Control of Gene Expression

• Totipotent cell- cells that can mature into any body cell
• Multipotent stem cells- found in mature mammals

Some genes are permantley expressed in some cells, whereas in others they are switched off.

Genes in specialised cells become switched off since it would be wasteful to synthesise unnecessary protiens

Ways which genes are prevented from expressing themselves:

• preventing transcription so prevents the productin of mRNA and polypeptides
• Breakingdown mRNA before translation

Only a few multipotent cells exsist in mature animals:

• Adult stem cells may be found in inner lining of intestine, bone marrow and skin. They can specialise and develop into certain cells
• Emryonic stem cells that occur in the earliest stage of development of an embryo

Mature plants have many totipotent cells. Growing cells outside of organism is known as invitro development

For transcription to start, the gene needs to be stimulated by a specific molecule (transcriptin factor) that moves from the cytoplasm into the nucleas. Each type of transcription factor has a site capable of binding to a specific region of DNA

When it binds, transcription can begin and mRNA forms, so a polypeptide can be synthesised

An inhibitor molecule can bind to a transcription factor where it woul bind to DNA. It therefore blocks the site where DNA binds, so transcriptin can't occur

Oestrogen is lipid-soluble so can pass through the phospholipid bi-layer of the plama meebrane into the cyctoplasm

Once inside the cytoplasm, it binds to a complementory receptor site on the transcriptinal factor molecule

When it does, the transcriptional factor changes shape and releases the inhibitor molecule from DNA binding site.

The transcriptional factor can now enter the nucleas and bind to a specific region of DNA where it will stimulate transcription 

siRNA= small interfering RNA. Double stranded.

Involves breaking down mRNA before translation

• An enzyme cuts the large double stranded RNA molecule into 2 smaller sections called siRNA
• One of the 2 strands of siRNA now combines with an enzyme
• Since the siRNA molecule has complementry bases to a region of mRNA, it can guide the enzyme to complementory section of mRNA
• Once enzyme is in correct postition, it cuts the mRNA into smaller sections that can no longer be translated

Uses

Used to identify genes in a biological pathway by adding siRNA that can block a particular gene, the effects of the gene can be deduced as a certain function can no longer take place

siRNA can also be used to block genes that are causing disease