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Transcript of Cell Growth!!!!
How do we get "big"?
When a living thing grows, what happens to its cells? When we get larger, is it because our cells are getting larger in size or is it because we have a larger number of cells? In most cases, the cells of an adult animal are no larger than those of a young animal - there are just more of them. Does this mean there are limitations to the size a cell can achieve?
Limits to cell growth
In order for an organism to grow larger, it will need to perform cellular division rather than enlarge its currently existing cells. There are two main reasons why cells divide rather than continuing to grow indefinitely. Can you think of the reasons?
As you know, the information that controls a cell's function is stored in a molecule called DNA. In eurkaryotic cells, the DNA is stored in the nucleus of the cell. In a small cell, the info stored in DNA can meet all of the cell's needs. However, when a cell grows larger, it usually does not make extra DNA. This can lead to an information crisis within the cell.
DNA overload is a lot like a town that only has one movie rental place. A small town may have only one option for movie rentals. If a lot of people move into the town (cell grows larger), people may have to wait longer for popular titles as the demand for these titles has risen along with the population increase.
Another reason for the limit on cell size is the exchange of materials. You may recall that food, oxygen, and water enter a cell through its membrane and that waste products exit the same way. The rate at which these exchanges occur depends on the surface area of the cell. However, the rate at which they are used depends on the cell's volume.
Surface Area / Volume Ratio
Imagine a cell that is shaped like a cube, like those in the picture below. Next, assume that all sides are 1 cm long. Use the equations below to calculate both the surface area and the volume for this particular cell. To get the ratio, divide the surface area by the volume. Next, determine the ratio if the cell's side are 2cm, then 3cm, then 4cm. Remember, the surface area determines how quickly the materials are exchanged, the volume determines how quickly they are used. Do you see the problem?
Surface Area / Volume Ratio
Note that the volume increases much more rapidly than the surface area, causing the ratio to decrease. This decrease creates serious problems for the cell. When the volume gets too great, it becomes like a highway during rush hour. Normally, one can drive 65mph along the highway, but when traffic volume builds the speeds of the cars must decrease. If a cell got too large, it would take longer and become more difficult to get food and oxygen in and waste products out.
Division is the Answer!!
Since growing indefinitely larger is not an option, a growing cell divides forming two "daughter" cells before becoming too large. The process by which a cell divides into two new daughter cells is called cell division. Understand, you have been doing this since you were a single cell. Today, you are comprised of trillions of cells and they all came from the first cell!
Division is the Answer!!
Before cell division occurs, the cell replicates its DNA. This replication of DNA gives each daughter cell its own complete set of genetic material. Also, each daughter cell has an increased ratio of surface area to volume ratio. Since each cell has its own DNA and increased ratio, both limitations for cell growth have been addressed through cell division.
Eukaryotic vs Prokaryotic
As we have previously learned, cells make duplicate copies of their DNA before they divide. This duplication ensures that the two new cells are identical. In most prokaryotes, the rest of the process of division is merely the separation of the remaining parts of the cell into two parts. However, for eukaryotes, the process of cell division is more complex and occurs in two stages.
Mitosis & Cytokinesis
The first stage of cell division is called mitosis. Mitosis is the division of the cell's nucleus. The second stage, cytokinesis, is the division of the cytoplasm. Many organisms, especially unicellular ones, reproduce by means of mitosis and cytokinesis. This form of reproduction is called asexual reproduction since the daughter cell is exactly like the parent cell. In humans, mitosis begins shortly after the egg has been fertilized.
In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes. Chromosomes are made up of DNA and proteins. The cells of every organism have a specific number of chromosomes. For example, the cells of a fruit fly have only 8 chromosomes and carrot cells have 18 chromosomes. Do you know how many chromosomes your cells have?
Show me the Chromosomes!
Chromosomes are not visible in most cells except during cell division. This is because the DNA and protein molecules that make up the chromosomes are spread throughout the nucleus. At the beginning cell division, however, the chromosomes condense into compact, visible structures that can be seen through a microscope.
Well before cell division, each chromosome is replicated, or copied. Because of this, each chromosome consists of two identical chromatids. When the cell divides, the "sister" chromatids separate from each other. One will go into each of the two new cells.
Each pair of chromatids is attached at an area called the centromere. Centromeres are typically located in the middle of the chromosome, but can sometimes lie near the end.
Fertilization and Cell Division
The Cell Cycle
Biologists have learned that a great deal happens in the time between cell divisions, and use a concept known as the cell cycle to represent recurring events in the life of the cell. During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again.
The Cell Cycle
The G1 phase is the phase in which the cells do most of their growing. During this phase, cells increase in size and synthesize new proteins and organelles. G1 is followed by the S phase, in which chromosomes are replicated and the synthesis of DNA molecules takes place. Usually, once a cell enters the S phase and begins the replication of chromosomes, it completes the rest of the cell cycle.
The Cell Cycle
When the DNA replication is completed, the cell enters the G2 phase. G2 is usually the shortest of the three phases of interphase (the in-between period of growth). During the G2 phase, many of the organelles and molecules required for cell division are produced. When the events of G2 are completed, the cell is ready to enter the M phase.
