Control of Gene Expression
Totipotency and cell specialisation
- All cells contain the same genes and therefore all cells are capable of making everything that the body can produce.
- Even though cells contain the same genes as every cell in the body, only some genes are expressed (switched on) in a certain cell; this is why the small intestine will produce maltase and not insulin, which is produced by the pancreas.
- Some genes are permanently expressed (switched on) in all cells, for example, the genes that code for essential chemicals, such as enzymes involved in respiration.
- Other genes are permanently not expressed (switched off), for example, insulin gene in the small intestine.
- Some genes are switched on and off as they are needed.
- Differentiated cells differ from each other; this is mainly because they each produce different proteins.
- The proteins that a cell produces are coded for by the genes it possesses or the genes that are expressed.
- These are cells which can mature into any form of body cells, such as embryonic stem cells where all genes have the potential to become active.
- Totipotent cells are cells before they have had the chance to differentiate and become specialised for a particular function.
- Found in mature humans, some genes are now switched off.
- Specialised cells:
- Only those few genes needed to function are now activatied.
- If genes express themselves in cells where they are not needed, things such as unneeded proteins are wasted and so there are two ways that make sure that genes do not express themselves:
- Preventing transcription and hence preventing the production of mRNA.
- Breaking down mRNA before its genetic code can be translated.
- Once cells have matured and specialised they can no longer develop into other cells and so lose their totipotency; the only cells which are still totepotent in mature animals are adult stem cells.
- Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced.
- They are found in the inner lining of the small intestine, in the skin and in the bone marrow, which produces red and white blood cells.
- Under certain conditions, stem cells can develop into any other types of cells.
- Because of this, they can be used to treat a variety of genetic disorders such as sickle cell anaemia.
- Mature plants have many totipotent cells. Under the right conditions, many plants can develop into any other cell.
Growth of plant tissue cultures:
- There are many factors that influence growth of plant tissue cultures from totipotent cells.
- One group consists of plant growth factors, which are chemicals involved in the growth and development of plant tissues.
- Plant growth factors have a number of features:
- They have a wide range of effects on plant tissue.
- The effects on a particular tissue depend upon the concentration of the growth factor.
- The same concentration affects different tissues in different ways.