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Cell Signaling
written by Michael
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Cell Signaling
Cells are in constant communication with one another. Cells communicate by sending and receiving signaling molecules called ligands. If ligands are the way that a cell speaks then cell surface receptors are the way that cells listen. If a ligand connects to the cells receptor then a message is conveyed to the cell. Different forms of communication depend on the ligand because a receptor can only take its particular form of ligand.

The picture below represents a cell. Such a cell may have many types of receptors on its surface. In order to see these receptors we would have to get closer to the cells surface, until we can see the actual molecules that make up the cell.
cell.gif

This is what we might see if we could zoom in enough times to see cell surface receptors. As you can see the receptors have a section outside of the cell and a section within the cell. When a ligand binds to the receptor the message is sent into the cell. Ligands are shaped in order to fit exactly into a particular receptor. Different shaped receptors can only bind to the same shaped ligands.

cell_receptor.jpg

Lets take the example of receptors that regulate cell growth. These are called the Tyrosine Kinase receptors and the ligands are referred to as growth factors.

In our diagram we see two receptors that have not yet received a message. Sitting inside the cell is a chemical called RAS that is connected to GDP. When RAS is connected to GDP it is turned off and does not send the growth signal.

InactiveRasSmall.jpg

When the growth factors fit into the receptors the receptors connect together. This then causes P chemicals to connect to the receptors surface within the cell. A chemical reaction occurs in which SH2 connects to P, SH3 connects to SH2, SOS connects to SH3 causing GDP to be released from RAS and GTP to replace it. This is when the growth signal is sent to the neucleus. The GTP is then lost and replaced with GDP and the whole thing goes back to normal.

ActiveRASSmall.jpg

When RAS has mutated it takes on a different shape. This means that the GTP gets stuck within RAS and so the cell is permanently in the ON position constantly sending the signal to grow. Therefore once initially triggered, the cell will behave as if growth messages are constantly being received.

MutantRasSmall.jpg


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