Control of Gene Expression

Although all cells contain all genes, only certain genes are expressed in any one cell at a time. 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 that are switched on.

An organism develops from a single fertilised egg. A fertilised egg is known as a totipotent cell as it can mature into any body cell. They later differentiate and become specialised for a particular function as only some of the genes are expressed. This means that only part of the DNA of a cell is translated into proteins. So the cell only makes proteins that it requires to carry out its specialised function. 

The ways that genes are prevented from expressing themselves include:

• preventing transcription and so preventing the production of mRNA
• breaking down mRNA before its genetic code is translated

When a totipotent cell becomes specialised it loses its totipotency and so cannot develop into other specialised cells. Only a few totipotent cells exist in mature animals: adult stem cells.

Stem cells are undifferentiated dividing cells that occur in adult animal tissues and embryos that require constant replacement.

They occur in the inner lining of the small intestine, in the skin and also in bone marrow that produces red and white blood cells. Stem cells can develop into any other type of cells, so they can be used to treat a variety of genetic disorders.

Embryonic stem cells occur at the earliest stage of the development of an embryo, before the cells have differentiated.

However, many plants have totipotent cells. Under the right conditions, many plant cells can develop into any other cell. 

Preventing and expressing a gene by preventing transcription:

• for transcription to begin the gene needs to be stimulated by specific molecules that move from the cytoplasm into the nucleus. These are called transcriptional factors
• each factor has a site that binds to a specific region of the DNA in the nucleus
• when it binds, it stimulates this region of DNA to begin transcription
• mRNA is produced and the genetic code it carries is translated into a polypeptide
• when a gene is not expressed, the site of the transcriptional factor that binds to DNA is blocked by an inhibitor molecule
• the inhibitor molecule prevents the transcriptional factor binding to DNA so prevents transcription and polypeptide synthesis

Oestrogen is a hormone that can switch on a gene and therefore start transcription. It does this by combining with the receptor on the transcriptional factor, which releases the inhibitor molecule.

• oestrogen is a lipid-soluble molecule and therefore easily diffuses through the phospholipid portion of the cell-surface membrane
• once inside the cytoplasm of a cell, oestrogen combines with a site on a receptor molecule of the transcriptional factor. The shape of this site and the shape of oestrogen molecule complement eachother
• by combining with the site, the oestrogen changes the shape of the receptor molecule. This change of shape releases the inhibitor molecule from the DNA binding site on the transcriptional factor
• the transcriptional factor can now enter the nucleus through a nuclear pore and combine with DNA
• the combination of the transcriptional factor with DNA stimulates transcription of the gene that makes up the portion of DNA

Gene expression can be prevented by breaking down mRNA before its genetic code can be translated into a polypeptide. Essential to this process are small double-stranded sections of RNA: small interfering RNA (siRNA). 

• an enzyme cuts large double-stranded molecules of RNA into smaller sections called siRNA
• one of the two siRNA strands combines with an enzyme
• the siRNA molecule guides the enzyme to a mRNA molecule by pairing up its bases with the complementary ones on a section of the mRNA molecule
• once in position, the enzyme cuts the mRNA into smaller sections
• the mRNA is no longer capable of being translated into a polypeptide
• this means that the gene has not been expresses (has been blocked)

The siRNA has uses:

• could be used to identify the role of genes in a biological pathway. Some siRNA that blocks a particular gene could be added to cells. By observing the effects we could determine what the role of the blocked gene is
• possible to use siRNA to block these genes and so prevent the disease