Unit #5 Cell Cycle

Chapter 9 Pages 242-257

Introduction

Many events in nature occur in cycles: the seasons, the tides, etc. Living organisms are no exception. One cycle common to most living things is the cell cycle. As a cell goes through this cycle, it will pass through two periods: a period of growth and a period of division. This unit will be divided into these corresponding sections—here we go!

Part #1 Interphase

I. The Cell CycleA. A cell’s life follows the cell cycle. The cell cycle can be depicted in Figure 1. The cell cycle is the sequence of growth. DNA replication, growth, and cell division are events that all cells go through.

Figure 1

1. Interphase

a. Interphase can be divided into 3 phases: i. G1 phase At the beginning of interphase, the

cells are brand new. They are very small and do not have very much chemical energy (ATP). During this phase, the cell grows rapidly and makes ATP. Many metabolic activities are occuring. For example, proteins are being made and enzymes are being produced. Of the 3 phases of interphase, the G1 phase lasts the longest. G stands for growth.

1. Interphase

ii. S phase After acquiring sufficient energy, the cell enters into the S phase of interphase. During this phase, the cell copies or replicates its chromosomes (DNA). This process takes a lot of energy in the form of ATP. This must be done so the cell can divide. The S stands for synthesis.

1. Interphase

iii. G2 phase Because the cell used a lot of its energy in the S phase as it duplicated its chromosomes, the cell undergoes a second growth and energy-producing phase called the G2 phase. The energy made during this phase is used in the upcoming mitosis. During this time, mitochondria and other organelles are made. Cell parts (such as the centrioles that are needed for cell division) are made at this time. Chromosomes become tightly coiled.

1. Interphase

b. Some cells have a cell cycle that lasts for 20 minutes (bacteria), while other cells have a cell cycle that lasts for 87 hours. Others, such as nerve cells, lose their ability to divide once they reach maturity. Liver cells maintain their ability to divide, but do not normally do so unless part of the liver is removed. Then the liver cells will divide until the liver reaches its former size.

II. Cell Size Limitations

A. A cell spends a good amount of time in interphase getting the cell ready to undergo mitosis/cell division.

B. Why does a cell need to divide? Why can’t it just keep growing until it is 14 ft tall and 5 ft wide?1. Most cells are between 2 and 200 microns in diameter. A blood cell is pretty small, with a diameter of just 8 microns. Nerve cells can be much larger—up to 1 mm, but they have very small diameters. The cell with the largest diameter is the yolk of an ostrich egg—8 cm!

2. Diffusion of nutrients into the cell and waste out of the cell limit cell size. The cell membrane allows a steady supply of nutrients such as oxygen and glucose to enter the cell. Glucose has to enter the cell and travel to the mitochondria, where it can then be converted to energy. As long as the distance that something (like glucose) has to travel is short, diffusion is an effective way for things to get from one place to another in and around the cell.

Let’s say, however, that we had a cell with a diameter of 20 cm (about 8 inches)—basically, a really big cell! It would actually take months for glucose and oxygen to reach a mitochondria at the center of the cell—the cell would die if it took nutrients that long to reach the appropriate organelles!

This point can be illustrated in Figure 2. Notice how the surface area (the size of the cell’s surface) does not increase at as fast of a rate as the volume (inside) of the cell.

Figure 2:

3. DNA limits cell size. We have discussed how the DNA contains the blueprints for which proteins should be made in the cell. There is a limit as to how quickly the blueprints for these proteins can be copied in the nucleus and made on the ribosomes. The cell cannot survive unless there is enough DNA to support the protein needs of the cell. Some cells that are big have more than one nucleus in order to help with protein production.

Questions to answer:

On the back of your notes paper, please record and answer the following:

In the U.S., 1 in ____ people develop cancer at some point in their lives.

The number of people that I have known with cancer = _____

Types of cancers that I’ve heard of:

III. Control of the Cell Cycle

A. Normal control of the cell cycle1. Certain enzymes control the cell cycle.2. Which enzymes get made in the cell is

controlled by the DNA (located on the chromosomes in the nucleus). A certain segment of DNA that codes for a certain enzyme or any type of protein is called a gene.

