Subtitles prepared by human
Epigenetics – changes beyond the genetic code The human body contains around 200 different cell types. And although all the cells have the same DNA, a heart cell differs fundamentally from a liver cell, for example. The reason: different genes are active in each cell type. Shown in red here. Those that are active control the cell. Special proteins then bind to the DNA, and the target gene is read. This process is controlled by means of epigenetic changes. These changes do not affect the genetic code of the DNA but are nevertheless inheritable. Thus, the cell can switch genes on or off – by attaching small chemical molecules to them. One example is DNA methylation. It mainly affects the promotor region – shown here in yellow – the actual starting point for reading genes. Cytosine molecules are particularly common here. Enzymes, such as methyl transferases, attach a methyl group to them.
Other proteins are then able to bind to the promotor and prevent the gene from being read. It is thus disabled. DNA methylation plays a key role in the development of cancer. In many cancer cells, the DNA carries more methyl groups than usual. As a result, genes are switched off that normally prevent cancer from developing. This, too, is an epigenetic effect. However, methylation is only part of the story. Hydroxyl groups are also used. They undo the whole process. Hydroxymethylation is an intermediary step in the removal of DNA methyl groups. In this way, previously disabled genes are reactivated. Another way to control the activity of genes lies in the way DNA is packed. In order for the two-metre-long strand of DNA to fit inside the cell nucleus,... ... which measures just a few micrometres across, it is wrapped around proteins - the histones. There are several types of histone: H2A, H2B, H3 and H4.
They dock in pairs, and because there are two of each pair, an eight-element complex forms, to which histone H1 also attaches. This complex of proteins and DNA is known as chromatin. The packing density of the chromatin then determines whether the reading machinery is able ... ... to access the DNA, because only if DNA and histones are loosely packed ... ... is a gene accessible to enzymes and can therefore be read. Once again, the packing density of the DNA is determined by small tags, this time attached to the histones. Methyl acetyl or phosphate groups. The chromatin opens or closes depending on where such chemical groups are attached. If histones are acetylated, for example, the packing is generally looser. This means that the chromatin opens and becomes what is known as euchromatin. Enzymes can now bind to the DNA and read the genes. This general rule does not apply to a methyl group.
Here, the effect depends on where exactly the methyl group is placed. It can activate or, as in this example, switch off a gene. The chromatin structure closes, and the gene can no longer be read. The DNA is present as heterochromatin. But this state is also reversible. Epigenetic changes therefore do not affect the genetic code itself. That is reflected in the prefix “epi” - meaning “beyond” the genetic code. Nevertheless, they can be passed on to the next generation. Whereas DNA mutations are irreversible, epigenetic changes can easily be undone. This is a remarkable mechanism that enable organisms to respond to changing environmental factors.
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