THE CELL CYCLE



THE CELL CYCLE

INTRODUCTION

The processes that occur during the life of a cell occur in a predictable, repeating pattern. As a cell proceeds through this process, it will grow in size. Once the cell has reached its growth potential, it will begin to divide. This division is referred to as mitosis.

You will observe the stages of the cell cycle, including mitosis, using the onion root tip. Growth occurring at the tip of an onion root requires cells to divide. Special stains have been applied to a very thin section of the root tip so that nuclear material is easily distinguished from the other parts of the cell. By observing and counting the numbers of cells in each phase of the cell cycle, you may estimate the amount of time a cell spends in each stage of the life cycle.

If you consider that the slide is a second of the cell’s life frozen in time, you will see that if a phase lasts a long time, there should be many cells in this stage. If a phase is very short, there should be very few cells caught in this stage. Today’s lab will allow you to estimate the percent of time spent in each phase of the cell cycle.

Events in the cell cycle occur in the following order:

Interphase

Events - The cell increases in size, carries on metabolism, and the chromosomes duplicate.

Appearance - The cell will contain a lightly stained round nucleus. The nucleus appears to be smooth. Most of the cells you will observe are in this stage.

Prophase (Mitosis Begins)

Events - Cell division begins. The nuclear membrane disintegrates. The chromosomes thicken and the mitotic apparatus (centrioles-only in animal cells-and spindle) is assembled.

Appearance - The nucleus still appears to be round, but now appears to be more darkly stained. The chromosomes have condensed and this often causes the nuclear area to appear as if it is very coarse. There may seem to be a mass of twisted thread where the nucleus should be.

Metaphase

Events - Each sister chromatid is attached to a spindle fiber, the chromosomes are pushed and pulled by these spindle fibers to line up along the central axis.

Appearance - The chromosomes appear lined up in the middle of the nucleus.

Anaphase

Events - Separation of sister chromatids occurs as the centromeres split. The chromatid pairs migrate to opposite sides of the cell.

Appearance - The chromosomes are migrating to opposite ends of the cells. They look like spiders doing pushups on a mirror.

Telophase

Events - The chromatids reach opposite poles (sides of cells) and the chromosomes unwind to direct metabolic activities, spindle fiber breakdown, nucleolus reappears and the nuclear envelop forms around the chromosomes.

Appearance - The chromosomes are organized in the darkly stained bundles on opposite sides of a cell. You will sometimes see the beginning of a new cell wall forming between these two bundles.

Cytokinesis

Events - In animal cells, the cellular membrane pinches in along the equator and the cell separates creating two identical daughter cells. Plant cells have a rigid cell wall and the cytoplasm is divided by the construction of a cell plate.

Appearance - There will be two dark spots of nuclear material at either end of the cell. A faint line (the cell plate) will appear in the middle of the cell.

PURPOSE

1. To get a better understanding of the cell cycle and mitosis you will use prepared onion root tip slides to identify the stages of the cell cycle and calculate the length of time of each phase of the cell cycle.

2. You will use pipe cleaners and/or modeling clay to construct models of chromosomes, which will be used to demonstrate the movement of chromosomes during mitosis.

SAFETY

1. Slides will break! Be sure to use good microscope skills.

2. Pipe cleaners have sharp ends. Use care during manipulation.

3. Keep clay and pipe cleaners out of your mouth and away from you eyes and noses.

4. Make sure the electrical cords are properly positioned to prevent tripping or electrocution

MATERIALS

2 pipe cleaners of one color and 2 of another color

Drawing paper

Pencil

Microscope

Onion root tip slide

Colored pencils

PROCEDURE

PART A. Building chromosomes with pipe cleaners

1. Obtain 4 pipe cleaners (two of 2 different colors) and a cell sheet.

2. Using a pencil, draw a circle that takes up most of the space on your paper. This circle represents the cell membrane. Draw a second circle about the size of a grapefruit in the center of the cell membrane. This represents the nuclear envelop.

