We started off with some further discussion of the complementation test: Crosses among several strains can be used to define "complementation groups" corresponding to specific genes affecting a trait.
Chromosomes were described and characterized between Mendel’s report (1866) and the "rediscovery " of Mendel in 1900. The characteristics of chromosomes were consistent with what one might expect of the genetic material. The number of chromosomes is characteristic for a given species. The number in humans was not finally identified as 46 until 1956; people had been wrong for over 30 years about the number and hadn’t realized it. Somatic cells have a higher (diploid = 2N) chromosome number than do germ cells. Germ cells and gametes have half the chromosome number (haploid = N) of somatic cells.
We then did an overview of the results of mitosis and meiosis. Mitosis takes place in somatic cells. It is preceded by one replication and involves one division. The number of chromosomes stays constant and the daughter cell is identical to the parent cell. Meiosis takes place during the formation of gametes in germ cells. It is preceded by one replication and involves two divisions. The number of chromosomes is halved and genes are reshuffled by crossing over and independent assortment.
Mitosis is a stage in the cell cycle, the others are G1, S and G2 (all parts of Interphase). Checkpoints for the cell cycle involve cyclin proteins and are at the boundary of G1 and S and G2 and M. The four stages of mitosis are prophase, metaphase, anaphase and telophase. Terminology: sister chromatid, nucleolus, centromere, kinetochore (spindle fiber attachment site), spindle fibers.
Meiosis differs from mitosis in that the chromosome is reduced in half during the first division of meiosis. In prophase I of meiosis, the homologous chromosomes pair (synapsis) and cross over. Chromosomers are visible along the paired chromosomes. Exchange points are termed chiasmata. Reshuffling of genes also occurs by the independent assortment of chromosomes at Anaphase I of meiosis. The second division of meiosis is much like mitosis; sister chromatids separate.
The products of meiosis vary among organisms. In animals: males have 4 sperm produced from a single meiosis. Females have a single egg and three nonfunctional polar bodies from a single meiosis. In plants, cell divisions may take place at the haploid (gametophyte) stage. (The diploid is called the sporophyte stage). Sperm nuclei are produced in the male, while multiple haploid nuclei are produced in the female. One sperm nucleus fertilizes one female nucleus to form the embryo, while another sperm nucleus joins two other female nuclei to form the endosperm.
Chromosomes contain one long piece of DNA. Autoradiography studies supported this by showing chromosome-sized pieces in Drosophila. Pulsed-field electrophoresis studies supported this by showing chromosome-sized pieces in yeast.