B. A mistake in the cell cycle may lead to cancer1. Cancer can be the result of a change to one or more

genes that code for enzyme(s) that are involved in controlling the cell cycle. When the cycle of cell division is not controlled, cells divide more rapidly. Cancer cells are rapidly dividing cells that can eventually form masses of tissue called tumors. These tumors (if not treated or if treatment does not work) cause other vital organs and organ systems not to work; this can lead to death.

2. Cancer is the second leading cause of death in the U.S.—it is second only to heart disease.

a. In the U.S., breast, colon, prostate, and lung cancer are the most

common types of cancer.

3. What causes cancer?a. Environmental factors: i. Different people, different countries = different types of cancer at different rates ii. Breast = high in U.S., low in Japan Stomach = low in U.S., high in Japan iii.Smoke, air & water pollution, UV radiation

cause genes to be damaged or mutated, causing cancer

b. Genetic factors:i. A small number are hereditary…

People who inherit specific gene mutations have a higher risk of developing certain forms of cancer as compared to the general population.

4. How can cancer be prevented?a. A healthy lifestyle! i. A diet that is low in fat and high in

fiber (lots of fruits and veggies) ii. No tobacco iii.Vitamins (A, C, E, Ca, carotenes)

Part #2--Mitosis

I. Mitosis defined A. Mitosis is another word for cell division.

When a cell divides, it is a continuous process. During mitosis, things are constantly moving to the two ends of the cell. However, mitosis can be divided into 5 distinct phases.

B. Mitosis is a type of cell division in which the number of chromosomes in the 2 resulting daughter cells is the same as the number and kind of chromosomes in the original parent cell. For example, if the parent cell started out with 46 chromosomes, then after mitosis, each daughter cell would have 46 chromosomes. How many chromosomes does every human cell (except egg and sperm cells) have?

II. Phases of Mitosis

A. Refer to the cell cycle drawing. Does a cell spend more of its time in interphase or mitosis?

B. Why does a cell divide (undergo mitosis)?

So, it is pretty important that a cell undergoes mitosis or it will die. What if we could stop the process of mitosis in cancer cells—would that be good? YES! One cancer-fighting drug used right now is taxol.

C. The 5 phases of mitosis

1. Phase #1 = Prophase a. First phase b. Longest phase c. Prophase occurs right after the G2 phase

of interphase is complete, so the chromosomes have duplicated themselves. Each chromosome is attached to its duplicate with a structure called a centromere. The original chromosome and its duplicate are called sister chromatids.

d. What happens during prophase?1. The chromosomes begin to coil up.2. At the beginning of prophase, the nuclear

membrane begins to disintegrate; by the end of prophase, it is totally gone.

3. The nucleolus disappears.4. In animal cells, centrioles begin to move the

poles of the cell.5. The spindle fibers begin to form. These are

structures that will help the chromosomes move to the opposite ends of the cell.

2. Phase #2 = Metaphase

a. Second phase b. What happens during metaphase? 1. The doubled chromosomes become attached to the spindle fibers 2. The chromosomes are pulled by the spindle fibers and line up on the equator 3. The chromosomes line up such that each new cell will receive an identical set of

chromosomes that is identical to the parent cells. 4. Asters (in animal cells) become more visible.

3. Phase #3 = Anaphase

a. Third phaseb. What happens during anaphase?

1. The chromosomes separate from each other. They are pulled apart from each

other and move towards the poles by the shortening of the spindle fibers.

4. Phase #4 = Telophase

a. Fourth phaseb. What happens during telophase?

1. As the chromosomes reach the opposite poles of the cell, telophase begins.

2. The chromosomes begin to uncoil. 3. The spindle fibers begin to break down, as they

are no longer needed. 4. The nucleolus reappears. 5. A new nuclear membrane forms around each

new set of chromosomes as they sit at the poles.

5. Phase #5 = Cytokinesisa. ‘Cyto’ stands for cytoplasm, and ‘kinesis’ means movement.b. What happens during cytokinesis?

1. Equal amounts of cytoplasm and the organelles within it move to each of the two opposite halves of the cell. In an animal cell, a furrow will form as the cleavage furrow pinches across the cell. In a plant cell, a cell plate is formed across the equator. A new cell wall will form on each side of the cell plate. 2. The two new cells are called daughter cells. They are very small.