Mitosis & Meiosis (Lab 3) Review

Some Key Review Concepts

Mitosis

Mitosis is the division involved in the development of an adult organism from a single fertilized egg, in growth and repair of tissues, in regeneration of body parts, and in asexual reproduction. The parent cell produces two genetically identical "daughter cells". Mitosis can occur in both diploid (2n) and haploid (n) cells.

Meiosis

Meiosis is the division of diploid (2n) parent cells to produce the gametes or spores that give rise to new individuals. The process includes two divisions, unlike Mitosis, in which cells divide only once. The parent cell will produce four haploid (n) daughter cells, each with one half of the chromosomes as the parent cell. 

In spermogenesis (male) the four cells will be equal in size.  In oogenesis (female) there will be one large cell and three smaller cells.

Crossing Over (Meiosis)

Each parent cell has pairs of homologous chromosomes, one from the father and one from the mother. In meiosis, the maternal and paternal chromosomes are put into the daughter cells in many different combinations (in humans there are 2²³ possible combinations). This ensures genetic variation in sexually reproducing organisms. Further genetic variation comes from crossing over, which may occur during prophase I of meiosis.

Lab Design

Part I - Mitosis

Sketch the events of cell division in either plant or animal cells using a microscope slide of cells at various stages in the process of division.

Interphase: (The resting phase) The nucleolus and the nuclear envelope are distinct and the chromosomes are in the form of threadlike chromatin.

Prophase: The chromosomes appear condensed, and the nuclear envelope is not apparent.

Metaphase: Thick, coiled chromosomes, each with two chromatids, are lined up on the metaphase plate.

Anaphase: The chromatids of each chromosome have separated and are moving toward the poles.

Telophase: The chromosomes are at the poles, and are becoming more diffuse. The nuclear envelope is reforming. The cytoplasm may be dividing.

Cytokinesis: Division into two daughter cells is complete.

Past II - Meiosis

In this exercise you observe the results of crossing over in a fungus, Sordaria. A cross between two haploid strains of Sordaria produces spores of different colors.

Where the growing filaments of the two strains meet, fertilization occurs and zygotes form. Meiosis occurs within fruiting bodies to form haploid ascospores, spores contained in asci (special sacs). Then one mitotic division doubles the number of ascospores to eight.

Analysis of Results

Part I - Mitosis

Once the approximate duration of a particular cell cycle is known, it's possible to calculate the amount of time the cell spends in each phase. To do so you need to (1) Determine the approximate duration of the entire cycle for the cells you are studying. This information should be provided. (2) Looking at the slide, count and record the number of cells in the field of view that are in each phase. (3) Determine the total number of cells counted. (4) Determine the percent of cells in each stage. (5) Calculate the time (in minutes) for each phase by multiplying the percent of cells in that phase by the number of minutes for the whole cycle.

Part II - Meiosis

If the ascospores are arranged 4 dark/4 light, count the ascus as "No crossing over." If the arrangement of ascospores is in any other combination, count it as "Crossing over."