3.8.2.1 Most of a cell’s DNA is not translated

In multicellular organisms cells are specialized to perform specific functions. For example red blood cells carry oxygen whilst phagocytes carry out phagocytosis. The process by which cells develop into a specialized structure suited to their function is cell differentiation. In early development all cells are the same. As the organism matures it adapts to the function it will perform when it's mature.

All multicellular cells are derived by mitotic divisions f the zygote (fertilised egg) meaning they all contain the same genes. However, only certain genes are expressed in any certain cell at a time. Certain genes are always expressed such as genes that code for enzymes required for respiration.

As mentioned above, all cells develop from a single fertilised egg. Cells which can differentiate into any body cell (such as fertilised eggs) are known as totipotent cells. In mature mammals very few cells retain the ability to differentiate into any cell - these are known as stem cells. These are undifferentiated dividing cells that must be constantly replaced - they have the ability to divide and form an identical copy of themselves (self-renewal). There are a variety of sources of stem cells in mammals:

• Embryonic stem cells
• Come from embryos in the early stages of development. Can differentiate into any type of cell
• Umbilical cord blood stem cells
• Are derived from umbilical cord blood and are similar to adult stem cells
• Placental stem cells
• Found in the placenta and develop into specific types of cells
• Adult stem cells
• Found in the body tissues of the fetus through to the adult. Are specific to a particular tissue/organ within which they produce the cells to maintain and repair tissues throughout an organism's life

There are a few different types of stem cell we need to know about:

• Totipotent stem cells
• Found in the early embryo
• Can differentiate into any type of cell
• Pluripotent stem cells
• Found in embryos
• Can differentiate into almost any type of cell
• Examples include embryonic stem cells and fetal stem cells
• Multipotent stem cells
• Found in adults
• Can differentiate into a limited number of specialized cells - usually of a particular type (e.g stem cells in bone marrow can poduce any type of blood cell)
• Examples include adult stem cells and umbilical cord lood stem cells
• Unipotent stem cells
• Can only differentiate into a single type of cell
• Are derived from multipotent stem cells and are made in adult tissue
• Examples include cardiomyocytes (muscle cells that make up the cardiac muscle)

Induced pluripotent stem cells (iPS cells)

These are a type of pluripotent cell that is produced from unipotent stem cells. The unipotent stem cell may be almost any body cell. It is then genetically altered to make it acquire the characteristics of embryonic stem cells. To make it aquire the charateristics we induce genes andtranscriptional factors. 

iPS cells are usually not exact copies of embryonic tem cells. They are capacble of self-renewal meaning that potentially they can divide to produce an unlimited supply.

Pluripotent cells in treating human disorders

Pluripotent cells have endless uses. they can be used to treat...

• Burns and wounds (skin cells)
• Heart damage (cardiac muscle cells)
• Muscular dystrophy (skeletal muscle cells)
• Type 1 diabetes (beta cells of the pancreas)
• Parkinsons disease/multiple sclerosis/strokes/Alzheimer's disease/paralysis due to spinal injury (nerve cells)
• Leukaemia/inherited blood diseases (blood cells)
• Osteoperosis (bone cells)
• Osteoarthritis (cartilage cells)
• Macular degeneration (Retina cells of the eye)