) were the first to demonstrate that recombination is the result of a physical exchange between homologous chromosomes On chromosome 9 in corn there were two markers: Endosperm composition: Seed color: Wx waxy C colored wx starchy c colorless In addition, the chromosomes were morphologically distinct. Some had a cytologically visible structure known as a knob at the telomere and others had an interchange such that it is longer W C W C X w c w c W C w c F1

26. 26 The genetic recombinants were also cytological recombinants. This strongly supported the model that recombination involves a physical exchange between homologous chromosomes

27. 27

28. 28 Mitosis and meiosis compared: