Science Homework Help

Refer to the laboratory models and charts, your slides, or your textbook’s diagrams of the mitotic phases in plants. In the spaces below, make realistic rough sketches (not cartoons) of each stage of the cell cycle (including interphase) as one onion root tip cell divides into two daughter cells.

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Make diagrams here:

List the main actions that happen during this phase here. Use arrows and words to label important parts:

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Prophase:

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Metaphase:

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Anaphase:

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Telophase:

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Interphase:

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Table 1. Number tally of onion root tip cells in different phases of the cell cycle
		Group Tally	Other Group Counts	Class Total	Percent of Time	Class Avg.
	Interphase		X	x		x
	Prophase		X	X		X
	Metaphase		x	x		X
	Anaphase		X	X		X
	Telophase		x	x		X
	Total Cells		X	X		X

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• Calculate the percent of the cells’ total cycle time spent in each phase using the following formula:

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percent of time spent in phase =

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• Record the values in Table 1 and in the class data table on the board.

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Question 1. Based on your calculations which phase plant cells spend the most time. Be able to explain why.

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Question 2. Why is a class average more likely (than your group’s data alone) to reflect the true percentage of time spent in each phase?

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Question 3. Using the class averages for each phase (Table 1, previous page), draw a “pie chart” to graphically demonstrate the relative amount of time an average onion root tip cell spends in each phase of mitosis:

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Using the whole “pie” below to represent 100% of the cell cycle time, sketch in “wedges” which reflect the portion of the pie (relative percent of time) spent in the different phases by the cells in your onion root tips.

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Figure 3. Pie Chart: Relative Amounts of Time

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Spent in the Various Phases of Plant Mitosis

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Question 4. Compare your chart with a general pie chart for plant mitosis found in your textbook or some other reference. Describe the similarities and differences.

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Question 5. Speculate on why our class’ pie chart percentages may not agree exactly with those seen in your reference.

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Question 6. Are the aboral (top side) and oral (under side) surfaces similar? If not, how do they differ?

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Question 7. Closely observe the flat oral surface. Locate the main opening in the center, surrounded by the five-pointed star-like structure. The star-like structure is often difficult to see in a sand dollar as it covered by many of the moveable spines. However, this structure is much more easily seen in a relative of the sand dollar, an urchin. View the following video to observe this structure. https://www.youtube.com/watch?v=hLg4EbxWqNo (Links to an external site.)What is this structure’s function?

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Question 8. Observe the following short video: Video”A Sea Biscuit’s Life) 3:36 minuteshttps://www.youtube.com/watch?v=KeIvSE5S2yQ (Links to an external site.)

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Use the above Video to answer questions 8, 9, 10, and 12.

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Above is a Sand Dollar Egg, notice the difference between it and the Sea Biscuit egg (35 seconds into the video). What do you see as the major differences between the two species?

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Question 9. Describe the sperm movement (31 to 33 seconds in video). Does it appear to be directed or random?

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Question 10. In the spaces below, briefly describe the form (shape) of an egg and a sperm. Then describe the mobility (movement), if any, of the two kinds of cells. Now compare the gametes’ sizes (make sure you are viewing them both at the same magnification).

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	Sperm	Egg
Form
Mobility
Size

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Suggest a possible explanation for the observed differences.

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Question 11. Surrounding the egg cells, you will notice a protective gelatinous material containing colored granules. This material is outside the cell membrane of the egg. (Label this material in the image labeled Unfertilized Egg in Figure 4.)

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• Fertilization

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Question 12. It is usually not possible to observe fertilization (nuclear union) but you can tell when it has occurred, for a fertilization membrane (40seconds) will develop around each fertilized egg, inside the egg’s gelatinous cover. (Label this on the image of the Fertilized Egg in Figure 4.) This halo-appearing membrane emerges from the surface of the cell and signals the formation of the zygote. Draw a fertilized egg showing the fertilization membrane. What might be its function?

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Question 13. Draw arrows between each of the stages in Figures 4 and 5. Then on the top of the arrow, label what process is happening (formation of fertilization membrane, first cleavage, etc.) based on what is listed in Table 2 below. On the bottom of the arrow, label how long the process takes (e.g. 2-5 minutes, etc.). See the example on Figure 4.

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Question 14. On entering the egg, the sperm nucleus carrying the male chromosomes unites with the nucleus of the egg to produce a zygote with the chromosomes of both egg and sperm. What is the relative chromosome number of the zygote compared to the gametes? THINK!

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Question 15. Were the photographs of all the stages up to gastrulation pictured in the video of the Sea Biscuit seen earlier taken at the same power magnification?

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• Within individual stages in the early cleavage stages, how do the individual cell sizes compare?

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• How does the size of the entire cell mass compare at these different stages?

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Question 16. Find the mass of cells called a morula (1:22 in the Video) and on figure 5 Draw the morula. Be sure to label it as a solid sphere of cells.

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Question 17. Draw a blastula (only seen in Figure 5). Be sure to label it as a hollow, fluid-filled sphere.

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Question 18. Compare the size of a blastula to that of an unfertilized egg. Has the embryo actually grown? Estimate the number of cells the blastula contains:

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Question 19. Describe how you tell if an embryo is a simple mass of cells (“morula“) or actually a blastula (fluid-filled sphere of cells).

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Question 20. Draw a gastrula. Label the ectoderm and endoderm (using arrows to point to parts).

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Question 20. A gastrula will develop into free-swimming “pluteus” larvae (1:34 minutes) in a day or two. In contrast a sea star forms a bipinnaria larva. What does gastro- (as in gastrointestinal tract) mean?

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Questions 22. At what point in development would you logically expect the larvae to be capable of growth (growing in size)? Explain your answer.

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• Review of Sand Dollar Development

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Answer the following questions with reference to the letter designations in the chart below.

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Processes	Stages
a. fertilization	f. larva
b. morphogenic migration	g. gametes
c. hatching	h. blastula
d. cleavage	i. zygote
e. metamorphosis	j. gastrula

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Question 23. Which of the following represents the sequence of occurrence for the developmental processes?

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• a, b, c, d, e
• a, d, b, c, e
• b, e, d, c, a
• e, d, b, a, c
• a, d, e, c, b

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Question 24 . Which of the following represents the sequence of occurrence for the developmental stages?

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• f, g, h, i, j
• j, h, i, f, g
• g, h, i, j, f
• g, i, h, j, f
• i, g, f, j, h