Stem cells and totipotencyAQA A-Level Biology: Revision notes
Section 1
Totipotent cells and specialisation
Totipotent cells can divide and produce any type of body cell (in mammals, including the placenta). They occur only for a limited time in early mammalian embryos.
During development, cells become specialised because they translate only part of their DNA. Different genes are switched on in different cells, so different proteins are made and each cell type gets its own structure and function. A totipotent cell can translate any part of its DNA.
Specialised cells have not lost genes. They still hold all the DNA but translate only some of it.
Section 2
Pluripotent, multipotent and unipotent cells
Pluripotent cells are found in embryos. They can divide in unlimited numbers and can form most or all of the body's cell types.
Multipotent and unipotent cells are found in mature mammals. They divide to form a limited number of cell types. Multipotent cells form several types, for example bone marrow stem cells forming red blood cells, white blood cells and platelets. Unipotent cells can form only one type, for example the formation of cardiomyocytes (heart muscle cells).
Order by range of potential: totipotent, then pluripotent, then multipotent, then unipotent.
Section 3
Induced pluripotent stem (iPS) cells
iPS cells are made from adult somatic cells, such as skin cells. The cells are treated with appropriate protein transcription factors. These switch on genes that were not being translated, so the cells return to a pluripotent state.
The patient's own cells can be used, so iPS cells are genetically identical to the patient.
Section 4
Treating human disorders
Pluripotent stem cells divide in unlimited numbers, so many cells can be grown. They can be directed to become specialised cells to replace damaged tissue, for example retinal cells for sight loss or cardiomyocytes after heart damage. Multipotent bone marrow stem cells are used to restore blood cells in leukaemia.
Section 5
Evaluating the use of stem cells
For: can replace damaged cells in many disorders; iPS cells are not rejected because they are genetically identical to the patient, and avoid destroying embryos.
Against: embryonic stem cells need embryos to be destroyed, which raises ethical objections; they may be rejected, so immunosuppressant drugs are needed; uncontrolled division can cause tumours; cells may not become fully specialised; trials are often small and short, so long-term safety is uncertain.
For an evaluate question give benefits and risks, then finish with a justified conclusion.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Stem cells and totipotency
- A fertilised mammalian egg divides several times to form a ball of cells. If the cells are separated at the 4-cell stage, each one can develop into a complete embryo, including the placenta. If cells are taken from the same embryo a week later, they can form only some of the tissues of the body.Explain why a totipotent cell can form many different cell types, but a specialised cell in the same embryo cannot.2 marks
- Bone marrow in an adult contains stem cells that divide to produce red blood cells, several types of white blood cell and platelets. These cells never produce nerve cells or muscle cells. Bone marrow transplants are used to treat patients with leukaemia, whose own blood-forming cells have been destroyed by treatment.Explain why a transplant of bone marrow stem cells can replace the blood cells of a leukaemia patient but could not be used to replace damaged nerve cells.2 marks
- A patient has a heart muscle disorder. Researchers took skin cells from the patient, treated them with a set of protein transcription factors and obtained cells that behave like embryonic stem cells. They then induced these cells to form cardiomyocytes (heart muscle cells) to test drugs and possibly to transplant into the patient's heart.Describe how induced pluripotent stem (iPS) cells can be produced from the patient's cells.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).