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1.3.1 The Cell Cycle and Mitosis
Daniel Williamson
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
Abstract
The cell cycle and mitosis.
Unit 1.3 Cell Division - Mitosis
The Cell Cycle and Mitosis
Animation on mitosis: http://www.sumanasinc.com/webcontent/animations/content/mitosis.html 1
Khan: Phases of mitosishttp://www.khanacademy.org/video/phases- ofmitosis?playlist=Biology
Neok12: Mitosis games and videos http://www.neok12.com/Cell-Division.htm 2
Cell Cycle:http://www.daviddarling.info/encyclopedia/C/cell_cycle.html
Khan: Chromosomes, Chromatids, Chromatin, etc: http://www.khanacademy.org/video/chromosomes
chromatidschromatin tc?playlist=Biology
1 Introduction
The cell cycle is the series of events that takes place in a cell 3 leading to its division and duplication
(replication). In cells without a nucleus ( prokaryotic 4 cells e.g. bacteria), the cell cycle occurs through
a process termed binary ssion 5 . In cells with a nucleus ( eukaryotes 6 ), the cell cycle can be divided
in two brief periods: interphase 7 during which the cell grows, accumulating nutrients needed for mitosis
and duplicating its DNA 8 and the mitosis 9 (M) phase, just after which the cell splits itself into two
distinct cells, often called "daughter cells". The cell-division cycle is a vital process by which a single-celled
fertilized egg 10 develops into a mature organism, as well as the process by which hair 11 , skin 12 , blood
cells 13 , some internal organs are renewed and wounds are healed
∗ Version
1.1: Feb 16, 2012 6:11 pm -0600
† http://creativecommons.org/licenses/by/3.0/
1 http://www.sumanasinc.com/webcontent/animations/content/mitosis.html
2 http://www.neok12.com/Cell-Division.htm
3 http://en.wikipedia.org/wiki/Cell_(biology)
4 http://en.wikipedia.org/wiki/Prokaryotic
5 http://en.wikipedia.org/wiki/Binary_ssion
6 http://en.wikipedia.org/wiki/Eukaryotes
7 http://en.wikipedia.org/wiki/Interphase
8 http://en.wikipedia.org/wiki/DNA_replication
9 http://en.wikipedia.org/wiki/Mitosis
10 http://en.wikipedia.org/wiki/Fertilized_egg
11 http://en.wikipedia.org/wiki/Hair
12 http://en.wikipedia.org/wiki/Skin
13 http://en.wikipedia.org/wiki/Blood_cell
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Figure 1
Diagram - Cell division.
2 Phases
The cell cycle consists of four distinct phases: G 14 1 15 phase 16 , S phase 17 (synthesis), G 18 2 19
phase 20 (collectively known as interphase 21 ) and M phase 22 (mitosis). M phase is itself composed of two
tightly coupled processes: mitosis, in which the cell's chromosomes 23 are divided between the two daughter
cells, and cytokinesis 24 , in which the cell's cytoplasm 25 divides in half forming two distinct cells.
14 http://en.wikipedia.org/wiki/G1_phase
15 http://en.wikipedia.org/wiki/G1_phase
16 http://en.wikipedia.org/wiki/G1_phase
17 http://en.wikipedia.org/wiki/S_phase
18 http://en.wikipedia.org/wiki/G2_phase
19 http://en.wikipedia.org/wiki/G2_phase
20 http://en.wikipedia.org/wiki/G2_phase
21 http://en.wikipedia.org/wiki/Interphase
22 http://en.wikipedia.org/wiki/Mitosis
23 http://en.wikipedia.org/wiki/Chromosomes
24 http://en.wikipedia.org/wiki/Cytokinesis
25 http://en.wikipedia.org/wiki/Cytoplasm
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Figure 2
Diagram - Schematic of the cell cycle. outer ring: I = Interphase 26 , M = Mitosis 27 ; inner ring: M =
Mitosis 28 , G 1 = Gap 1 29 , G 2 = Gap 2 30 , S = Synthesis 31 ; not in ring: G 0 = Gap 0/Resting
32
. [1]
Phase
Abbreviation
Description
quiescent/senescent Gap 0
dividing.
