Stem cells and cell specialisationEdexcel A-Level Biology A: Revision notes
Section 1
Stem cells and potency
A stem cell is an undifferentiated cell that can divide by mitosis to produce more cells and can differentiate into specialised cells.
- Totipotent cells can differentiate into any cell type, including the cells of the placenta, and can form a complete organism. Cells of the very early embryo (up to about the 8-cell stage) are totipotent.
- Pluripotent cells can differentiate into almost any cell type in the body but cannot form a whole organism or the placenta. Embryonic stem cells from the inner cell mass of the blastocyst are pluripotent.
Adult (tissue) stem cells, such as those in bone marrow, can form only a limited range of cell types.
Do not say pluripotent cells can make a whole organism; that is totipotent. Pluripotent cells cannot form the placenta.
Section 2
Using stem cells in medicine
Stem cells may be used to replace damaged or lost tissue, for example retinal cells in macular degeneration, or beta cells in type 1 diabetes. Bone marrow stem cells are already used to treat blood disorders.
Benefits: treat conditions that currently have no cure; cells from the patient are not rejected by the immune system.
Risks and problems: donor cells may be rejected; cells that divide uncontrollably could form tumours; the techniques are new and costly.
Section 3
How society decides
Science can show what stem cells can do and describe the risks and benefits, but it cannot say what is right. Obtaining embryonic stem cells involves destroying embryos, and some people object because they believe an embryo has a right to life. Others argue that using surplus IVF embryos that would otherwise be discarded, to treat disease, is acceptable. Adult stem cells avoid this concern but are more limited.
Decisions are made by society: governments and regulatory bodies listen to scientists, ethicists, religious groups, patients and the public, and make laws that control and license the research. Different countries reach different decisions.
In a discuss question give a benefit, a concern, who decides, and a justified conclusion.
Section 4
Differential gene expression
All the body cells of an organism come from the zygote by mitosis, so they contain the same genes. Cells differ because they express different genes. In a given cell some genes are switched on and others are switched off, in response to chemical signals.
A gene that is expressed is transcribed to produce active mRNA, which is translated to make a protein. The proteins then control the processes of the cell, as enzymes do, or determine its structure.
Examples. In animals, beta cells in the pancreas express the insulin gene and make insulin; red blood cell precursors express the haemoglobin gene. In plants, developing xylem vessels express genes for enzymes that make lignin, so their walls become thickened.
Specialised cells have not lost the genes they do not use. They still contain them but do not express them.
Section 5
The lac operon
In bacteria such as E. coli, genes can be switched on and off in response to the environment. The lac operon controls the use of lactose.
The operon has a promoter (where RNA polymerase binds), an operator, and structural genes such as lacZ (beta-galactosidase) and lacY. A separate regulatory gene, lacI, makes a repressor protein.
- No lactose: the repressor binds to the operator, blocking RNA polymerase, so the structural genes are not transcribed.
- Lactose present: lactose binds to the repressor and changes its shape, so it cannot bind to the operator. RNA polymerase transcribes the structural genes, and the enzymes are made.
This saves the cell energy by making the enzymes only when they are needed.
Remember the sequence: lactose binds the repressor, the repressor leaves the operator, RNA polymerase transcribes, enzymes are made.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Stem cells and cell specialisation
- A human zygote divides by mitosis. Cells taken from the embryo at the four-cell stage can each develop into a complete, separate individual, which is how identical twins and triplets can arise. A few days later the embryo is a hollow ball called a blastocyst, and cells from its inner cell mass can form almost any tissue in the body but cannot form the placenta.State what is meant by a stem cell and explain how a stem cell becomes a specialised cell.2 marks
- A medical team proposes treating patients with macular degeneration, a disease that destroys retinal cells and causes blindness, using retinal cells grown from human embryonic stem cells. The stem cells would be taken from surplus embryos created during IVF treatment that would otherwise be discarded.Suggest why decisions about stem cell therapies are not made by scientists alone.2 marks
- The bacterium Escherichia coli can use lactose as a respiratory substrate. In the lac operon, the regulatory gene lacI codes for a repressor protein. The operon also contains a promoter, an operator, and structural genes including lacZ, which codes for the enzyme beta-galactosidase that hydrolyses lactose. The repressor protein can bind to the operator, and lactose can bind to the repressor protein.Explain why beta-galactosidase is made by E. coli only when lactose is present.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).