Time For the Cell Cycle
Name ______________________________
Do all phases of the Cell Cycle require the same amount of time for completion? This question can be
answered by counting the number of onion root tip cells in the four phases of mitosis and also interphase. Many
cells found in one specific phase indicate that a long period of time is required for completion of that phase.
Few cells found in a specific phase indicate a short period of time is required for completion of that phase.
Using this measure, you will analyze various aspects of the cell cycle stages.
In this investigation, you will
a. Count the number of cells in each of the phases of mitosis and in interphase.
b. Compute the length of time in minutes needed to complete each phase.
c. Compare data of the time needed for normal cells to complete each phase with that of abnormal cancer cells.
Materials
- Onion Root Tip Sample Diagram
- calculator
Procedure
Part A / Locating and Counting Onion Cells in Various Phases of the Cell Cycle: Interphase and Mitosis
1. Examine the “Onion Root Tip Section Diagram” which shows the portion of an onion root tip.
2. Examine the “MAGNIFIED INSET SECTION”
3. Using the key shown there, count and record the NUMBER OF CELLS IN EACH PHASE of the cell cycle
in Table 1.
4. Count or use math to determine the TOTAL NUMBER OF CELLS found in the inset section and record in
Table 1.
Part B / Determining the Time Required for Each Phase of Onion Root Tip Cell Cycle
Assume that the number of cells in a phase is an indication of the time spent in that phase during mitosis. Time
spent in a mitotic phase and in interphase can be calculated if the total time for mitosis is known. Onion cells
require 12 hours (720 minutes) to complete mitosis (from interphase to interphase). The amount of time needed
for a phase can be calculated using the formula:
time spent
in a phase =
number of cells in a phase
total number of cells
x
720
minutes
For example: If 109 cells were counted in metaphase and 980 total cells were counted, then
(109 cells / 980 total cells) x 720 minutes = 80 minutes of time spent in metaphase
1. Calculate the time spent in each phase of the cell cycle using the data in column 1 and the math formula
above. Assume that the total time for mitosis is 720 minutes.
2. Record the calculated times in Table 1.
Table 1
Number of Cells
In Each Phase
M
I
T
O
S
I
S
Math to Determine Time
In Each Phase (see above)
Time In Each Phase
Interphase
Prophase
Metaphase
Anaphase
Telophase
Total
Number
Of Cells
1. Which phase requires the longest time for completion? __________________________________________
2. Using your text notes or the text book, describe the 3 main events of the phase you identified above.
A. ___________________________________________________________________________________
B. ___________________________________________________________________________________
C. ___________________________________________________________________________________
TABLE 2. TIME SPENT IN EACH PHASE OF THE CELL CYCLE
NORMAL AND CANCEROUS CHICKEN STOMACH CELLS (IN MINUTES)
NORMAL CHICKEN STOMACH CELLS
CANCEROUS CHICKEN STOMACH CELLS
Time spent in each phase (min.)
Time spent in each phase (min.)
Interphase
540
380
Prophase
60
45
Metaphase
10
10
Anaphase
3
3
Telophase
12
10
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Part C / Table 2 Analysis: Normal Cells vs. Cancerous Cells
1. Examine / compare the time spent in each phase of the cell cycle for a normal chicken stomach cell vs. the
cancerous one. Which phases show a significant difference in the time spent? __________________________
2. Calculate the TOTAL amount of time it takes for the cell cycle (cell division) to complete for each type of
cell and record below:
Normal Stomach Cell Cycle: ___________min.
Cancerous Stomach Cell Cycle: ___________min.
3. If you could observe the activity of the cells in adult chicken stomach tissue, you would find that cell cycle
only happens to maintain healthy tissue. In other words, a cell divides and becomes 2 cells when a pre-existing
cell dies off naturally. This is the normal way things happen in an adult organism – the total number of cells in
an area of tissue stays about the same. Hence, a cell dies and a new one is developed through the cell cycle (cell
division) to replace it. Let’s say that, on average, in healthy chicken stomachs there is at least one cell dying and
a new one produced every 625 minutes. (from # 2 above)
4. Consider the following: A NORMAL chicken stomach cells naturally die off and are replaced every 625
min. For reasons unknown, a single functioning CANCEROUS chicken cell develops among the normal ones,
HOWEVER it has a cell cycle time of 448 min. Let’s also say that cancerous cells will die off at the same rate
as normal cells are generated. In other words, the cancerous cells will reproduce (divide) - even though no
cancer cell has died off – every 448 min. and we’ll assume the number of cancer cell deaths equals the number
of normal cells that die. What kind of a problem arises from this? ___________________________________
5. Lets analyze the question from # 4 using some math. Some basic calculations are needed before we can
analyze the problem, Calculate the following:
Number of minutes in a day: ________________ Number of minutes in a week: _______________
Number of CANCEROUS cell cycles in a week: ______________ (round answer)
Number of NORMAL cell cycles in a week: ________________ (round answer – given the rules in step 4, this
number of normal cell cycles ALSO equals the number of cancerous cells that die off in a week)
Now analyze the problem:
6. How many MORE cell cycles does the CANCEROUS cell(s) go through one week after the cancer cell
developed?_______________.
