Stem cells are undifferentiated cells that can divide continually and develop into other types of cell. The range of cell types a stem cell can produce is called its potency.Totipotent cells can divide and produce any type of body cell. In mammals they also give rise to the extra-embryonic tissues, such as the placenta, so a single totipotent cell can develop into a whole organism. The zygote is totipotent, and so are the cells produced by its first few mitotic divisions. In mammals, totipotent cells exist only for a limited time in the very early embryo. After a few days the cells have become pluripotent.Every totipotent cell contains the whole genome. During development, totipotent cells translate only part of their DNA, resulting in cell specialisation. Some genes are switched on (transcribed and translated) and others are switched off, so each cell makes only the proteins it needs. For example, a cell developing into a red blood cell makes large amounts of haemoglobin, while the gene for keratin, a protein of skin cells, stays switched off. A red blood cell and a skin cell from the same person contain the same genes; they differ because different genes are switched on.
In animals, specialisation is usually permanent: a muscle cell does not normally turn back into a stem cell. Which genes are expressed is controlled by transcription factors and by epigenetic marks, which you will meet later in this topic.
In animals, specialisation is usually permanent: a muscle cell does not normally turn back into a stem cell. Which genes are expressed is controlled by transcription factors and by epigenetic marks, which you will meet later in this topic.