Why are chromosomes important?

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Experiment 1: Microscopic Anatomy of the Reproductive System

Visualizing the microscopic anatomy of the reproductive system will aid in your understanding of its function.

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Materials

Penis (Cross-Section) Digital Slide Image Testis (Cross-Section) Digital Slide Image Sperm Digital Slide Image

Ovary Digital Slide Image Uterus Digital Slide Image

Procedure

1. Examine each of the digital slide images.

2. Label the images provided at the end of the digital slide images.

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Penis (Cross-Section) 100X. The urethra is lined with stratified, squamous epithelium near the bottom of the tubule. The corpus spongiosum, which surrounds the urethra, includes blood sinuses which are often filled with blood. These sinuses are also lined with simple, squamous epithelium. The corpus cavernosa (not pictured) is located just above the corpus spongiosa, and contains erectile tissue. This tissue is filled with empty spaces which fill with arterial blood in a process called tumescense.
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Penis (Cross-Section) 1000X. Blood cells in the corpus spongiosum are visible in this image.
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Testis (Cross-Section) 100X. Testes are dense with seminiferous tubules (approximately 800- 1600 tubules per testis; or, approximately 600 meters of tubules when added together). These tubules are the site for spermatogenesis, and are lined with Sertoli cells. Septa reside between these tubules, and are comprised of connective tissue.
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Testis (Cross-Section) 1000X. Sertoli cells are referred to as “nursery cells” because they help create a healthy environment for spermatogenesis. These cells are directly atop the boundary tissue which surrounds the seminiferous tubules, and are ovular in shape. Meiotic activity produces, primary spermatocytes, secondary spermatocytes, and spermatids. Spermatids are located near the lumen within the tubules, and appear morphologically different based on their respective phases of maturation. Young spermatids have elongated, tail-like structures while more developed spermatids appear boxy and dense.
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Sperm 1000X. Sperm cell anatomy includes a head, a midpiece, and a flagella. The head appears dense and includes the nucleus. The midpiece has a filamentous core with many mitochondrial organelles present on the outside. The flagella is used for motility.
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Ovary 100X. The surface layer of the ovary is composed of a single layer of epithelium, referred to as germinal epithelium. The tunica albuginea is directly below the germinal epithelium and creates a connective tissue capsule surrounding the ovary. The outer layer of the ovary, shown above, is referred to as the cortex and is where follicles reside. Ovaries contain different types of follicle cells referred to as primordial follicles, primary follicles, secondary follicles, and tertiary follicles. A central medulla also exists within the ovary.
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Uterus 100X. The endometrium is a mucosal layer used for egg implantation, and consists of simple columnar epithelium; this includes both ciliated and secretory cells). Note that the precise composition of the endometrium varies by physiological state. The myometrium is a fibromuscular layer. Uterine glands are located in the endometrium
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Uterus 1000X. Uterine glands are lined by ciliated columnar epithelium. They function to secrete biochemical substances required for healthy embryonic development, and become enlarged after impregnation occurs in the uterus.

Post-Lab Questions

1. Label the slide images

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2. What type of epithelium did you observe in the prepared slide of the penis?

3. Which layer of the uterus forms a new functional layer each month?

Experiment 1: Observation of Mitosis in a Plant Cell

image19.jpgIn this experiment, we will look at the different stage of mitosis in an onion cell. Remember that mitosis only occupies one to two hours while interphase can take anywhere from 18 – 24 hours. Using this information and the data from your experiment, you can estimate the percentage of cells in each stage of the cell cycle.

Materials

Onion (allium) Root Tip Digital Slide Images

Procedure

1. The length of the cell cycle in the onion root tip is about 24 hours. Predict how many hours of the 24 hour cell cycle you think each step takes. Record your predictions, along with supporting evidence, in Table 1.

2. Examine the onion root tip slide images on the following pages. There are four images, each displaying a different field of view. Pick one of the images, and count the number of cells in each stage. Then count the total number of cells in the image. Record the image you selected and your counts in Table 2.

3. Calculate the time spent by a cell in each stage based on the 24 hour cycle:

Hours of Stage = 24 x Number of Cells in Stage
Total Number of Cells Counted

4. Locate the region just above the root cap tip.

5. Locate a good example of a cell in each of the following stages: interphase, prophase, metaphase, anaphase, and telophase.

6. Draw the dividing cell in the appropriate area for each stage of the cell cycle, exactly as it appears. Include your drawings in Table 3.

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Onion Root Tip: 100X
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Onion Root Tip: 100X
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Onion Root Tip: 100X
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Onion Root Tip: 100X
Table 1: Mitosis Predictions
Predictions:  
Supporting Evidence:  
Table 2: Mitosis Data
Number of Cells in Each Stage Total Number of Cells Calculated % of Time Spent in Each Stage
Interphase: Interphase:
Prophase: Prophase:
Metaphase: Metaphase:
Anaphase: Anaphase:
Telophase: Telophase:
Cytokinesis: Cytokinesis:
Table 3: Stage Drawings
Cell Stage: Drawing:
Interphase:  
Prophase:  
Metaphase:  
Anaphase:  
Telophase:  
Cytokinesis:  

Post-Lab Questions

1. Label the arrows in the slide image below with the appropriate stage of the cell cycle.

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2. What stage were most of the onion root tip cells in? Does this make sense?