7. What is happening to the total cell count in the area where the cancer cell developed? (Check # 4 above)
_____________________________ If you said the total cell count is increasing, you’re right! If the cell count
is increasing, which type will the extra ones be? _____________________. NOW FOR THE TRICKY PART:
How many more cells will there be one week after the cancer cell developed? _____________ If you guessed
the same number that you gave in #6 above (number of extra cell cycles), YOU’RE INCORRECT. If cell deaths
don’t equal cell births, the cell count can increase exponentially. In the first week of chicken stomach cancer,
there will be 7 extra cancerous cell cycles, but for EACH CYCLE the number of extra cells DOUBLES since
each new cell will make a copy of itself in that time. Given 7 extra cycles, the math would be 27 where the
exponent “7” equals the number of cell cycles. The number of cancer cells = 128
8. How many EXTRA cancer cells could possibly develop given 6 months of time (180 days)? __________
9. The result you get in # 8 is very, very unrealistic since there are many other factors involved which lower the
number like space, food and resources available as well as many others, but it illustrates how fast the number of
cells can increase when they freely grow exponentially. Given the opportunity to divide out of control, there
will be an extraordinarily high number of extra cells that form a MASS or LUMP in the area where it all began
called a TUMOR. See the extra “Read Further” page for more realistic details on this topic.
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Part D / Graphing the Time for The Cell Cycle of an Onion Root Tip Cell.
Using your data from Table 1 and the sectioned circle below, prepare a circle graph which shows the number of
minutes that onion cells spend in each phase of mitosis. The following suggestions may aid you in preparing
your graph.
a. Graph your data using the “Time in each Phase” column from Table 1 (the first table).
b. The circle is divided into 18 minute sections. Each section of the graph equals 18 minutes. If a phase is
not exactly 18 minutes long (or some interval close to a multiple of 18 minutes), approximate the
position of the line on the graph.
c. Shade each phase on your graph with colored pencils.
d. Identify each phase by shading the key to correspond with the shading on your graph.
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ONION ROOT TIP SAMPLE DIAGRAM / PHASES OF THE CELL CYCLE
KEY
LEAVE IN CLASS
LEAVE IN CLASS
MAGNIFIED
ONION ROOT TIP
INSET SECTION
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Read Further:
How fast does an invasive breast cancer grow?
From Wiki Answers:
Most women are shocked to find out that they have had their breast cancer for years before it was discovered.
Cancer growth is measured in doubling times. One doubling time is the amount of time that it takes for the mass
of cancer cells to double in size. It takes about 23 doubling times to go from one cancer cell to become a large
enough mass to be seen on a mammogram, and about 30 doubling times to become large enough to be felt as a
lump, about one billion cells. Doubling times may be as short as 10 days or as long as several years. Four
months is an average doubling time. A cancer's doubling time is an indication of how fast it grows. The
doubling time is very important in determining the time course of the cancer. For example, a cancer first starts
when a woman is 40. It is a fast growing cancer with a doubling time of 2 months. It takes 46 months (2x23), or
almost four years, before it is large enough to be detected by a mammogram and 60 months, or five years,
before she feels it. She is diagnosed with cancer at age 44 or 45. If, on the other hand, she has a slow growing
cancer with a doubling time of two years, she would be 86 before it could be found on a mammogram and 100
before she could feel it. She probably would die of other causes before she ever knew she had cancer. Carefully
performed autopsy studies show undetected invasive breast cancer in over 3% of women age 40 and older.
Since most breast cancers grow at an intermediate pace, usually the cancer has been present for 8-10 years
before being discovered. Metastatic cancer, like the original cancer in the breast, must double in size at least 23
times before it can be found.
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