28 March 2005

Lecture 30

Reading, Chapter 6 and 7

VI. Genes

E. Cell division

3. Meiosis

a. Stages of meiosis

Like mitosis, meiosis has stages. Unlike mitosis, meiosis consists of two cell divisions rather than one, yielding four daughter cells instead of two. The two divisions of meiosis are called Meiosis I and Meiosis II. The stages of these two divisions have the same names as those of mitosis except that they are specified as I or II, for example metaphase II is the metaphase stage of the second meiotic division.

Meiosis I consists of the separation of homologous chromosomes. It has the following stages:

Prophase I - Before prophase I, the chromosomes of the dividing cell have been replicated so that each consists of a pair of attached chromatids. Like prophase of mitosis, chromosomes condense, the spindle begins to form and the nuclear envelope breaks down. Unlike mitosis, the homologous chromosomes of the cell pair up and stick together a little bit during prophase I. For example, Chromosome 7 from Dad will pair with Chromosome 7 from Mom in your cells.

Metaphase I - During metaphase I, the pairs of homologous chromosomes are attached to the spindle and line up in the middle of the cell.

Anaphase I - During anaphase I, the homologous pairs of chromosomes are separated by the spindle and drawn into opposite poles of the dividing cell. The chromatids of each chromosome remain together.

Telophase I - During telophase I, the cell pinches in the middle and divides. There are now two daughter cells that are haploid, having one each of the 23 human chromosomes (in the case of human meiosis). Each chromosome still consists of a pair of chromatids.

Meiosis II is very similar to mitosis. It separates the chromatids so that there are now four haploid daughter cells having one copy of each of the 23 human chromosomes. These are gametes (eggs and sperm). they can fuse with the opposite gamete and recreate a diploid cell that is genetically unique

The reduction division accompished by meiosis is necessary for sexual reproduction. Most cells, and animal cells in particular, are very sensitive to having extra copies or extra sets of chromosomes. For example, having 3 of Chromosome 21 (a very small one with relatively few genes) instead of the normal 2 leads to Down's syndrome. Having an extra Chromosome 1 or 8 or a whole extra set of chromsomes (triploid) or lacking one or more chromosomes is nearly always deleterious to cells, especially animal cells.

During sexual reproduction, two cells from two different individuals fuse to produce a new, genetically unique individual. If these two cells were diploid, the resulting zygote would not survive on a consistent basis because it would have four sets of chromosomes. For this reason, cells must be made haploid, having only one set of chromosomes instead of two, before they can act as gametes. Meiosis accomplishes this.

b. Trisomy

Most cells, and animal cells in particular, are very sensitive to having extra copies or extra sets of chromosomes. For example, having 3 of Chromosome 21 (a very small one with relatively few genes) instead of the normal 2 leads to a condition called trisomy 21, also known as Down's Syndrome. In humans, extra or absent sex chromosomes (X and Y) can also occur. The phenotypes conferred by extra or absent sex chromosomes are described on pages 176 and 177 of your text.

Having an extra Chromosome 1 or 8 or a whole extra set of chromsomes (triploid) or lacking one or more of these large chromosomes is nearly always fatal to zygotes, especially animal zygotes.

E. Patterns of inheritance

Genes govern the appearance and function of organisms. In other words, the genotype of an organism confers the phenotype. The genotype is all the genes on the chromsomes of an organism. The phenotype is how the organism looks and works.

Discrete elements of the phenotype are called traits, for example flower color is a trait in pea plants. Most traits are determined by mulitiple genes acting together. Your height or your eye color are two such traits. A few traits are determined by single genes however. They are the exception but study of the inheritance of such simple traits gave rise to the whole concept of genes in the first place, long before anything was known about DNA.