Question

Experiment 2: Tracking Chromosomal DNA Movement through Mitosis

Although mitosis and meiosis share similarities, they are different processes and create very different results. In this experiment, you will follow the movement of chromosomes through mitosis to create somatic daughter cells.

Materials 2 Sets of Different Colored Pop-it® Beads (32 of each - these may be any color) (8) 5-Holed Pop-it® Beads (used as centromeres)

Procedure?

Genetic content is replicated during interphase. DNA exists as loose molecular strands called chromatin; it has not condensed to form chromosomes yet.

Sister chromatids begin coiling into chromosomes during prophase. Begin your experiment here:

1. Build a pair of replicated, homologous chromosomes. 10 beads should be used to create each individual sister chromatid (20 beads per chromosome pair). Two five-holed beads represent each centromere. To do this...

Figure 5: Bead set-up. The blue beads represent one pair of sister chromatids and the black beads represent a second pair of sister chromatids. The black and blue pair are homologous.

a. Start with 20 beads of one color to create your first sister chromatid pair. Five beads must be snapped together for each of the four different strands. Two strands create the first chromatid, and two strands create the second chromatid.

b. Place one five-holed bead flat on a work surface with the node positioned up. Then, two of the four strands into the bead to create an “I” shaped sister chromatid. Repeat this step with the other two strands and another five-holed bead.

c. Once both sister chromatids are constructed, connect them by their five-holed beads creating an “X” shape.

d. Repeat this process using 20 new beads (of a different color) to create the second sister chromatid pair. See Figure 5 for reference.

2. Assemble a second pair of replicated sister chromatids; this time using 12 beads per pair (six beads per each complete sister chromatid strand).

Figure 6: Second set of replicated chromosomes.

3. Repeat this process using 12 new beads (of a different color) to create the second set of sister chromatids. See Figure 6 for reference.

4. Configure the chromosomes as they would appear in each of the stages of the cell cycle (prophase, metaphase, anaphase, telophase, and cytokinesis). On the following page diagram the images for each stage in the section titled “Cell Cycle Division: Mitosis Beads Diagram”. Be sure to indicate the number of chromosomes present in each cell during each phase.

Cell Cycle Division: Mitosis Beads Diagram:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Post-Lab Questions

A.How many chromosomes did each of your daughter cells contain?

B. Why is it important for each daughter cell to contain information identical to the parent cell?

C. How often do human skin cells divide? Why might that be? Compare this rate to how frequently human neurons divide. What do you notice?

D.Hypothesize what would happen if the sister chromatids did not split equally during anaphase of mitosis.

Answer 1

In all, each daughter cell got 4 chromosomes, but two homologous pairs. (black and blue being one pair, red and pencil the other).

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It is important for each daughter cell to have identical genetic information to the parent cell in order to function and replicate accurately, and in some cases, depending on the genetic defect even survive(since some genetic defects can be lethal).

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The function of skin is to form a natural physical barrier for the body which means it is highly susceptible to a lot of environmental damage like injury, bruises and bacterial infections. Hence it is a place where high cell turnover is necessary and this necessity is provided for by the availability of stem cells in the dermis and keratinocyte stem cells in the stratum basale layer of the skin. The average lifespan of a skin epidemal cell is 2 - 4 weeks.

Neurons on the other hand form during fetal development and childhood. The function of neurons is to pass and store information. All neurons in an adult are in a terminal G₀ phase (resting phase of replication) and remain so until the death of the cell. A few exceptions to this is a few peripheral axons that can heal themselves in case of mild damage and some neural stem cells that reside in the subventricular zone and hippocampus (both are parts of the brain). That is why there is no cure for neurodegenerative diseases like ALS, Parinson's and Alzheimer's disease.

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In this case, one daughter cell will end up with three copies of the same gene while the other will have only one. This type of error is called nondisjunction of sister chromatids. So you may have 5 total chromosomes in one cell and 3 in the other. This is called Aneuploidy (abnormal number of chromosomes). There are several human diseases associated with this defect, for example, Turner's syndrome in which the person has only one copy of the X chromosome and as a result is female but sterile. In Down's syndrome the person has three copies of chromosome 21.