Totipotent cells

A totipotent cell can divide and produce every type of cell in an organism, including, in mammals, the cells of the placenta. In animals, totipotent cells exist only for a short time at the very start of development: the fertilised egg (zygote) and the cells from its first few divisions. In plants, totipotent cells stay in the meristems throughout life, which is why a whole plant can be grown from a small piece of tissue.

What does totipotent mean?

"Toti" comes from the Latin for whole: a totipotent cell has the potential to make the whole organism. AQA A-level Biology says totipotent cells can divide and produce any type of body cell, and that in mammals they occur only for a limited time in early embryos.

Every cell in your body has the same DNA as the zygote it came from. What makes a cell specialised is which genes it uses. AQA puts it this way: during development, totipotent cells translate only part of their DNA, which results in cell specialisation. A cell that has switched off most of its genes can no longer make other kinds of cell.

Totipotent, pluripotent, multipotent and unipotent

Stem cells are unspecialised cells that can divide and become other types of cell. How many types a stem cell can become is called its potency, and it falls as an embryo develops:

AQA says pluripotent cells are found in embryos, while multipotent and unipotent cells are found in mature mammals, and gives the formation of cardiomyocytes (heart muscle cells) as its example of unipotent cells.

Totipotent vs pluripotent

This is the comparison exam questions ask for most. A totipotent cell can form the embryo and the tissues that support it, such as the placenta, so on its own it could develop into a whole organism. A pluripotent cell can form any cell of the body itself, but not the placenta, so it cannot develop into a whole organism by itself.

Match specialised cells to their features and stem cells to what they become in this question from the Cell Biology course.

Totipotent cells in plants

Plants keep totipotent cells all their lives, in meristems at the tips of roots and shoots. This is why a cutting can grow roots, and why plants can be cloned by tissue culture (micropropagation):

1. Cut a small piece of tissue, called an explant, from the parent plant, often from a meristem.
2. Sterilise it and put it on a growth medium with nutrients and plant hormones.
3. Its cells divide to form a mass of unspecialised cells, called a callus.
4. Different hormones make the callus grow shoots and roots.
5. The plantlets are moved to soil. Every one is a clone of the parent.

AQA suggests producing tissue cultures from pieces of cauliflower as a practical. For how cells become specialised once they stop being stem cells, see specialised cells.

Stem cells in medicine

Because pluripotent stem cells can divide in unlimited numbers and become many cell types, they could replace damaged cells in disorders such as some kinds of heart damage, type 1 diabetes or spinal cord injury. Embryonic stem cells raise ethical questions because the embryo is destroyed to get them.

Induced pluripotent stem cells (iPS cells) avoid that. AQA says they can be produced from adult somatic cells using appropriate protein transcription factors, which switch back on the genes that keep a cell unspecialised. The discovery that mature cells can be reprogrammed to become pluripotent won John Gurdon and Shinya Yamanaka the 2012 Nobel Prize in Physiology or Medicine. Cells made from a patient's own body are also less likely to be rejected.

Check your understanding.

Common mistakes

• Mixing up totipotent and pluripotent. Totipotent includes the placenta; pluripotent does not.
• Saying specialised cells have lost genes. They keep all their DNA; they only translate part of it.
• Saying adult stem cells are pluripotent. In mature mammals they are multipotent or unipotent; iPS cells are made pluripotent in the lab.
• Thinking only animals have stem cells. Plant meristems contain totipotent cells.
• Writing "any cell" for multipotent cells. They make a limited range of related cells.

Practise with a short timed paper

Stem cells and gene expression are in the second year of A-level Biology, and stem cells in GCSE Biology. The daily A-level Biology: Year 2 and GCSE Biology Higher papers are short timed papers of 8 questions, with a new one every day.

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