Lesson 10.1
Cell Growth, Division, and Reproduction
Do cells just keep growing or do they divide?
We Divide!
The larger a cell becomes, the more demands the cell places on it's DNA. A larger cell is less efficient in moving nutrients and waste materials across the cell membrane.
2 Reasons Cell Size is Limitied:
1. Information overload
2. Exchanging materials across the membrane
Information Overload
As a cell grows, the information in it's DNA is used to build the molecules needed for cell growth. As the cell increases in size, the DNA does not. This places too much "stress" on the DNA.
Similarly,
if a town library represents the cell's DNA, as the town grows it may outgrow its library's supply of books. It may be time to build another library!
Exchanging Materials
Food, Oxygen, and water enter a cell through its cell membrane while waste products leave the same way. The rate this can take place depends on the surface area of a cell membrane.
As shown below, the rate at which the surface area grows is not as fast as the rate at which the inside cell volume grows. In other words, the inside outgrows the outside!
The ratio of cell membrane surface area to cell volume decreases as the cell gets larger. Therefore, the cell membrane cannot keep up with all the transportation needs of the larger cell.
This causes traffic problems for the cell. The membrane transport system becomes jammed!
So, rather than causing traffic jams we divide!
Before a cell becomes too large, it divides and creates two "daughter cells".
This is called cell division.
Before cell division can occur, the cell must copy all of it's DNA so that each new daughter cell has a full copy, or set of instructions. Cells without a full copy will probably not thrive.
Because of cell division, cell membranes are easily able to support the transport of these smaller cells.
Asexual Reproduction
Many organisms are single-celled organism, like bacteria. Bacteria don't have to go out and meet a mate, conduct a courtship, or fight off rivals. All they have to do is divide!
For these types of organisms, reproduction of organisms is simply based on cell division. The process is simple, efficient, and effective helping populations to increase in number very quickly.
The cells produced in cell division are almost always genetically identical to the "parent" cell.
(variations only happens due to mutations)
Therefore,
Asexual Reproduction is the production of genetically identical offspring from a single parent.
Sexual Reproduction
Unlike asexual reproduction, where cells separate into a new individual, sexual reproduction involves the fusion of two separate parent cells. Offspring, are produced by the fusion of special reproductive cells formed by each of the two parents.
Offspring produced by sexual reproduction inherit some of their genetic information from each parent!
This reproduction includes most animals and plants.
Animals and plants also use asexual reproduction of cells to grow and heal!
Compared to asexual reproduction, in which all offspring are genetically identical, sexual reproduction allows for much genetic variation of each species.
This is beneficial for a species' survival so that some of the same species may have more "survivable" traits than others of the same species.
Example: If there were a great flood and the flood waters were up to 6 feet, only the people who stand over 6 feet tall could hold their heads above water (assuming there was nothing to stand on) and would survive and reproduce a taller species of people. This is how species evolve over time.
Whereas, if people were all genetically identical and we were all 5'10", no one of the species would survive the flood and the species would be extinguished.
Therefore, organisms who reproduce sexually and have genetic diversity may be able to adapt to environmental changes better than those who reproduce asexually and are genetically identical.
Yay us!
For 10.1 Powerpoint click here:
https://docs.google.com/presentation/d/1Q600ysVmruDb67oX-Tyo16fYWr-kuWBf4ZMwNxZR4oQ/edit?usp=sharing
Lesson 10.2
The Process of Cell Division
What process does cell division play in your life?
Humans use asexual cell division (mitosis) to grow and repair. We use sexual reproduction (meiosis) to make new organisms.
If cells just split in two without any prior organization of it's DNA, the outcome would be a mess!
The Prokaryotic Cell Cycle
The prokaryotic cell cycle is a regular pattern of growth, DNA replication, and cell division that can take place very rapidly under ideal conditions. This is a form of asexual reproduction known as binary fission.
Eukaryotic Chromosomes
Eukaryotic chromosomes generally have much more DNA than prokaryotes. Therefore, they contain many more chromosomes. Fruit flies have 8. Humans have 46. Carrots have 18. Eu-chromosomes form a close association with histones, a type of protein. The complex of chromosome and protein is called chromatin. DNA tightly coils around the histones, and together, the DNA and histone molecules form beadlike structures called nucleosomes. Nucleosomes coil into Coils which coil into Supercoils. Supercoils become chromosomes, tightly wounds strands of DNA.
Chromosomes make it possible to separate DNA precisely during cell division.
The Eukaryotic Cell Cycle
The eukaryotic cell cycle consists of four phases:
What process does cell division play in your life?
