Stem cells

What is a stem cell?

Stem cells are undifferentiated cells which can keep dividing to give rise to other types of cell types. They can either: 

• Divide by mitosis to produce more stem cells (self-renewal) 

• Differentiate into specialised cells (exits the cell cycle and loses the capacity to divide) 

What are stem cells needed for in the body?

1. Growth 

2. Repair of tissues  

3. Replacement of dead cells  

Totipotent stem cells

• Can differentiate into all types of specialised cells in the body; have the capacity to form an entire organism. In animals, only stem cells from the very early embryo (less than 3 days old) are totipotent

Pluripotent stem cells

• Can differentiate into all types of cell but not a whole organism. Embryonic stem cells from blastocyst stage embryos (3-5 days old; approx. 150 cells stage) are pluripotent. Pluripotent stem cells in mammals can’t form the placenta

Multipotent stem cells

• Can differentiate into multiple types of specialised cells, but not all types. Tissue stem cells are multipotent 

Unipotent stem cells

• Stem cells that can only become one type of cell, happens at the end of specialisation 

iPS

• Induced pluripotent stem cells (don’t exist naturally as pluripotent) 

• Adult somatic cells were de-differentiated/reprogrammed them, so that the cells regain the capacity to differentiate into any type of cell in the body 

• No ethical issues (an adult that can give consent), will also be recognised as self-cells and not induce an immune response 

What are transcription factors?

• Proteins that physically bind to the ‘promoter’ regions upstream of genes to activate (or inhibit) the transcription of the gene (they usually do this by helping RNA polymerase to bind or by stopping RNA polymerase from binding). 

• They bind during transcription to the exposed DNA strand  

•  Transcription factors are globular proteins therefore they are synthesised in the ribosomes (free ribosomes as they are internal proteins, not to be secreted outside of the cell). Transcription factors are produced in the cytoplasm, but operate within the nucleus, therefore, to control the action of transcription factors, their movement between these two locations can be controlled. 

What is oestrogen, and what does it do?

• Can control the transcription of around 100 different genes 

• Oestrogen is a steroid hormone, meaning it is lipid-based and hydrophobic  

How does oestrogen work to control transcription factors?

• Oestrogen can diffuse directly through the cell surface membrane phospholipid bilayer into the cytoplasm 

• In the cytoplasm it binds to an internal oestrogen receptor (a protein with a complementary shape to oestrogen) 

• Binding to the receptor causes the receptor to undergo a conformational shape change 

• The receptor is now able to diffuse through a nuclear pore into the nucleus 

• The new shape of the receptor allows it to bind, in combination with other proteins, to the DNA of the promoter region of the gene to be expressed 

• This stimulates RNA polymerase binding and gene transcription (where the receptor acts as the transcription factor) 

How can expression of a gene be reduced?

• Expression of a gene can be reduced by either preventing transcription (transcription factors), and hence preventing the production of mRNA, or by the breakdown of mRNA before its genetic code can be translated 

Epigenetics 

The study of how environmental factors can cause heritable changes in gene activity without changing the base sequence of DNA. An important concept is that the shape of the DNA-histone complex, including how tightly wound the DNA is around the histone proteins, can be changed