3. As a cell grows, what happens to its surface area : volume ratio? (Think of a balloon being blown up). How is this changing ratio related to cell division?

4. What is the function of mitosis in a cell that is about to divide?

5. What would happen if mitosis were uncontrolled?

6. How accurate were your time predication for each stage of the cell cycle?

7. Discuss one observation that you found interesting while looking at the onion root tip cells.

Experiment 3: Following Chromosomal DNA Movement through Meiosis

In this experiment, you will follow the movement of the chromosomes through meiosis I and II to create gametes

Materials

2 Sets of Different Colored Pop-it® Beads (32 of each – these may be any color) 4 5-Holed Pop-it® Beads (used as centromeres)

Procedure Trial 1

As prophase I begins, the replicated chromosomes coil and condense…

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

a. For example, suppose you start with 20 red beads 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 the five-holed bead flat on a work surface with the node positioned up. Then, snap each of the four strands into the bead to create an “X” shaped pair of sister chromosomes.

c. Repeat this process using 20 new beads (of a different color) to create the second sister chromatid pair. See Figure 4 (located in Experiment 2) for reference.

2. Assemble a second pair of replicated sister chromatids; this time using 12 beads, instead of 20, per pair (six beads per each complete sister chromatid strand). Snap each of the four pieces into a new five-holed bead to complete the set up. See Figure 5 (located in Experiment 2) for reference.

3. Pair up the homologous chromosome pairs created in Step 1 and 2. DO NOT SIMULATE CROSSING OVER IN THIS TRIAL. You will simulate crossing over in Trial 2.

4. Configure the chromosomes as they would appear in each of the stages of meiotic division (prophase I and II, metaphase I and II, anaphase I and II, telophase I and II, and cytokinesis).

5. Diagram the corresponding images for each stage in the sections titled “Trial 1 – Meiotic Division Beads Diagram”. Be sure to indicate the number of chromosomes present in each cell for each phase.

6. Disassemble the beads used in Trial 1. You will need to recycle these beads for a second meiosis trial in Steps 7 – 11.

Trial 1 – Meiotic Division Beads Diagram

Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

Trial 2

7. Build a pair of replicated, homologous chromosomes. 10 beads should be used to create each individual sister chromatid (20 beads per chromosome pair). The five-holed bead represents the centromere. To do this…

a. For example, suppose you start with 20 red beads 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 the five-holed bead flat on a work surface with the node positioned up. Then, snap each of the four strands into the bead to create an “X” shaped pair of sister chromosomes.

c. Repeat this process using 20 new beads (of a different color) to create the second sister chromatid pair. See Figure 4 (located in Experiment 2) for reference.

8. Assemble a second pair of replicated sister chromatids; this time using 12 beads, instead of 20, per pair (six beads per each complete sister chromatid strand). Snap each of the four pieces into a new five-holed bead to complete the set up. See Figure 5 (located in Experiment 2) for reference.

9. Pair up the homologous chromosomes created in Step 6 and 7.

10. SIMULATE CROSSING OVER. To do this, bring the two homologous pairs of sister chromatids together (creating the chiasma) and exchange an equal number of beads between the two. This will result in chromatids of the same original length, there will now be new combinations of chromatid colors.

11. Configure the chromosomes as they would appear in each of the stages of meiotic division (prophase I and II, metaphase I and II, anaphase I and II, telophase I and II, and cytokinesis).

12. Diagram the corresponding images for each stage in the section titled “Trial 2 – Meiotic Division Beads Diagram”. Be sure to indicate the number of chromosomes present in each cell for each phase. Also, indicate how the crossing over affected the genetic content in the gametes from Trial 1 versus Trial 2.

Trial 2 – Meiotic Division Beads Diagram:

Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

Post-Lab Questions

1. What is the state of the DNA at the end of meiosis I? What about at the end of meiosis II?

2. Why are chromosomes important?

3. How are meiosis I and meiosis II different?

4. Why do you use non-sister chromatids to demonstrate crossing over?

5. What combinations of alleles could result from a crossover between BD and bd chromosomes?

6. How many chromosomes were present when meiosis I started?

7. How many nuclei are present at the end of meiosis II? How many chromosomes are in each?

8. Identify two ways that meiosis contributes to genetic recombination.

9. Why is it necessary to reduce the number of chromosomes in gametes, but not in other cells?

10. Blue whales have 44 chromosomes in every cell. Determine how many chromosomes you would expect to find in the following: Sperm Cell:

Egg Cell:

Daughter Cell from Mitosis:

Daughter Cell from Meiosis II:

11. Research and find a disease that is caused by chromosomal mutations. When does the mutation occur? What chromosomes are affected? What are the consequences?

12. Diagram what would happen if sexual reproduction took place for four generations using diploid (2n) cells.

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