3. When mitosis begins, the chromosomes have duplicated themselves and exist as dyads. A dyad is a single chromosome, which has two identical halves called sister chromatids. These sister chromatids are held together by a centromere. Bend the pipe cleaners into a V and hook them together to form two dyads. These two dyads represent two homologous chromosomes. The connected point represents the centromere and the arms of the pipe cleaners represent the sister chromatids.

4. We will begin with a simulation of mitosis with prophase. During prophase, the chromosomes thicken and shorten by tightly coiling and the nuclear membrane disappears. Coil individual chromatids around your finger to represent the coiling of chromosomes in prophase. Place these coiled chromosomes in the nucleus and draw mitotic apparatus on opposite sides of the nucleus and erase the nuclear membrane.

5. During metaphase, the chromosomes will align themselves in the center of the cell and attach to the spindle apparatus. Center the chromosomes between the spindle apparatus so that point of the V of each sister chromatid is pointing toward opposite spindle apparatus. Draw lines representing spindle fibers to each sister chromatid to the spindle apparatus. Each sister chromatid will be attached to its own spindle fiber.

6. The sister chromatids will migrate along the spindle fibers to opposite sides of the cell during anaphase. Place your finger on the V and pull the sister chromatids along the spindle fiber towards opposite spindle apparatus.

7. During telophase chromatids have reached the opposite sides of the cell and begin to uncoil. Each of these sister chromatids is now called a chromosome. Uncoil the chromosomes and erase the spindle fibers. Draw a nucleolus then draw a nuclear envelope around the nucleolus and the chromosomes. During this stage of mitosis, chromosomes are monads. They will not duplicate and become dyads until mitosis is over.

8. Cytokinesis occurs when the cellular membrane pinches together and two identical daughter cells are formed. This represents the end of mitosis. From opposite sides of the cell membrane, draw V’s that meet in the middle representing the pinching together of the cellular membrane to form two identical cells.

PART B. Drawing and quantifying the phases of the cell cycle

1. Obtain a prepared slide of onion root tip.

2. Using the high power objective locate and sketch an example of each of the five phases.

3. Observe the number of cells in each stage and predict which stage will be the longest. Record your prediction on the analysis sheet.

4. Select a row of cells. Scan down the row and identify the stage of the cell cycle for each cell. As you identify the stage, report the stage to your partner. Your partner will keep a record of the stage of each cell by placing a tally mark in the appropriate column on the data sheet. Count all the cells that are in the row. Do not skip any cell that can be identified. Continue counting until you have identified the stages of 50 cells.

5. After identifying 50 cells, have your partner identify 50 more while you make the tally marks.

6. Total the tally marks for each stage. Since you counted 100 cells, this number represents the percent of cells in each stage.

7. Use the formula on your data sheet to calculate the number of hours the cell spends in each phase of the cycle.

8. Clean your microscope and slide.

STUDENT DATA SHEET Name: ___________________________

THE CELL CYCLE Date: ___________________________

OBSERVATIONS

Sketch each phase here.

INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE

DATA

Place tally marks in the appropriate columns for each cell counted.

|INTERPHASE |PROPHASE |METAPHASE |ANAPHASE |TELOPHASE |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

|Total ________ |Total ________ |Total ________ |Total ________ |Total ________ |

________% ________% ________% _______% ________%

Interphase Prophase Metaphase Anaphase Telophase

Number of hours = 24 hr x Number of Cell Counted in each phase

100

Number of hours in each phase:

Interphase _____ Prophase _____ Metaphase _____ Anaphase _____ Telophase ______

ANALYSIS

Part A.

1. How can one dyad become two monads?

2. What is the difference between chromatids and chromosomes?

3. Why does mitosis occur?

4. What is the significance of the process of mitosis?

5. What is the final stage of mitotic division?

6. What is the fundamental difference between a monad and a dyad?

Part B.

1. I predict the longest stage of mitosis to be _____________ because:

2. Compare your prediction to the outcome.

3. In which phase of the cell cycle does a cell spend most of its time? The least time?

4. Describe the cell plate that was present during telophase. How would telophase differ in an animal cell?

5. Prepare a bar graph of the data you collected.

THE CELL CYCLE

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ALABAMA COURSE OF STUDY

Biology Content Standards: 5, 6

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