AQA Biology unit 5 Chapter 15

Differentiated cells differ from one another. (mainly due to different proteins they produce)

An organism develops from a single fertilised egg which has the ability to give rise to all types of body cell - these cells that can mature into any type of cell are totipotent. 

Early cells derived from fertilised eggs are also totipotent, they later specialise for a specific function (e.g. mesophyll cells for photosynthesis & muscle cells for contraction.

This is because not all genes are expressed during cell specialisation - not all DNA is translated into proteins. The cell only produces the proteins needed to carry out its specialised function. 

NB: All cells can produce all proteins but as they aren't needed, this would be wasteful

The ways in which genes are prevented from expressing themselves include:

• Preventing transcription & hence production of mRNA
• Breaking down mRNA before its genetic code can be translated

If all cells have all the genes of an organism, can they still develop into another type of cell? It depends. Xylem vessels (transport water in plants) & red blood cells lose their nuclei once specialised - they can't develop into another type of cell

Specialisation irreversible in most animal cells - cells lose totipotency.

Only a few totipotent cells exist in mature animals, they are adult stem cells. They're undifferentiated dividing cells that occur in adult animal tissues & need to be constantly replaced. Found in: lining of small intestine, skin & bone marrow. 

Under certain conditions stem cells can develop into any other type of cell - as a result they can be used to treat a variety of genetic disorders such as the blood disease sickle cell anaemia.

As well as these, stem cells also occur at the earliest stage of development of an embryo, before the cells have differentiated. These are embryonic stem cells 

The situation in plants is different - mature plants have many totipotent cells. 

Under the right conditions, many plant cells can develop into any other cell. For example, if a cell is taken from a carrot root & placed in a nutrient medium & given certain chemical stimuli at the right time, a complete new carrot plant can be developed. 

Growing cells outside of a living organism in this way is called in vitro development

This new plant is genetically identical to the one from which the single root cell came, it's a clone.

Cells from most plant species can be used to clone new plants in this way. 

Many factors influence the growth of plant tissue from totipotent cells, one group are growth factors (chemicals involved in growth & development). Plant GF have many features: wide range of effects on plant tissue, the effects on a particular tissue depend on concentration of the GF, same concentration effects different tissues differently & effect of one GF can be modified by another

• The first few stem cells formed in division of the egg have the greatest potential to treat disease
• They can be grown in vitro & introduced to a range of human tissues.
• Many uses of cells grown in this way, they can be used to regrow tissues that have been damaged (e.g. skin graft)
• Currently, embryonic stem cell research is only allowed in the UK under licence & specialised conditions (conditions include its use as a way of researching embryonic development & diseases)
• One issue surrounding this is the argument as to whether or not a human embryo should be treated as a foetus or an adult - some feel this research undermines respect for human life & could lead to cloning
• Others disagree - it's only a ball of identical undifferentiated cells that has no resemblence to a human being - they also say it's wrong to allow suffering to continue when we could stop it
• Embryos are produced for other reasons (e.g. fertility treatments) - why not use those instead of destroying them?
• Stem cells from bone marrow, however, raises no real ethical issues, as long as the donor consents

Effect of oestrogen on gene transcription - general principles of preventing transcription:

• The gene needs to be stimulated by specific molecules - transcriptional factors (TF)
• Each TF has a site that binds to a specific region of DNA in the nucleus. 
• When it binds, it stimulates this region of DNA to begin transcription
• mRNA is produced & the genetic code it carries is translated into a polypeptide
• When a gene is not expressed, the site on the TF is blocked by an inhibitor molecule
• The inhibitor prevents the TF binding to DNA & prevents transcription & polypeptide synthesis

Hormones can switch on a gene by combining with a receptor on the TF that releases the inhibitor, this process occurs as follows:

• Oestrogen diffuses through phospholipid bilayer of membranes
• Once in cytoplasm, oestrogen combines with a site on a receptor molecule of the TF
• This changes the shape of receptor molecule which releases the inhibitor
• The TF can now enter the nucleus & bind with DNA
• Combination of TF & DNA stimulates transcription of the gene on that portion of DNA 

Gene expression can be stopped by breaking down mRNA before it's translated - this needs small double stranded sections of siRNA. Process happens as follows:

• An enzyme cuts big double stranded mRNA molecules into siRNA
• One of the 2 siRNA strands combines with an enzyme
• The siRNA molecule guides enzyme to mRNA molecule by pairing up its basrs with complementary ones on the mRNA 
• 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 the gene is not expressed

The siRNA has a number of potential scientific and medical uses:

• It could 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 (or lack of them) we can determine what the role of the blocked gene is
• As some diseases are caused by genes, it may be possible to use siRNA to block these genes and prevent the disease