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

Stem cells  E – Describe how stem cell technologies work  C – Analyse data on the subject  A – Evaluate technology and methods

 Some genes are permanently switched on in all cells – can you think of any?  Some genes are permanently switched off in certain cells e.g. Insulin genes in the cells lining the small intestine.  Some genes can be switched on and off when they are needed.

 Organisms can develop from a fertilised egg, known as a zygote.  The zygote then starts to divide, it is symmeterical in its shape as well as its contents up to the eight cell stage.  At this point the cells are said to be totipotent.  This means that they are potentially capable of expressing all of the genes of their genome.

 At the 8 cell stage if the cells are separated from each other then the can grow into a normal individual.  It is a process used to make twins/triplet/quads etc... (up to octuplets) in cattle.

 The reverse is true as well, two different eight cell stage embryos can be combined and will develop into a single normal individual.  Such individuals are known as chimeras because it contains cells from different genetic lines.

 The totipotent zygote, which is capable of producing the entire organism, will first divide into pluripotent cells.  Pluripotent cells that do not have this ability but can nevertheless produce all the tissues of the organism.  Next will come multipotent cells, which can produce various cells within a particular tissue.  Last will come specialized cells.

 During the process of specialisation only some genes are switched on.  This means that only part of the DNA is translated into proteins, those which are required to carry out specialised functions.  Though the cell is capable of producing other proteins it would be wasteful to do so, so the genes for the proteins are not expressed. This is done in the following ways:  Prevention of transcription  Prevention of translation

 In animals – very few, can be for reasons such as the fact that red blood cells have lost their nucleus therefore no longer have all of the genes, others just loose their totipotency.  A few exists in mature animal cells these are known as adult stem cells. Can you think where they are likely to be found?  In plants many more cells are totipotent (main exception = xylem for the same reasons as the red blood cell)  Under the right conditions many plant cells can develop into any other cell, this can be the bases of cloning plants

 Using page 237, explain how plant growth factors could be used to grow a whole new plant from a single plant cell

 Using the following websites, you books and your own opinions in pairs/threes produce a presentation (and I mean a presentation not just a slide show!) where you explain:  what stem cell therapy is,  how it can be used to treat different diseases  the ethical, legal and social issues surrounding stem cell therapy.  c_intro.htm c_intro.htm    x.html x.html