33
G 0 A resting phase where the cell has left the cycle and has stopped
26 http://en.wikipedia.org/wiki/Interphase
27 http://en.wikipedia.org/wiki/Mitosis
28 http://en.wikipedia.org/wiki/Mitosis
29 http://en.wikipedia.org/wiki/G1_phase
30 http://en.wikipedia.org/wiki/G2_phase
31 http://en.wikipedia.org/wiki/S_phase
32 http://en.wikipedia.org/wiki/G0_phase
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Interphase 34 Gap 1 35 G 1 Cells increase in size in Gap 1. The G 1 checkpoint control
mechanism ensures that everything is ready for DNA 36 synthesis.
Synthesis 37 S
DNA replication 38 occurs during this phase.
Gap 2 39 G 2 During the gap between DNA synthesis and mitosis, the cell will continue to grow.
The G 2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase
and divide.
Cell division 40 Mitosis 41 M
Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter
cells. A checkpoint in the middle of mitosis ( Metaphase Checkpoint ) ensures that the cell is ready to
complete cell division.
Phase
Abbreviation
Description
quiescent/senescent Gap 0
dividing.
33
G 0 A resting phase where the cell has left the cycle and has stopped
Interphase 34 Gap 1 35 G 1 Cells increase in size in Gap 1. The G 1 checkpoint control
mechanism ensures that everything is ready for DNA 36 synthesis.
Synthesis 37 S
DNA replication 38 occurs during this phase.
Gap 2 39 G 2 During the gap between DNA synthesis and mitosis, the cell will continue to grow.
The G 2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase
and divide.
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Cell division 40 Mitosis 41 M
Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter
cells. A checkpoint in the middle of mitosis ( Metaphase Checkpoint ) ensures that the cell is ready to
complete cell division.
Phase
Abbreviation
Description
quiescent/senescent Gap 0
dividing.
42
G 0 A resting phase where the cell has left the cycle and has stopped
Interphase 43 Gap 1 44 G 1 Cells increase in size in Gap 1. The G 1 checkpoint control
mechanism ensures that everything is ready for DNA 45 synthesis.
Synthesis 46 S
DNA replication 47 occurs during this phase.
Gap 2 48 G 2 During the gap between DNA synthesis and mitosis, the cell will continue to grow.
The G 2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase
and divide.
Cell division 49 Mitosis 50 M
Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter
cells. A checkpoint in the middle of mitosis ( Metaphase Checkpoint ) ensures that the cell is ready to
complete cell division.
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State
Phase
quiescent/senescent
Interphase
6
34
Gap 0
33
Gap 1
35
Synthesis
Gap 2
Cell division
40
Mitosis
39
41
37
Abbreviation
G 0
G 1
S
G 2
M
Description
A resting phase where the cell has left the cycle and has stopped
Cells increase in size in Gap 1. The G 1 checkpoint control m
DNA replication
38
occurs during this phase.
During the gap between DNA synthesis and mitosis, the cell wil
Cell growth stops at this stage and cellular energy is focused on
Table 1
Table Phases of the cell cycle (the details of the G and S phases are not required but are included to
give an overview).
33 http://en.wikipedia.org/wiki/G0_phase
34 http://en.wikipedia.org/wiki/Interphase
35 http://en.wikipedia.org/wiki/G1_phase
36 http://en.wikipedia.org/wiki/DNA
37 http://en.wikipedia.org/wiki/S_phase
38 http://en.wikipedia.org/wiki/DNA_replication
39 http://en.wikipedia.org/wiki/G2_phase
40 http://en.wikipedia.org/wiki/Cell_division
41 http://en.wikipedia.org/wiki/Mitosis
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3 Stages of Mitosis
Figure 3
Diagram Allium (Onion) cells in the dierent cycle of mitosis.
3.1 Interphase
The cell spends most of its life in the interphase. During this phase the cell grows to its maximum size and
performs its normal functions. Many scientists do not count interphase as part of mitosis.
3.2 Prophase
The chromatin (a special protein (actually a nucleoprotein) that chromosomes are made of) condenses into
chromosomes (human cells have 46 chromosomes 23 from your father and 23 from your mother). Each
chromosome eventually can be seen to consist of two strands or chromatids joined at a central centromere
in an X shape. The nuclear membrane disappears. The centriole splits and starts to move to opposite poles.
Spindle threads form between the poles.
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3.3 Metaphase
Chromosomes lie on the equator of the cell. Each chromosome is attached to the spindle microbers by its
centromere. The chromosomes appear in a straight line across the middle of the cell.
