Lecture 6 - Monday, Febuary 3, 2003 R. Jones Chapters 8 & 9
Meiosis summary and begin Inheritance and genetics
1. In metaphase I of Meiosis chromosomes align at the equator on the cell as homolgous pairs, one homolog facing one pole of the cell, the other facing the other pole. The way in which parental chromsomes are arranged at the metaphase plate is random and the number of possibilities of their organization is 2 to the power of the number of chromsomes, so in humans there are 2 to the power of 23 possible arrangements of human chromsomes (i.e. only 1 in 8,388,608 possibilities that two eggs or sperm will receive an exact maternal or paternal complement of chromosomes).
2. Anaphase I: The microtubules pulls one set of each homolgous pair of chromosomes to the poles and telophase ensures that two separate groups of chromosomes accumulate at the poles. Each pole has one set of the homologous pairs of chromosomes.
3. Cytokinesis occurs as telophase ends and this separates the chromosomes into two cells, EACH OF WHICH NOW HAS HALF OF THE NUMBER OF CHROMOSOMES THAT WERE FOUND IN THE PARENT CELL. The cells are now haploid (N).
4. Meiosis II is indistinguishable from mitosis. Meiosis II is divided into prophase II, metaphase II, anaphase II and telophase II. At metaphase II of meiosis all chromosomes lie at the metaphase plate in an arrangement that is similar to that in mitosis and contrast swith metaphase I of meiosis.
5. Anaphase moves chromsomes to the poles of the cell and telophase and cytokinesis occur. Cytokinesis results in the formation of cells with half the number of chromosomes and where the chromsomes are composed of only one chromatid.
6. A combination of meiosis I and II produces 4 haploid cells from one diploid cell. In spermatogenesis in animals, meiosis gives rise to 4 haploid (N) sperm from one diploid (2N) spermatogonium. In oogenesis only 1 haploid egg is formed becuase three polar bodies are also made. Polar bodies play no role in fertilization.
7. Fertilization-1 N sperm and 1 N egg gives rise to a diploid zygote having homologous pairs of chromosomes.
8. Defects in meisois (or mitosis) and in chromosome structure have serious consequences for human development. If the defects happen duing formation of gametes they canlead to congenital disorders and similar changes in somatic cells can cause defects such as cancers.
9. The Cell Theory was proposed in the 19th century and the details of mitosis and meiosis were worked out in the early parts of the 20th century. Theories about inheritence in the middle of the 19th century were based on casual observations and were incorrect. Gregor Mendel's (1822-1884) experiments with garden peas were the first experiments that established how characters (traits) were inherited.
10. Mendel showed that genetic information existed in discrete units in cells that he called genes. His work established that two copies of each gene existed, one form each parent, and these different forms of the geen he called alleles. Each parent transmitted one copy of each gene to offspring because sperm and egg carry only oen alllele of each gene. When an organism carried the same allele of a gene it is known as homozygous for that trait and when it carries diffferent copis it is known as heterozygous.
11. Mendel's experiments with monohybrid crosses (inheritance of only one character or trait) allowed him to draw many conclusions including the idea that some traits are dominant and other recessive.
12. Mendel worked with easily scorable traits (the phenotype) such as tall/dwarf, round/wrinkled seed, yellow/green seed, yellow/green pods, purple/white flowers. In a cross between purple flowered and white flowered parents he discovered that the first filllial generation (F1) were all purple flowered. When F1 plants were self-pollinated, i.e., purple x purple F1 the resulting progeny (now called F2, the second fillial generation) were found in the ratio of 3 purple flowered plants to 1 white flowered plant. This simple experiment showed that allthouh the white phenotype was lost in the F1 generation it reappeared in the F2.
13. Mendel rationalized the results as follows: in the original true breeding parents flower color represented by the P gene in purple plants and by the p gene in white plants. P is a domoinant gene because all F1 plants are purple. Because there are two copies of the P and p gene in the parents he represented the cross this way: one parent is PP the other pp and the gametes produced by these plants would carry the P and p alleles. The F1 progeny of the cross were all Pp, that is heterozygous and because P is dominant over p the color was purple.
14. Mendel then crossed F1 plants with themselves i.e. Pp parent female x Pp parent male. The gametes from either parent in this cross could be either P or p and by doing a Punnett square one can find that the progeny of this cross will be: PP, Pp, Pp or pp. PP and Pp are all purple and pp are white and a ratio of 3 purple to one white is obtained
15. Mendel devised a method that would alow him to distinguish plants with the same phenotype, i.e. purple flowers from those with different genotypes (i.e. PP and Pp). He called this the Test Cross. In the Test Cross, plants whose genotypes are not known are crossed with a homozygous recessive parent, in the case of flower color a true-breeding white-flowered plant (pp).
16. When homozygous dominant PP is crossed with pp the progeny are all Pp, i.e. purple flowered but when heterozygous Pp and pp are crossed half of the progeny are purple and Pp, the other are white and pp in the ration of 1 to 1.
Next Wednesday: Dihybrid crosses. What happens when you cross plants having two traits?