Life on Earth is composed of individual single-celled organisms, colonies of single-celled organisms, or organisms composed of many cells (multicellular).

Pond water is a typical environment for single-celled organisms.

 
Living things organize atoms into the molecules needed to sustain them, such as carbohydrates, amino acids, proteins, lipids, and nucleic acids.

They also exhibit several levels of cellular organization.

• cells - the basic units of life
   
• tissues - groups of different types of cells that perform a common function
   
• organs - groups of different types of tissues that perform a common function
   
• organ systems - groups of different types of organs that perform a common function
   
• organism - any complete living thing - no matter the number of cells.

The joining of sperm and egg in sexual reproduction produces a zygote.

A human egg cell is about 1/100^th of a centimeter in diameter, about the width of a human hair. Shortly after fertilization, the zygote begins dividing, replicating itself again and again. Before long, a mass of cells forms called a blastula, consisting of dozens - then hundreds - then thousands of stem cells. Each stem cell in the blastula is nearly identical.

There are about 200 different types of cells - many highly specialized - making up the tissues and organs of the human body. The cells that line the retina of your eye, for example, have a structure and function that is very different from those of the muscle cells in your biceps.

Stem cells begin their transformation into the different types of cells during a phase in the development process called cell differentiation. In vertebrates, differentiation begins during a stage called gastrulation, when distinct tissue layers first form.

Like most other developmental processes, differentiation is controlled by genes, the genetic instructions encoded in the DNA of every cell. Genes instruct each cell how and when to build the proteins that allow it to create the structures, and ultimately perform the functions, specific to its type of cell.

Every nucleus of every cell has the same set of genes. A heart cell nucleus contains skin cell genes, as well as the genes that instruct stomach cells how to absorb nutrients. This suggests that in order for cells to differentiate - to become different from one another - certain genes must somehow be activated, while others remain inactive.

Much of today's research is with embryonic stem cells. These are the "inner mass" cells of a 4 to 5 day old embryo in the blastocyst stage.

The term fetal stem cells simply means means they are taken from a fetus, which develops about eight weeks after the joining of sperm and egg.

Research with both embryonic and fetal stem cells is controversial. Their only source is from the destruction of a developing "baby", and many object to that.

Scientists have found that the umbilical cord and placenta both contain stem cells. These are known as cord blood stem cells. Since these stem cells can be obtained after a baby is born, their use is not as controversial.

There are even adult stem cells. One of the first places in the adult body where stem cells were identified was in bone marrow. Several types of specialized white blood cells are produced from one type of cell in the bone marrow.

Stem cell research has greatly increased our understanding of how cells coordinate the activation and inactivation of their genes. Success inducing these changes artificially has been almost completely limited to embryonic stem cells. There is something, as yet unknown, that makes them different from other types of stem cells. Gaining such control over the developmental process may eventually result in cures for a wide variety of diseases, including cancer.

 
Cancer can be simply defined as the uncontrolled division of cells. It begins in a single cell with a change in a gene that is responsible for cell growth or repair.

Anything that damages cells might cause cancer. Carcinogens are agents that are known to lead to the production of cancer cells. These are divided into three general groups:

• Physical - such as UV and ionizing radiation
   
• Chemical - such as asbestos and tobacco
   
• Biological - such as viruses, bacteria, fungi, and parasites

A tumor is a mass of cells resulting from uncontrolled, abnormal cell division

• All the cells of a benign tumor remain attached to the tumor.
   
• Some of the cells of a malignant tumor will metastasize - break away from the parent tumor. These cells are carried to other parts of the body by the blood, where they begin forming new tumors.

• Carcinoma - occurs in skin and nerve cells
   
• Sarcoma - occurs in bone and muscle cells
   
• Lymphoma - occurs in lymph cells

10 Facts About Cancer

 
Cancer cells are sometimes refered to as immortal, because they seem to have broken through the "biological clock" that limits normal cells to a specific life span. The "average" human body cell divides 30 to 50 times before it dies.

Biologists have determined the average life span for many human body cells. Some divide slowly and live for a long time while others divide quickly and live for a short time. Cancer cells not only divide faster than normal body cells, they do so for a much longer period of time.

Telomeres are bits of DNA at the ends of chromosomes - think of the hard ends of your shoelaces. While they do not contain genes, they do act as starting points for replication during cell division. Every time a cell divides, its telomeres are shortened, until they are so short that no futher cell division takes place. A cell that is unable to divide will eventually malfunction and die.

Almost 90% of all cancer cells have been found to contain large amounts of the enzyme telomerase, which seems to "rewind" our cellular clocks. Telomerase strengthens and lengthens telomeres. If telomerase stops telomere shortening, then in theory, those cells can live forever.

The shortening of telomeres have been associated with a number of diseases, many of them age-related, like the wrinkling of skin, the loss of action in neurons of the central nervous system, a decline in the immune system, and decreased effeciency in cells of the cardiovascular system.

As our cells age, so do we. Although not everyone agrees, the discovery of a cure for cancer may be closely tied to an increase in the expected human life span.