In the other form of cell division, meiosis, homologous chromosomes line up in pairs, side by side.
3.4 Anaphase
The centromere splits. Each chromosome divides into two sister chromatids. Each chromatid is moved to
opposite poles of the cell by the shortening of the spindle bres.
Chromatids (now called daughter chromosomes ) gather at opposite poles of the cell.
3.5 Telophase
A nuclear membrane forms around each of the daughter chromosomes that have gathered at the poles. The
daughter chromosomes uncoil to form diuse chromatin. The cytoplasm then divides during a process called
cytokinesis . Note cytokinesis is not a stage of mitosis but the process of the cytoplasm splitting into
two. There are now two genetically identical daughter cells. They are identical to the parent cell and to each
other. In an animal cell an invagination or infolding will divide the cytoplasm. In a plant cell a cross
wall divides the cytoplasm.
Animation Cell cycle and stages of mitosis 42
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animatio n__how_the_cell_cycle_works.
3.6 Summary of mitosis
Two identical daughter cells are formed from the mother cell.
Each daughter cell has the same number of chromosomes as the mother cell.
Each daughter cell will grow to its maximum size.
4 Biological importance of mitosis
Growth Living tissue grows by mitosis e.g. bone and skin.
Repair - Damaged and worn-out tissues are replaced with new cells by mitosis.
Asexual reproduction - Single-celled (unicellular) organisms and bacteria often reproduce asexually by
mitosis. Organisms like amoeba are able to split from a single individual into two and therefore can reproduce
without a mate and sexual reproduction.
5 Chromosomes
In mitosis at the end of prophase the chromosomes appear as X-shaped threads. Each thread is in fact a
chromatid and they are joined in the centre at a point called the centromere. There are two of each chromosome and the full set of chromosomes is often shown with the complimentary or homologous chromosomes
paired up.
42 http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_cell_cycle_works.html
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Figure 4
6 Activity 1 - Investigating mitosis in allium root tip squash
Introduction
Talking about what chromosomes do during mitosis could be very interesting, but seeing them for yourself
adds an extra dimension.
Video - Preparing Microscope Slides
http://www.youtube.com/watch?v=GHnndVuaync
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Lesson Organisation
The allium/onoin roots need to be prepared 1-10 days in advance of the lesson. Some practitioners
report that cutting the root tips around noon makes a dierence to the mitotic index, so you may want
your technician to cut and `x' the tips in ethanoic alcohol rather than ask your students to carry out this
step. If you have access to a video microscope it is worth capturing some images as this procedure can be
frustrating.
Method
1. Pour approximately 5 ml. methanol-acetic acid xative into a small beaker. Place 2-3 mm length
onion root tip into the xative. Incubate at 60 C for 15 mins. 2. At the end of the xative incubation period,
pour o the xative into a waste beaker. Be careful not to pour o your xed onion root tip. Now add
approximately 5 ml. 1 M HCl to your xed onion root tip to partially hydrolize the cells. Incubate at 60
C for exactly 10 mins. 3. At the end of the hydrolysis incubation period, pour o the 1 M HCl into a
waste beaker. Be careful not to pour o your xed/hydrolyzed onion root tip. Now add approximately 1 ml.
43 http://www.youtube.com/watch?v=GHnndVuaync
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Feulgen stain to your xed/hydrolyzed onion root tip in order to stain the chromosomes. Bath your onion
root tip in the Feulgen stain for 20-30 mins. to allow the Feulgen stain to penetrate the chromosomes.4. To
make a slide of your stained onion root tip, transfer your onion root tip from the beaker to a microscope
slide and add a small drop of 45% acetic acid. Do not allow the onion root tip to dry out during
the subsequent steps. Add 45% acetic acid if you notice your specimen is drying out. 5.