Humans use asexual cell division (mitosis) to grow and repair. We use sexual reproduction (meiosis) to make new organisms.
If cells just split in two without any prior organization of it's DNA, the outcome would be a mess!
In order to make sure that each daughter cell is created equally, the cells must first make a complete copy of their genetic information before cell division begins. This takes place during Interphase.
Prokaryotic Chromosomes
Even a small cell like a bacterium has a tremendous amount of genetic information in the form of DNA. The prokaryotic DNA would be roughly 1000 times longer than the cell. This amount of genetic molecule has to be carefully packaged. DNA is bundled into packages of DNA called chromosomes.
Prokaryotes lack nuclei so their DNA molecules are found in the cytoplasm. Most prokaryotes contain a single, circular DNA chromosome.
The prokaryotic cell cycle is a regular pattern of growth, DNA replication, and cell division that can take place very rapidly under ideal conditions. This is a form of asexual reproduction known as binary fission.
 |
| Once the chromosome has been replicated, the two DNA molecules attach to different regions of the cell membrane. A network of fibers forms between them, stretching from one side of the cell to the other. The fibers constrict and the cell is pinched inward, dividing the cytoplasm and chromosomes between two newly formed cells. This results in the production of two genetically identical cells. |
Eukaryotic chromosomes generally have much more DNA than prokaryotes. Therefore, they contain many more chromosomes. Fruit flies have 8. Humans have 46. Carrots have 18. Eu-chromosomes form a close association with histones, a type of protein. The complex of chromosome and protein is called chromatin. DNA tightly coils around the histones, and together, the DNA and histone molecules form beadlike structures called nucleosomes. Nucleosomes coil into Coils which coil into Supercoils. Supercoils become chromosomes, tightly wounds strands of DNA.
Chromosomes make it possible to separate DNA precisely during cell division.
The Eukaryotic Cell Cycle
The eukaryotic cell cycle consists of four phases:
Interphase contains:
G1 Phase: Cell growth: Cells do most of their growing during this phase. Cells increase in size and synthesize new proteins and organelles.
S Phase: S stands for synthesis of new DNA when chromosomes are replicated. At the end of this phase the cell contains twice as much DNA as it did at the beginning.
G2 Phase: Preparing for cell division. Usually the shortest of the three phases of interphase. Molecules required for cell division are produced (centrioles and microtubules).
Then:
M Phase: Cell Division. One parent cell produces two daughter cells. Mitosis takes place quickly.
M Phase has two parts: Mitosis (division of the nucleus) and Cytokinesis (division of the cytoplasm)
Phases of Mitosis
Prophase: Usually the longest phase of mitosis.
Metaphase: "meet in the middle"
Anaphase: "away or apart"
Telophase: "need a tele to talk"
Cytokinesis
After Mitosis, all that remains to complete the M phase of the cycle is cytokinesis, the division of the cytoplasm. This often occurs during Telophase.
Cytokinesis completes the process of cell division-- splitting one cell into two.
Cytokinesis in Animal Cells: in animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each contains its own nucleus and cytoplasm.
Cytokinesis in Plant Cells: The cell membrane is not flexible enough to draw inward because of the rigid cell wall that surrounds it. Instead, a structure known as a cell plate forms halfway between the divided nuclei. The cell plate gradually becomes the cell membrane that separates the daughter cells. A cell wall then forms in between the two new membranes.
To view the 10.2 powerpoint click here:
https://docs.google.com/presentation/d/1j3110Mpo9n1ShGYB1PVd24g_xrlKgeeWvyZD8d3ZGos/edit?usp=sharing
Lesson 10.3 and 10. 4
Regulating the Cell Cycle
and Cell Differentiation
Not all cells move through the cell cycle at the same rate!
In the human body, there are four types of tissues.... epithelial, connective, muscle and nervous tissue.
If you get a cut, your epithelial cells get to work replacing the injured and dead cells very quickly.
But, if you injure your nervous tissue (brain and spinal cord) these cells don't replace themselves. Nerve cells don't undergo mitosis anymore once a person has stopped growing into an adult.
Then again, your red blood cells are being produces at the astounding rate of 2.4 million cells/second!
What controls the cell cycle?
When scientists grow cells in a lab, they learned that the cells will grow and divide until they come into contact with each other. Then they stop growing.
This is why cells at a wound will rapidly divide until the wound is healed, then stop dividing.
Cyclins
For many years, scientists looked for answers to explain what the controllers of the cell cycle are. Finally, in the early 1980s they discovered that an increase in proteins called cyclins (cycle) seemed to cause a cell to go into mitosis.
Cyclins - regulate the timing of the cell cycle in eukaryotes
Since the discovery of cyclins, many more regulatory proteins have been discovered.
