embryo development and cell differentiation what happens after fertilisation

Fertilised call or zygote is said to be totipotent it has the potential to from 216 different cell types (however future cell roles are decided early in development)

• 1 = cleavage, involves special mitosis where the cells divide rapidly without interphase for growth between division, result mass of small undifferentiated cells forming hollow sphere. called a blastocyst
• 2= blastocyst, takes about a week is an early embryo
• Tiny cells in embryo are stem cells they have the potential to differentiate into any type of cell so they are totipotent like the zygote
• the outer layer of cells in the blastocyst become placenta cells
• 3= Pluripotent inner layer of cells have already lost some of the ability to differentiate they can become any type of cell for future but not placenta tissue so they are pluripotent embryonic stem cells
• 4= cell ditermination closely linked to the position of the cell in teh embryo ( e.g if you remove cells and grow on they will still produce predetermined cell type)
• 5= differentiate to form organs and tissue

Totipotent plant cells - cloning

• you can start with just one cell and grow a new plant
• the original cell must be kept totipotent with no genes perminently switched off
• comercially = cloning has increasing number of uses, comercially produced plants by tissue culture garuntees the quality of a new plant e.g rare orchids
• in reasurch = its of great value to have plants and animals genetically identical so differences in behaviour or development can be directly attributed to the experimental variables.

problems

• cloning plants is common place and easy because plant cells remain totipotent throughout their lives
• however cloning animals is harder as they are only totipotent for a short time in embryonic development becoming pluripotent then fully differentiated.

Embryo splitting

• one type of animal cloning is easier to carryout embryo splitting - relying on the fact that every cell in the early embryo is totipotent - used mostly in cattle breeding
• 1 embryo is created by ivf or in the mother natrully
• 2 the individuals of the early embryo are cloned by seperating them ionto individual cells, producing large numbers of genetically identical embryos
• 3 each are then placed back inside an ordinary cow which carries the calf to term and dilivers offspring not genetically theres
• alllows high quality cow to be produced on large scale using ordinary cows
• also allows transport of prize stock cheaply due to transporting embryos in surragate mothers such as rabbits (who are smaller and easier to move) then being extracted - ethics = rabits are killed to remove embryo also playing god

• cell differentiation happens when cells switch different genes on and off when needed  - may be due to an internal stimulus, or a change in the outside environment an external stimulus
• positions of the cells in the embryo result in different chemical gradients in their cytoplasm its this which triggers the start of cell differentiation
• once dfferentiation starts, particular proteins are made chemical differences in the cell increase and so differentiation continues
• common way of controlling gene expression is by switching on and off the transcription of certain genes

Transcription

• describes the process which the genetic cde of the DNA is copied to a complementary strand of RNA before protein synthesis can take place
• brought about by supercoiling some areas of the DNA sequence preventing genes being transcribed and uncoiling other areas so they can be transcribed and new proteins made



gene probes

• allows csientists to identify particular pieces of DNA in a cell
• gene probes find a unique sequence of nucleotides on the DNA that make up the gene
• this can be recognised using a stretch of complementary RNA sequence in a process known as DNA hybridisation

The process

• the DNA for some cell is isolated and heated gently
• this breaks the weak hydrogen bonds holding together the 2 strands of DNA
• radioactively labled mRNA for the required gene is added (this is the probe)
• any DNA-RNA hybredisation that takes place shows that the required gene is present
• this hybredisation is pinpointed using the radioactive lable on the mRNA which shows up on x-ray film (autoradiography)

Haemoglobin in the fetus=

• during fetus development different versions of the globin gene can be switched on and off
• adult haemolobin contains two alpha and 2 beta globin chains
• fetal haemoglobin which has a stronger affinity with oxygen contains 2 alpha and 2 gamma globin chains
• levels of different types of globin change throughout human fetus development
• due to genes from different proteins being switched on and off
• the genes are also activated in different tissue as development progresses

importance of a single molecule froming a single protein

• in most cases a single molecule controls the formation of a single protein
• but in some cases a singal molecule can set up a cascade of other signals which result in the formation of an entire organ a good example in the eye.
• mice and even humans a single protein controls the formation of eye development the protein produced as a result of the activation of the eye gene in the cell triggers the formation of a compleate organ
• this was tested moving the region which would code for the eye to the leg region in the fly embryo when it hatched it had an eye on its leg 

• allows us to clone animals from adult mammalian cell dolly the sheap was first
•  we can also produce clones of modifies animal which have been modifies to produce human proteins in their milk this is called transgenic animals and it allows us to culture large amounts of useful genes like insulin for diabetics ect...

where they are found :

•  embryonic stem cells = in embryo become increasingly specialised from early embryonic cells which are totipotent to blastocytes stage where inner cells become pluripotent to developed tissues and organs at the differentiation stage 
• Problems = ethics donated embryos from unwanted ivf traetment
• umbilical cord stem cells : blood that drains from the plecenta and umbilical cord after birth is a rich source of pluripotent stem cells. if blood is frozen and stored the cells would be available for the child or family if they needed them
• Problems= space not enought to do it for everyone so who is privilaged,  also genetic diseases like leukimia is said to be altready present in these cells pre birth
• Adult stem cells :found in the white bone marrow are undifferentiated  called somatic stem cells
• probems = only small number, very difficult to extract, most from limited range differentiated cells said to be multipotent, hard to grow in labs, also need to find the cell trigger to get them to differentiate into a specific cell.

stem cell cloning or therapeutic cloning :used to produce healthy tissue to treat someone with a disease caused by faulty cells  first step is to produce healthy cloned cells from the patient

• removing the nucleus from one of the normal ody cells, transfering it to a human ovum where the nucleus has already been removed
• mild electric shock the new pre embryonic cell starts to develope, producing a collection of embryonic cells with the same genetic infomation as the patient (cloned human embryo)embryo is simply a source of stem cells with the DNA that matches the patient perfectly
• stem cells are harvested from the embryo, which destroys it in the process
• the cells will be cultured in a suitable environmnet so they differentiate into the required tissue
• tissue will be transfered to the patient where they can do there job without risk of rejection from the bodys immune system.

problems = cancer increased risk, not sure exactly howe to switch genes on and off advantages = reduces isk of rejection if we can make organs from patiens own cells potenmtial cure for conditions we can't cure at the moment

• Parkinsons disease nerve cells which produce dopimine are lost so growing new one could slove problems
• Diabetes insulin secreating cells in the pancreas are destroyed or used out so can't control blood sugar levels
• damaged nerves e.g spinal cord problems
• organs for transplant

Ethics

some relativists think its ok if helping mankind as whole ( depending on the sircumstances  (e.g if the embryos will be destroyed anyway) others think its against human right and playing god. Absolutists are either always with it or against it with no grey areas.