Pulverize your onion root tip into a ne pulp on the microscope slide by tapping it with a glass rod. Try
to produce as ne a pulp as possible to prevent large cell clumps which will not be useful for microscopic
examination.6. Now place a microscope cover slip on top of your pulverized onion root tip. Put two layers
of paper towel on top of the microscope cover slip and press down hard enough to squash the root but not
enough to break the cover slip. This should result in the onion root tip cells from forming a monolayer which
is ideal for microscopic examination.7. Using the scanning objective focus on the onion root tip cells and
identify a cell undergoing mitosis by looking for pink-staining bodies (chromosomes) within the cell.8. Switch
to the low-magnication10X objective and ne-focus.9. Switch to the high-magnication 40X objective and
ne-focus. At this magnication you should be able to identify cells in several stages of mitosis. Identify a
specic stage of mitosis and go on to step 10.10. Add a drop of oil to your prepared slide and switch to the
oil-immersion 100X objective and ne-focus. Repeat steps 9 and 10 for all four stages of mitosis.
As part of your Lab Exit Quiz you will be asked to: 1) show your prepared onion root tip slide
to a lab instructor under the microscope.2) identify several stages of mitosis on your prepared
onion root tip slide.
Mitotic Index
Figure 5
The duration of each stage of mitosis has been recorded and the data (see table below) could be used to
compare the observed frequencies of the dierent stages as recorded by students.
7 Activity 2 Stages of Mitosis
Task Look at Cells 1 -5. Decide which stage of mitosis each cell is in. For each cell describe the features
that make you think it is in this stage.
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Figure 6
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8 Activity 3 - Quiz on Onion Tip Mitosis (with Answers)
1. 1. Why do we study the root tip to nd mitosis instead of any other part of the onion plant? We study
the root tip because it is growing therefore cells are dividing rapidly. This makes it the best part of the plant
to observe various stages of mitosis.
2. Based on you data what can you infer about the relative length of time an onion root-tip cell spends
in each stage of the cell cycle? Most cells are in interphase because most time is spent in this phase.
3. Based on your understanding of the structure of the chromosome, why might it take longer to complete
prophase than the other phases of nuclear division? Prophase is the longest phase of mitosis because the
chromosomes have to coil up into organized bodies. It takes a long time for the chromatin to coil or condense
into chromosomes.
4. How do you account for the dierences between the slides made by dierent groups? Possible answers:
Not all lab groups had the same slide so there can be dierences among the growth rates of the plants that
were used to prepare the slide. The groups may have been looking at dierent areas of the root. Some
groups may not have followed the instructions as carefully as others.
5. If you examined cells in the Zone of Dierentiation (Zone of Maturation) would you expect to get
similar results? No Why or why not? These cells are starting to specialize into mature tissues. They are
no longer meristematic cells.
6. Why did we use the pie chart to graph the data? The pie chart was used because the data represented
the parts of a whole and it is relatively easy to show proportions of the whole event.
9 Assignment 1 - Animation of Mitosis and Multiple Choice Questions on Mitosis
http://bealbio.wikispaces.com/Genetics 44
Description: Gives a detailed description of the steps involved in mitosis with animated videos and
narrative voice-over. Short multiple choice quiz provided containing the following multiple choice questions:
1
A)
Figure 7
continued on next page
44 http://bealbio.wikispaces.com/Genetics
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B)
Figure 9
C)
Figure 11
D)
Figure 13
E)
Figure 15
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2
A)
Figure 17
B)
Figure 19
C)
Figure 21
D)
Figure 23
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E)
Figure 25
3
A)
Figure 27
B)
Figure 29
C)
Figure 31
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D)
Figure 33
E)
Figure 35
4
A)
Figure 37
B)
Figure 39
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5
A)
Figure 41
B)
Figure 43
Table 2
10 Assignment 2 Observing Mitosis in an Onion Root Tip Activity
Mitosis Slides
Identify the stage of mitosis for each of the onion root-tip slides below (most stages are
represented more than once).
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Identify st
__i)_____
Figure 45
______iii
Figure 47
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_______
Figure 49
Table 3
Practice locating each of the stages of mitosis in the following slides of the onion root tip.
Each picture contains at least one cell at each stage of mitosis (and some stages are represented
by multiple cells).
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Figure 51
(vii)
Answers
i. Prophase; ii) Anaphase iii) Metaphase iv) Interphase v) Anaphase vi) Metaphase
Answers to (vii)
The numbered arrows indicate cells at various stages of mitosis (most of the rest of the
cells are in interphase). The key to the stages is to the right of each gure. You will not
be asked to distinguish early from late; this is provided to help clarify the more ambiguous
stages.
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Key:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Figure 52
Table 4
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late P
Metap
Teloph
early T
early T
Metap
Proph
Anaph
late A