Regulatory Proteins
The cell cycle is controlled by regulatory proteins both inside and outside the cell.
Internal Regulators: respond to events occurring inside a cell by allowing each phase to progress only after the prior phase has been completed.
Apoptosis: Some cells are programmed to die. :( Apoptosis is programmed cell death.
First the cell and its chromatin shrink, then parts of the cell's membranes break apart.
Other cells quickly clean up the cell's remains (macrophages).
One of the reasons our bone marrow produces 2.4 million RBCs per second is that our RBCs only last for 100-120 days.
The foot of a mouse during embryonic development looks quite different after birth thanks to apoptosis of the cells between the toes, allowing for greater dexterity of the mouses foot.
When apoptosis doesn't occur as it should, diseases can result.
Cancer
Why are there cell cycle regulators? The results of uncontrolled cell growth are quite severe.
Cancer - a disorder in which body cells lose the ability to control growth. This is caused by defects in the genes that regulate cell growth and division.
Cancer cells do not respond to the signals that regulate the growth of most cells.
As a result, the cells divide uncontrollably forming a mass called a tumor.
The human body contains an estimated 60 to 100 trillion cells. Amazingly, this is the result of only 47 cell divisions starting from the first cell!
The first few cell divisions create an embryo from which will grow into an adult organism. During the development process, an organism's cells become more and more differentiated and specialized for particular functions.
Differentiation: the process by which cells become specialized. Cells become specialized to perform certain tasks. Once a cell becomes specialized (like a nerve cell), they cannot change into another kind of cell (like a skin cell).
Unicellular organisms do not differentiate!
Totipotent: literally able to do everything; the ability of a cell to develop into any type of cell in the body.
Blastocyst : a hollow ball of cells with a cluster of cells inside known as the inner cell mass. The inner cell mass will become the embryo. The outer ball of cells will become the outer membranes that hold a fetus and the placenta in the uterus.
The cells of the inner cell mass, or embryo, are made up of stem cells. Stem cells have the ability to develop into any type of cell.
Researchers have proven that embryonic stem cells can been coaxed into many different specialized cells including neurons, muscle cells, fat cells, and even sperm and egg cells.
Adult stem cells do not have quite the versatility that embryonic stem cells do. Adult stem cells can develop into similar tissues that they come from. For instance, the adult bone marrow stem cell can make any kind of blood cell.
Scientists would like to learn exactly which signals tell a cell to become specialized, and how other cells remain multipotent.
The importance stem cells might have for human health make stem cell research a top priority among researchers. They offer the potential benefit of using undifferentiated cells to repair or replace badly damaged cells and tissues.
But, harvesting (gathering) stem cells is not without its controversy. Because harvesting embryonic stem cells usually kills the embryo they come from, there are ethical issues of life and death involved both for and against it.
Whereas, harvesting adult stem cells does not require taking a life so adult stem cell research is not controversial and may allow potentially lifesaving research to go forward.
Regulating the Cell Cycle
and Cell Differentiation
Not all cells move through the cell cycle at the same rate!
In the human body, there are four types of tissues.... epithelial, connective, muscle and nervous tissue.
If you get a cut, your epithelial cells get to work replacing the injured and dead cells very quickly.
But, if you injure your nervous tissue (brain and spinal cord) these cells don't replace themselves. Nerve cells don't undergo mitosis anymore once a person has stopped growing into an adult.
Then again, your red blood cells are being produces at the astounding rate of 2.4 million cells/second!
What controls the cell cycle?
When scientists grow cells in a lab, they learned that the cells will grow and divide until they come into contact with each other. Then they stop growing.
This is why cells at a wound will rapidly divide until the wound is healed, then stop dividing.
Cyclins
For many years, scientists looked for answers to explain what the controllers of the cell cycle are. Finally, in the early 1980s they discovered that an increase in proteins called cyclins (cycle) seemed to cause a cell to go into mitosis.
Cyclins - regulate the timing of the cell cycle in eukaryotes
Since the discovery of cyclins, many more regulatory proteins have been discovered.
Regulatory Proteins
The cell cycle is controlled by regulatory proteins both inside and outside the cell.
Internal Regulators: respond to events occurring inside a cell by allowing each phase to progress only after the prior phase has been completed.
External Regulators: direct cells to speed up or slow down the cell cycle.
- Growth Factors - type of external regulators which stimulate the growth and division of cells. Important during embryonic development and wound healing.
- Other external regulators on the surface of neighboring cells have the opposite effect. They cause cells to slow down or stop their cell cycles preventing excessive growth.
