Types of Cells

There are 3 types of cells when discussing cell division:

     Stem cells

          -Stem cells can divide and differentiate into any type of cell

          -They are great for repair and renewal

          Types of Stem Cells:

               Totipotent- These are the ultimate stem cells. They can become any type of cell. They are the first embryonic cells.

               Pluripotent- These types of stem cells derive from totipotent stem cells. Pluripotent stem cells are the stem cells we usually think of. They are in human tissue and can become any type of cell but are not able to sustain human life on their own like totipotent stem cells.

               Multipotent- These stem cells are capable of becoming different types of cells with in a certain lineage. For example a blood stem cell can become any type of blood cell; red blood cell, white blood cell, etc.

               Olgipotent- These stem cells can only turn into a select few types of cells.

     Other cells capable of dividing 

               -These cells are called unipotent. They can only divide into the type of cell they were.

          -Some cells can divide but can only differentiate into one (unipotent) or two (olgipotent) types of cells

          -For example liver cells can only make liver cells

     Permanently differentiated cells

          -Some cells cannot divide

          -They differentiate and never divide again

          -For example the human brain and heart cells

Class Activity:

As a class lets visit this page on Stem cells https://learn.genetics.utah.edu/content/stemcells/quickref

Lets read each section and discuss as a class what we are learning.

Chromosomes

     -There are autosomes which are chromosomes that both males and females have in common

     -And then there are sex chromosomes that are different depending if you are male or female

     -For example a human has 44 autosomes and 2 sex chromosomes or 22 pairs of autosomes and 1 pair of sex chromosomes.

     -In mammals the sex chromosomes are X and Y

     -The X chromosome is the female chromosome and the Y chromosome is the male chromosome

     -If you have a Y chromosome you are male

     -XX= Female and XY=Male

Chromosomes that contain the same genes are called homologous chromosomes

     -Just because they have the same genes does not mean that they are identical

     -They have the same genes but they have different alleles (different versions of those genes)

     -For example humans have two of chromosomes 1-22 so that means both of your chromosomes 1 are homologous, both of your chromosomes 2 are homologous and so on up to chromosome 22.

     -When it comes to the sex chromosomes, because females have two X chromosomes, we can say that their X chromosomes are homologous.

     -Because males have two different sex chromosomes, an X and a Y, they count as a pair but not a homologous pair. The X and the Y do not contain the same genes.

In each human cell...

     -A human has 23 pairs of chromosomes

     -A Human has 46 chromosomes all together

     -Even though a human has 46 they only have 23 different ones since a human has two of each chromosome. One set from mom one set from dad.

Diploid and Haploid

     -If an organism or cell has two of every chromosome it is called Diploid (Meaning Double)

     -Diploid organisms receive one set from each parent

     -Humans are a diploid organism

     -An organism or cell that only has one of every chromosome is called Haploid (Meaning Half)

     -There are some organisms that are haploid and usually reproduce asexually

     -Gametes are haploid cells because they will fuse with another haploid and become a diploid cell

     -Humans have two different types of cells

      -Gametes- sex cells like sperm and eggs which are haploid and contain half of your DNA

     -Somatic cells-body cells which are diploid and contain all of your DNA

Cell Growth and the Cell Cycle

Interphase is the normal growth and living stage of a cell

When a cell is not dividing it is in interphase

There are 3 steps to interphase.

     -G1 is the first growth stage where a cell grows

     -S phase is the synthesis stage. This is the time that a cell starts copying all of its DNA so when it comes time to divide it will have a copy for both cells

     -G2 is the 2nd growth stage where the cell grows for the purpose of division.

     -G0, if a cell is not going to divide, then once it is done with G1 it goes into G0, basically, zero growth. The G0 cell just lives out its function as long as it can. Like heart cells.

There are checks before many of the stages can continue

These checks are very important. They prevent a damaged cell from dividing. When damaged cells divide they could become a tumor and even cancer.

Stages of Mitosis

Prophase is the first phase is division. During prophase the nucleus breaks apart and the chromatin condenses into chromosomes. The chromosomes then begin to get pulled to the center of the cell

Metaphase is when the chromosomes are all aligned in the middle of the cell

Anaphase is when the sister chromatids are being pulled apart to opposite ends of the cell

Telophase is when 2 nuclei start to form around the separated chromatids, the chromatids unwind, and cytokinesis begins to split the cell

Student Exercise:

We are now going to go to the lab tables and practice using the microscopes. We will use the microscopes to locate and identify different stages of mitosis in cells.

Cancer is caused by unregulated cell division. It is basically mitosis gone wrong. 

Students Read:

Read this example of cancer and the cell cycle.

Cancer Arises from Homeostatic Imbalances

Cancer is an extremely complex condition, capable of arising from a wide variety of genetic and environmental causes. Typically, mutations or aberrations in a cell’s DNA that compromise normal cell cycle control systems lead to cancerous tumors. Cell cycle control is an example of a homeostatic mechanism that maintains proper cell function and health. While progressing through the phases of the cell cycle, a large variety of intracellular molecules provide stop and go signals to regulate movement forward to the next phase. These signals are maintained in an intricate balance so that the cell only proceeds to the next phase when it is ready. This homeostatic control of the cell cycle can be thought of like a car’s cruise control. Cruise control will continually apply just the right amount of acceleration to maintain a desired speed, unless the driver hits the brakes, in which case the car will slow down. Similarly, the cell includes molecular messengers, such as cyclins, that push the cell forward in its cycle.

In addition to cyclins, a class of proteins that are encoded by genes called proto-oncogenes provide important signals that regulate the cell cycle and move it forward. Examples of proto-oncogene products include cell-surface receptors for growth factors, or cell-signaling molecules, two classes of molecules that can promote DNA replication and cell division. In contrast, a second class of genes known as tumor suppressor genes sends stop signals during a cell cycle. For example, certain protein products of tumor suppressor genes signal potential problems with the DNA and thus stop the cell from dividing, while other proteins signal the cell to die if it is damaged beyond repair. Some tumor suppressor proteins also signal a sufficient surrounding cellular density, which indicates that the cell need not presently divide. The latter function is uniquely important in preventing tumor growth: normal cells exhibit a phenomenon called “contact inhibition;” thus, extensive cellular contact with neighboring cells causes a signal that stops further cell division.

These two contrasting classes of genes, proto-oncogenes and tumor suppressor genes, are like the accelerator and brake pedal of the cell’s own “cruise control system,” respectively. Under normal conditions, these stop and go signals are maintained in a homeostatic balance. Generally speaking, there are two ways that the cell’s cruise control can lose control: a malfunctioning (overactive) accelerator, or a malfunctioning (underactive) brake. When compromised through a mutation, or otherwise altered, proto-oncogenes can be converted to oncogenes, which produce oncoproteins that push a cell forward in its cycle and stimulate cell division even when it is undesirable to do so. For example, a cell that should be programmed to self-destruct (a process called apoptosis) due to extensive DNA damage might instead be triggered to proliferate by an oncoprotein. On the other hand, a dysfunctional tumor suppressor gene may fail to provide the cell with a necessary stop signal, also resulting in unwanted cell division and proliferation.

A delicate homeostatic balance between the many proto-oncogenes and tumor suppressor genes delicately controls the cell cycle and ensures that only healthy cells replicate. Therefore, a disruption of this homeostatic balance can cause aberrant cell division and cancerous growths.