3.8.2.1 Most of a cell's DNA is not tranlated

     3.8.2.1 Most of a cell's DNA is not translated

What do we need to know from the specification?

         What is a stem cell?

         1. Explain what totipotent cells are

         2. Explain how cells lose their totipotency and become specialised

         3. Describe the origins and types of stem cells

         4. Describe and explain induced pleuripotent stem cells and their significance in medical ethics

         5. Explain how pleuripotent stem cells can be used to treat human disorders

         

1. Explain what totipotent cells are

When an organism is developing from a signle fertilised egg, the resulting zygote has the potential to give rise to all of the types of body cell found in the organism.  A cell which can mature into any body cell is called a totipotent cell.

2. Explain how cells lose their totipotency and become specialised

The initial embryonic cells are totipotent, however as they mature, divide and develop, they lose this totipotency. Earlier cells which are derived form the initial embryonic cells are called pleuripotent, and again as these become more and more specialised, the cells lose their totipotency. The specialisation of cells allows it to be more suited to a specific function. For example mesophyll cells are more specialised for photosynthesis, and muscle cells are specialised for contraction. The process of specialisation is simultaneous with the loss of totipotency, and occurs through the selective expression of genes. This means only some of the cells DNA is translated into proteins. This DNA includes those for vital processes like respiration and membrane synthesis, and also those which are needed for the cell to carry out its specific function (e.g. the genes which allow B-cells of the pancreas to secrete insulin). The cell exclusively makes these proteins, and the production of unnecessary proteins would be a waste of energy and resources. The inhibition of such genes is carried out by 

• preventing transcription and thus the production of mRNA
• preventing translation and thus the productions of the protein.

In animal cells, while all body cells contain the entire genome for the body's proteome, once a cell has become specialised, they cannot develop into any cell. The act of specialisation in an animal cell is usually irreversible, due to the fact that genes outside of what it needs are switched off.

Few cells retain the ability to develop into other types of cell; these are called stem cells.

3. Describe the origins and types of stem cells

Stem cells are classed as stem cells because of their ability to differentiate into other cell types. However the extent of this ability can vary depending on the stem cell. Therefore stem cells can be classified according to their ability to differentiate;

• Totipotent stem cells- can differentiate into all types of body cell, the zygote is the first totipotent cell, and as it divided and matures, the resulting cells become slightly more specialised and become pleuripotent stem cells.
• Pleuripotent stem cells - can differentiate into almost any type of body cell, these are found in later stage embryos, and the fetus.
• Multipotent stem cells - can differentiate into a limited number of specialised cells. They usually develop into cells of a particular type, e.g. stem cells in bone marrow can differentiate into any type of blood cell. Adult stem cells, and umbilical blood stem cells are both multipotent.
• Unipotent stem cells- can differentiate into only one type of cell. They're a more specialised form of multipotent stem cells and are formed in adult tissue.

In mature mammals, on a few cells retain the ability to differentiate into other cells. These are called stem cells. Stem cells have various points of origination in mammals;

• Early embryonic stem cells. come from embryos in the early stages of development. These can differentiate into any type of cell in the initial stages of development, and so are totipotent.
• Umbilical cord blood stem cells,  found in umbilical cord blood and can differentiate into a limited range of cells, therefore are multipotent.
• Placental stem cells, come from the placenta and are multipotent.
• Adult stem cells (e.g. bone marrow cells) and found in body tissues of fetus through to adult and are specific to an organ or tissue. These are multipotent.

 4. Describe and explain induced pleuripotent stem cells and their significance in medical ethics

Induced pleuripotent stem cells (iPS) are a type of stem cell derived from unipotent stem cells. They are produced in a laboratory, in a process where they are genetically altered to make them acquire the characteristics of a later embyronic cell (which is pleuripotent). In order to do this, certain genes and transcription factors are induced, i.e. certain genes are reactivated.

Although they are not exact copies of embryonic cells, they are very similar in form and function.

One significant feature of them is that they are capable of self renewal. This mean they can potentially divide to form a limitless supply, the medical research profession would no longer require the use of real embryos which would solve some of the ethical issues surrounding embryonic use in labs.

5. Explain how pleuripotent stem cells can be used to treat human disorders

There are many possible uses of pleuripotent stem cells in treating disorders. The cells can be used to regrow tissues which have been damaged either by disease (e.g. parkinsons) or by accident (e.g. burns / wounds). 

Summary Questions

1. Explain what is meant by a totipotent cell.

Totipotent cells are cells with the ability to develop into any other cell of the organism.

2. Distinguish between totipotent, pleuripotent, multipotent and unipotent cells

Totipotent- can differentiate into any type of cell in the body and comprise the first few cells that form from the zygote.

Pleuripotent - can differentiate into almost any type of cell  and are found in the embryo and young fetus. 

Multipotent - can differentiate into a limited number of cells and are found in the umbilical cord, and some adult tissues (e.g. bone marrow)

Unipotent - can only differentiate into one type of cell and are found in adult tissues such as skin.

3. All cells possess the same genes and yet a skin cell can produce the protein keratin but not the protein myosin, while a muscle cell can produce myosin but not keratin. Explain why.

This is due to the inhibition (switching off) of certain genes. For example, in skin cells, the gene for keratin is switched on, but the gene for myosin is switched off. This means the keratin will be transcribed and translated into a protein, while myosin will not.

The inverse is true for the muscle cells.

4. Suggest a reason why skin cells retain an ability to divide by being unipotent when the cells of some other organs do not.

Skin cells, being on the outside of the body are much more prone to damage and so need replacing more frequently. Many other organs are less prone to damage and need little cell replacement.

Students should be able to evaluate the use of stem cells in treating human disorders

For the specification, it's necessary to know some arguments surrounding the ethical issue of the use of embryos in treating human disorder. Below are some reasons for and against.

For

• They have huge potential to cure debilitating diseases
• It is wrong to not cure suffering when it can be relieved
• If embryos are allowed to be created for other reasons (IVF), why can't they be produced for stem cells. 
• Embryos younger than 14 days are not recognisably human and so don't command the same respect as adults or fetuses.
• There is no risk of research escalating to include fetuses and current legislation prevents this.
• adult stem cells are not as suitable as embryonic stem cells and it may be years before they are, in the meantime many people suffer unnecessarily.

Against

• It is wrong to use humans (inc. potential humans) as a means to an end.
• Embryos are human as they have human genes and deserve same respect as an adult.
• Is a 'slippery slope' to the use of older embryos and fetuses.
• Could lead to research and development of human cloning.
• Undermines respect for human life.
• Adult stem cells are available and energies should be used developing these.