Cyclin Levels in Fertilized Clam Eggs
 |
| Scientists measured cyclin levels in clam egg cells as the cells went through their first mitotic divisions after fertilization. As the cyclin levels rose to a certain level mitosis was triggered. |
First the cell and its chromatin shrink, then parts of the cell's membranes break apart.
Other cells quickly clean up the cell's remains (macrophages).
One of the reasons our bone marrow produces 2.4 million RBCs per second is that our RBCs only last for 100-120 days.
The foot of a mouse during embryonic development looks quite different after birth thanks to apoptosis of the cells between the toes, allowing for greater dexterity of the mouses foot.
When apoptosis doesn't occur as it should, diseases can result.
Cancer
Why are there cell cycle regulators? The results of uncontrolled cell growth are quite severe.
Cancer - a disorder in which body cells lose the ability to control growth. This is caused by defects in the genes that regulate cell growth and division.
Cancer cells do not respond to the signals that regulate the growth of most cells.
As a result, the cells divide uncontrollably forming a mass called a tumor.
Tumors: benign - noncancerous, the cells grow into a mass but stop growing and do not spread to other tissues.
malignant - cancerous, cells invade and destroy surrounding healthy tissue absorbing the
nutrients needed by other cells, blocking nerve connections, and preventing
organs from functioning properly. Cancer is life-threatening.
External causes of cancer: radiation, carcinogens (cancer causing chemicals) such as tobacco, pesticides, asbestos, defective genes, and even some viruses.
p53 gene - a high number of cancer cells have a defective p53 gene which normally stops the cell cycle until all chromosomes have been properly replicated.
 |
| bloodstream or lymph vessels. The cancer then moves into other parts of the body and forms secondary tumors, called metastasis. |
Metastasis - When cancerous tumors spread from the primary tumor site to other parts of the body.
Treatments for Cancer
surgery to remove tumor
radiation
chemotherapy
Cancer cells divide much faster than normal cells hoarding the body's supply of energy and other materials for normal cell processes. This also makes cancer cells more susseptable to injury from radiation or chemotherapy. While normal cells might be injured or sickened by radiation and chemotherapy, the hope is that the cancer cells will be killed.
Cancer is a disease of the cell cycle, and conquering cancer will require a much deeper understanding of the processes that control cell division.
For powerpoint 10.3 click here
https://docs.google.com/presentation/d/1gb5uPWGlcP_25HqXU5nCfin0XzvxBLJGnbXxf-MZt0Y/edit?usp=sharing
The human body contains an estimated 60 to 100 trillion cells. Amazingly, this is the result of only 47 cell divisions starting from the first cell!
The first few cell divisions create an embryo from which will grow into an adult organism. During the development process, an organism's cells become more and more differentiated and specialized for particular functions.
Differentiation: the process by which cells become specialized. Cells become specialized to perform certain tasks. Once a cell becomes specialized (like a nerve cell), they cannot change into another kind of cell (like a skin cell).
Unicellular organisms do not differentiate!
Totipotent: literally able to do everything; the ability of a cell to develop into any type of cell in the body.
Blastocyst : a hollow ball of cells with a cluster of cells inside known as the inner cell mass. The inner cell mass will become the embryo. The outer ball of cells will become the outer membranes that hold a fetus and the placenta in the uterus.
Researchers have proven that embryonic stem cells can been coaxed into many different specialized cells including neurons, muscle cells, fat cells, and even sperm and egg cells.
Adult stem cells do not have quite the versatility that embryonic stem cells do. Adult stem cells can develop into similar tissues that they come from. For instance, the adult bone marrow stem cell can make any kind of blood cell.
Scientists would like to learn exactly which signals tell a cell to become specialized, and how other cells remain multipotent.
The importance stem cells might have for human health make stem cell research a top priority among researchers. They offer the potential benefit of using undifferentiated cells to repair or replace badly damaged cells and tissues.
But, harvesting (gathering) stem cells is not without its controversy. Because harvesting embryonic stem cells usually kills the embryo they come from, there are ethical issues of life and death involved both for and against it.
Whereas, harvesting adult stem cells does not require taking a life so adult stem cell research is not controversial and may allow potentially lifesaving research to go forward.
For powerpoint 10.4 click here
https://docs.google.com/presentation/d/1BrSc3Xo6vE_Say73dJ6axNKWECQiFz92UpPi-dbMLIo/edit?usp=sharing
An Overview of Stem Cell Research
by Center for Bioethics and Human Dignity
https://cbhd.org/stem-cell-research/overview
An Overview of Stem Cell Research
by Center for Bioethics and Human Dignity
https://cbhd.org/stem-cell-research/overview
