Differentiation and Stem Cells

A cell's structures and activities depend partly on the proteins it makes. During differentiation, changes in gene activity help produce the proteins needed for a specialised function. Muscle and nerve cells can therefore have different features even when they carry the same genes.

This does not require each cell type to discard all the genes it does not use. Most body cells with a nucleus retain the same gene set. They use different combinations of genes, and they also use many genes in common for shared tasks.

Gene expression means using a gene's instructions. One step is transcription, which copies information from DNA into RNA. Proteins called transcription factors help regulate this copying. Changing transcription can change the amount of a protein a cell produces.

The claim that every body cell retains the same genes is too broad. Mature human red blood cells lose their nucleus and do not keep the full gene set. They are an exception to the usual account of gene retention during differentiation.

Plants make new cells in regions called meristems. These regions contain cells that are unspecialised or only partly specialised. Division supplies descendants that can develop into plant tissues, while a population of meristem cells is maintained.

Location affects the kind of growth a meristem supports. Root and shoot tips allow growth in length. Other meristems allow roots and stems to become thicker. A description limited to root and shoot tips therefore leaves out some meristems.

Descendants can become protective tissue or tissue that transports water and sugars. As they differentiate, their structures and activities change to suit these functions.

Maintaining a meristem does not mean that every division has exactly one daughter that remains a stem cell and one that immediately specialises. It also does not mean that all meristem cells divide continuously. Their activity can change as conditions change.

Stem cells can produce more stem cells through self-renewal. They can also give rise to specialised cells. These two abilities let a stem-cell population be maintained while supplying cells with particular functions.

The possible specialised types vary. Embryonic stem cells are pluripotent and can form almost any body cell type. Adult stem cells usually have a narrower range. Blood-forming stem cells in bone marrow can produce several related blood cell types. This range concerns the kinds of cells produced, not their distance from the original stem cell.

Transplants of blood-forming stem cells are used for some blood diseases. They can help restore blood-cell production. Researchers also investigate other tissue-repair uses. A proposed use needs evidence of safety and effectiveness before it can be described as a proven treatment.

Some embryonic stem cells are obtained by methods that destroy an early embryo. People who believe embryos deserve protection may object to those methods. This concern is different from an objection to collecting adult stem cells from bone marrow. A discussion should identify both the intended use and the source of the cells.

Transcription produces RNA using information in a gene's DNA. Regulating this step changes how much RNA is made. For a gene that provides instructions for a protein, this can affect the amount of that protein a cell produces.

Transcription factors are proteins that recognise and bind particular DNA sequences. Activators can increase transcription. Repressors can decrease it. Their activity helps cells use different genes without replacing their gene set.

Enhancers and silencers are regions of regulatory DNA containing binding sites for transcription factors. They are not groups of transcription-factor proteins. A regulatory region can be near the gene it affects or farther away along the DNA.

Different combinations of active transcription factors help produce different patterns of gene activity. These patterns contribute to differentiation, so cells with the same genes can develop different structures and functions.