Maintaining Chromosome Number

1. Maintaining Chromosome Number

2. Processes that affect chromosome number • Meiosis • chromosome number is reduced by half • Aneuploidy • addition or deletion of one or a few chromosomes • Polyploidy • addition of complete sets of chromosomes

3. Meiosis • Meiosis is the type of cell division that accompanies gamete formation • Key features: • requires two cell divisions • produces cells in which chromosome number is reduced by half • produces cells that are genetically variable

4. Gametogenesis in animals Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

5. Gametogenesis and fertilization in plants Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

6. Steps in first meiotic cell division (Meiosis I) • Interphase: DNA duplicates. • Prophase I: Chromosomes condense; homologous chromosomes pair; crossing over occurs; nuclear membrane dissolves • Metaphase I: Homologous chromosomes line up on equator of cell; spindles form • Anaphase I: Homologous chromosomes are pulled to opposite poles of cell. • Telophase I: Chromosomes arrive at poles. Cells divide. Product is 2 cells, each with a single copy of each homologous chromosome.

7. Steps in second meiotic cell division (Meiosis II) • Prophase II: Chromosomes condense • Metaphase II: Chromosomes line up on equator of cell; spindles form at opposite poles • Anaphase I: Chromatids are pulled apart at the centromeres, to opposite poles of the cell. • Telophase I: Chromatids (now called chromosomes) arrive at the poles. Nuclear membranes reform. Cells divide. Product is 4 cells, each with half the number of chromosomes as the original cell.

8. Meiosis Animations • http://bcs.whfreeman.com/pierce2e • http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html

9. Result of Meiosis • Meiosis produces four cells that are genetically distinct from the original cell. • The daughter cells have half the number of chromosomes as the original cell. • One diploid (2N) cell gives rise to four haploid (N) cells.

10. Purposes of meiosis • Reduces chromosome number, so that when fertilization occurs, the zygote will maintain its proper chromosome number. • Introduces genetic variability • Independent assortment of chromosomes • Crossing over • http://bcs.whfreeman.com/pierce2e

11. I II S Non-disjunction S I II one too many chromosomes one too few chromosomes Mistakes in meiosis can lead to aneuploidy (wrong number of chromosomes) Normal meiosis

12. Fertilization of aneuploid gametes with normal gametes produces aneuploid zygotes Monosomic 2N - 1 chromosome + Trisomic 2N + 1 chromosome +

13. Effects of aneuploidy in animals • Usually lethal. • Lethality due to imbalance in dosage of chromosome expression. • Most genes are expressed from both chromosomal copies. • If there are too many or too few copies of a gene, expression levels are abnormal.

14. Trisomy in humans • Most trisomies are lethal. • Trisomy for 13, 18 or 21 can survive to birth. • Trisomy 13 and Trisomy18 infants do not usually live past a year. • Trisomy 21 (Down Syndrome) individuals can live productive lives. http://www.ncdsn.com/resources.html Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman. http://www.denverdsclinic.org/patients.htm

15. Aneuploidy in plants • Plants usually tolerate aneuploidy better than animals do. • For example, in Datura, trisomies are viable. • For each of the 12 chromosomes, the presence of one additional chromo-some leads to a difference in fruit morphology. Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

16. Polyploidy in plants: Extra chromosome sets • Autopolyploidy: chromosome sets derived from the same species • Allopolyploidy: chromosome sets derived from different species Some familiar polyploids potato (4n) soybean (4n) strawberry (8n)

17. Autoploidy • Can occur as result of error in mitosis • Non-disjunction or failure of cell division • Autopolyploids with even numbers of chromosome sets are fertile. Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

18. Autopolyploidy • Can occur as result of error in meiosis Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

19. Triploids are usually sterile Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

20. Triploids we know and love • The fruits of triploid plants have very small. very few or non-existent seeds. • Banana plants are propagated vegetatively. • Seedless watermelons are propagated by crossing a tetraploid female by a diploid male to produce triploid seeds. Banana Seedless watermelon

21. Allopolyploidy • Chromosome sets derived from different species • Example: The Triangle of U is proposed as a theory to explain the evolution of many of the Brassica species, which are thought to have hybridized with one another to produce new species http://en.wikipedia.org/wiki/Triangle_of_U

22. Bag-O’-BrassicasFun with Vegetables • Brassica rapa (AA = 20) • Brassica oleracea (CC=18) • Brassica juncea (AABB = 36) • Brassica napus (AACC = 38)

23. Consequences of polyploidy • Polyploid plants are bigger than diploid relatives. Consequently, many crops have been bred to be polyploid. • Larger plant mass • Bigger and / or more numerous fruits • Polyploids with odd numbers of chromosome sets produce few seeds. • Polyploids with even numbers of chromosome sets produce normal seed.

24. Summary of processes affecting chromosome number • Meiosis is the cell division process that reduces chromosome number by half and introduces genetic variability. • Aneuploidy (wrong number of chromosomes) is usually lethal in animals, but can be tolerated by plants. • Many plants are polyploids and have extra sets of chromosomes, derived either from the same or different species.