1.3.1 The Cell Cycle and Mitosis *

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1 OpenStax-CNX module: m The Cell Cycle and Mitosis * Daniel Williamson This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License 3.0 The cell cycle and mitosis. Abstract Unit 1.3 Cell Division - Mitosis The Cell Cycle and Mitosis Animation on mitosis: 1 Khan: Phases of mitosishttp:// ofmitosis?playlist=biology Neok12: Mitosis games and videos 2 Cell Cycle: Khan: Chromosomes, Chromatids, Chromatin, etc: 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, :11 pm

2 OpenStax-CNX module: m Figure 1 Diagram - Cell division. 2 Phases The cell cycle consists of four distinct phases: G phase 16, S phase 17 (synthesis), G 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

3 OpenStax-CNX module: m 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 33 G 0 A resting phase where the cell has left the cycle and has stopped dividing

4 OpenStax-CNX module: m 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 33 G 0 A resting phase where the cell has left the cycle and has stopped dividing. 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.

5 OpenStax-CNX module: m 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 42 G 0 A resting phase where the cell has left the cycle and has stopped dividing. 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.

6 OpenStax-CNX module: m State Phase Abbreviation Description quiescent/senescent Gap 0 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 m 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 Cell division 40 Mitosis 41 M 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)

7 OpenStax-CNX module: m 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.

8 OpenStax-CNX module: m 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 n how_the_cell_cycle_works. 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 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 how_the_cell_cycle_works.html

9 OpenStax-CNX module: m 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 43 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 43

10 OpenStax-CNX module: m 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. 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 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 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.

11 OpenStax-CNX module: m Figure 6

12 OpenStax-CNX module: m Activity 3 - Quiz on Onion Tip Mitosis (with Answers) 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 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

13 OpenStax-CNX module: m B) Figure 9 C) Figure 11 D) Figure 13 E) Figure 15 continued on next page

14 OpenStax-CNX module: m A) Figure 17 B) Figure 19 C) Figure 21 D) Figure 23 continued on next page

15 OpenStax-CNX module: m E) Figure 25 3 A) Figure 27 B) Figure 29 C) Figure 31 continued on next page

16 OpenStax-CNX module: m D) Figure 33 E) Figure 35 4 A) Figure 37 B) Figure 39 continued on next page

17 OpenStax-CNX module: m 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).

18 i) OpenStax-CNX module: m Identify st Figure 45 iii Figure 47 continued on next page

19 OpenStax-CNX module: m 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).

20 OpenStax-CNX module: m 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.

21 1. late P 2. Metap 3. Teloph 4. early T 5. early T 6. Metap 7. Proph 8. Anaph 9. late A OpenStax-CNX module: m Key: Figure 52 Table 4