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Cell signaling and Cell division

Cell signaling

The process by which a signal on a cell's surface is converted into a specific cellular response, a series of steps called a signal- transduction pathway , has been extensively studied in many types of cells.

Evolution and cell signaling

Cell signaling evolved early in the history of life.

The molecular details of signal transduction in most

cells are strikingly similar, even though the last

common ancestor of these different types of cells

lived over a billion years ago. These similarities

suggest that early versions of the cell-signaling

mechanisms used today evolved well before the first

multicellular creatures appeared on Earth.

The message carried by a ligand is often relayed through a chain of chemical messengers inside the cell. Ultimately, it leads to a change in the cell, such as alteration in the activity of a gene or even the induction of a whole process, such as cell division. Thus, the original intercellular (between-cells) signal is converted into an intracellular (within-cell) signal that triggers a response.

Forms of signaling

“ Communicating cells may be close together or far apart”

Cell-cell signaling involves the transmission of a signal from a sending cell to a receiving cell. However, not all sending and receiving cells are next-door neighbors, nor do all cell pairs exchange signals in the same way.

There are four basic categories of chemical signaling found in multicellular organisms: paracrine signaling , autocrine signaling , endocrine signaling , and signaling by direct contact. The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell.

Distance of signaling between Sender and

receptor cell

1- Endocrine: Act far distance, ex, Hormones.

2- Paracrine signaling: Act local distance.

3- Synaptic: More close distance.

4- Contact dependent: Cells are in contact.

At least three important classes of

signaling molecules are widely

recognized

• Hormones (occur in plant and animals).
• Neurotransmitters (Occur in Animals).
• Cytokines (Occur in Animals).

Many extracellular signals act via cell-surface receptors to change the behavior of the target cell. The receptor protein activates one or more intracellular signaling pathways, each mediated by a series of intracellular signaling molecules, which can be proteins or small messenger molecules; only one pathway is shown. Signaling molecules eventually interact with specific effector proteins , altering them to change the behavior of the cell in various ways.

Signal reception and the initiation of

Transduction

• G-Protein-Linked Receptors.

Most signal receptors are plasma-

membrane proteins

• Ion-Channel Receptors:
• Ligand-gated ion channels.

Extracellular signals can act slowly or rapidly. Certain types of cell responses—such as cell differentiation or increased cell growth and division involve changes in gene expression and the synthesis of new proteins; they therefore occur relatively slowly. Other responses—such as changes in cell movement, secretion, or metabolism— need not involve changes in gene expression and therefore occur more quickly.