Mitosis is considered the first stage of cell division, and is the stage in which the cell's nucleus is divided. Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase. Depending on the type of cell, the four phases of mitosis may last anywhere from a few minutes to a several days.
The first and longest phase of mitosis is the prophase. It can take up to 50-60% of the total time required to complete mitosis. During prophase, the chromosomes become visible. The centrioles separate and take up positions on opposite sides of the nucleus.
The centrioles lie in a region called the centrosome that helps to organize the spindle, a fanlike microtubule structure that helps separate the chromosomes. During prophase, the chromosomes become attached to the spindle at a point near the centromere of each chromatid. Near the end, the chromosomes coil more tightly, the nucleolus disappears, and the nuclear envelope breaks down.
The second phase of mitosis, metaphase, often last only a few minutes. During metaphase, the chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the two poles of the spindle.
Anaphase is the third phase of mitosis. During anaphase, the centromeres that join the sister chromatids split, allowing the chromatids to separate and become individual chromosomes. The two chromosomes continue to move until they separated into two groups near the poles of the spindle. Anaphase ends when the chromosomes stop moving.
Telophase the fourth and final phase of mitosis. In telophase, the chromosomes, which were distinct and condensed, begin to disperse into a tangle of dense material. A new nuclear envelope re-forms around each cluster of chromosomes. The spindle begins to break apart, and a nucleolus becomes visible in each daughter nucleus. This completes Mitosis, but not cell division.
As a result of mitosis, two nuclei - each with a duplicate set of chromosomes - are formed, usually within the cytoplasm of a single cell. All that remains to complete the M phase of the cycle is cytokinesis, the division of the cytoplasm itself. Cytokinesis normally occurs at the same time as telophase.
Cytokinesis can take place in a number of ways. In most animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each part contains its own nucleus and cytoplasmic organelles. In plants, a structure known as the cell plate forms midway between the divided nuclei. The cell plate gradually develops into a separating membrane and a cell wall then begins to appear in the cell plate.
Do all cells divide?
Regulating the Cell Cycle
One of the most striking aspects of cell behavior in a multicellular organism is how carefully cell growth and cell division are controlled. Some cells, like neurons, do not divide once they have developed, while skin cells may pass through the cell cycle every few hours.
Controls on Cell Division
Scientists can observe the effects of controlled cell growth in the laboratory by placing some cells in a petri dish containing nutrient broth. Most cells will grow and divide until they form a thin layer covering the bottom of the dish. When the cells come into contact with other cells, they respond by not growing. If a chunk of cells is removed from the middle, the cells begin dividing again until the empty space is removed.
Well....What does this mean?!?
What did these experiments tell scientists? What possibilities were created with this knowledge of cell behavior?
Controls on Cell Division
Something similar happens within the body. When an injury such as a cut or a break in a bone occurs, cells at the edges of injuries are stimulated to divide rapidly. This action produces new cells, starting the process of healing. When the healing process nears completion, the rate of cell division slows, controls on growth are restored, and everything returns to normal.
Who is the regulator?
For many years, biologist searched for a substance that might regulate the cell cycle - something that would "tell" cells when it was time to divide, duplicate their chromosomes, or enter another phase of the cycle. In the early 1980's, biologists found the substance.
The Regulator Revealed!!!
Several scientists discovered that cells in mitosis contained a protein that when injected into a non-dividing cell, would cause a mitotic spindle to form. To their surprise, they discovered that the amount of this protein rose and fell in time with the cell cycle. They decided to call this protein cyclin because it seemed to regulate the cell cycle.
Proteins that respond to events inside the cell are called internal regulators. These proteins allow the cell cycle to proceed only when certain processes have happened inside the cell. For example, several regulatory proteins make sure the cell doesn't enter mitosis until the chromosomes have been replicated.
Proteins that respond to events outside the cell are called external regulators. They tell the cell when to speed up or slow down the cell cycle. Growth factors such as wound healing and embryonic development are among the most important tasks of these external regulators. Also, these molecules found on the surfaces of other cells send signals that prevent excessive growth.
Why do we need regulators?
You may be asking yourself "why is cell growth regulated so carefully?". The principal reason may be that consequences of uncontrolled cell growth in a multicellular organism are very severe. Cancer, a disorder in which some of the body's own cells lose the ability to control growth, is one such example.
Cancer cells do not respond to the signals that regulate the growth of most cells. As a result, they divide uncontrollably and form masses of cells called tumors that can damage surrounding tissues. Cancer cells may break loose from tumors and travel to other parts of the body (metastasis) and cause further damage and even death. There are many causes for the development of cancer, however, all types of cancer have one thing in common: a disruption in the cell cycle.
Cancer is a serious disease. Understanding and combating cancer remains a major scientific challenge, but scientists at least know where to start. Cancer is a disease of the cell cycle, and conquering cancer will require a much deeper understanding of the processes that control the cell cycle.
When an individual is diagnosed with cancer, the individual has options depending on the type of cancer. Some cancer cells respond (are damaged by) to radiation treatments, some to chemotherapy, and some to both. Surgery is sometimes an option as well. One of the most important factors in cancer management is early detection and treatment.