Molecular Biology Wk 11

Question 1

GENERAL PRINCIPLES OF CELL SIGNALING

Answer 1

Most cells are specialized to do one or more specific functions. Many biological processes require various cells to work together and to coordinate their activities. To make this possible, cells have to communicate with each other, which is achieved by a process called cell signaling . Cell signaling makes it possible for cells to respond in an appropriate manner to a specific environmental stimulus.

Question 2

Cell Signaling allows cells to receive and respond to the extracellular environment allowing development, growth, immunity, etc.

Answer 2

Budding yeast cells responding to the mating factor

A- cells are normally spherical
B- in response to mating factor secreted by neighbouring yeast cells, they put out a protrusion toward the source of the factor in preparation to mating

Question 3

Three Stages of Cell Signaling

Answer 3

First, reception, whereby the signal molecule binds the receptor.

Then, signal transduction, which is where the chemical signal results in a series of enzyme activations.

Finally, the response, which is the resulting cellular responses.

Question 4

Signal transduction

Answer 4

Cells usually communicate with each other through extracellular messenger molecules. Extracellular messengers can travel a short distance and stimulate cells that are in close proximity to the origin of the message, or they can travel throughout the body, potentially stimulating cells that are far away from the source.

Question 5

Signals Can Act over a Long or Short Range

Answer 5

Cells typically communicate using chemical signals. There are four basic categories of chemical signaling found in multicellular organisms: endocrine signaling,
paracrine signaling, neuronal signaling and signaling by direct contact, so called contact- dependent signaling.

Question 6

Long distance signalling
Endocrine signalling

Answer 6

Specialised cells release hormone molecules which travel to target cells elsewhere in the organism

➢Part of the pancreas, for example, is an endocrine gland that produces several hormones—including insulin, which regulates glucose uptake in cells all over the body.

Question 7

Paracrine signaling

Answer 7

In this case, rather than entering the bloodstream, the signal
molecules diffuse locally through the extracellular fluid, remaining in the neighborhood of the cell that secretes them.
➢Many of the signal molecules that regulate inflammation at the site of an infection or that control cell proliferation in a healing wound function in this way.
➢Paracrine signals are especially important during development, when they allow one group of cells to tell a neighboring group of cells what cellular identity to take on.

Question 8

Autocrine signaling

Answer 8

In some cases, cells can respond to the local mediators that they themselves
produce, a form of paracrine communication called autocrine signaling;

In the case of autocrine signaling, the cell that is producing the messenger expresses receptors on its surface that can respond to that messenger. Consequently, cells releasing the message will stimulate (or inhibit) themselves. Cancer cells sometimes promote their own survival and proliferation in this way.

Question 9

Neuronal signaling
Long distance signaling

Answer 9

Like endocrine cells, nerve cells (neurons) can deliver messages over long distances. In the case of neuronal signaling, however, a message is not broadcast widely but is instead delivered quickly and specifically to individual target cells through private lines. When activated by signals from the environment or from other nerve cells, a neuron sends electrical impulses racing along its axon at speeds of up to 100 m/sec. On reaching the axon terminal, these electrical signals are converted into a chemical form: each electrical impulse stimulates the nerve terminal to release a pulse of an extracellular signal molecule called a neurotransmitter (often, opening ion channels and changing the electrical potential across the membrane).
(<100 nm)

Question 10

Contact-dependent signaling
Short distance signaling

Answer 10

➢In contact-dependent signaling, a cell-surface-bound signal molecule binds to a receptor protein on an adjacent cell.
➢Does not require the release of a secreted molecule. Instead, the cells make direct physical contact through signal molecules lodged in the plasma membrane of the signaling cell and receptor proteins embedded in the plasma membrane of the target cell.

Question 11

During embryonic development, for example, such contact-dependent signaling allows adjacent cells that are initially similar to become specialized to form different cell types .

Answer 11

Contact-dependent signaling controls nerve-cell production in the fruit fly Drosophila. The fly nervous system originates in the embryo from a sheet of epithelial cells.

Isolated cells in this sheet begin to specialize as neurons, while their neighbors remain non-neuronal and maintain the structure of the epithelial sheet. The signals that control this process are transmitted via direct cell–cell contacts: each future neuron delivers an inhibitory signal to the cells next to it, deterring them from specializing as neurons too—a process called lateral inhibition. Both the signal molecule (in this case, Delta) and the receptor molecule (called Notch) are transmembrane proteins.

Question 12

Signalling components

Answer 12

Signalling molecules
Receptors
Signalling intermediates
Effector proteins

First messengers are extracellular factors, often hormones or neurotransmitters, such as epinephrine, growth hormone, and serotonin.
Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. … Examples of second messenger molecules include cyclic AMP, cyclic GMP, inositol trisphosphate, diacylglycerol, and calcium.

Question 13

Signalling molecules

Answer 13

Signaling molecules are the molecules that are responsible for transmitting information between cells in your body. The size, shape, and function of different types of signaling molecules can vary greatly .
The signal molecules can be proteins, peptides, amino acids, nucleotides, steroids, fatty acid derivatives, or even dissolved gases.

Question 14

What are receptors

Answer 14

Are proteins associates with cell membrane or located in the cell

They recognise signalling molecules by binding to them

Binding of receptors by signalling molecules -> cell behaviour changes

Question 15

Two types receptors

Answer 15

• cell surface receptors
• intracellular receptors

Question 16

Receptor activation

Answer 16

LOOK AT GOODNOTES

Question 17

The same signal molecule can induce different responses in different target cells.

Answer 17

The extracellular signal molecule alone is not the message: the information conveyed by the signal depends on how the target cell receives and interprets the signal.

Question 18

Myasthenia gravis (MG)

Answer 18

Myasthenia gravis (MG) is a long-term neuromuscular disorder that’s usually caused by an autoimmune problem. It leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing.
Symptoms of MG can include:
●trouble talking
●problems walking up stairs or lifting objects
●facial paralysis
●difficulty breathing due to muscle weakness ●difficulty swallowing or chewing
●fatigue
●hoarse voice
●drooping of eyelids
●double vision
Not everyone will have every symptom, and the degree of muscle weakness can change from day to day. The severity of the symptoms typically increases over time if left untreated.

Question 19

A typical cell possesses many sorts of receptors—each present in tens to hundreds of thousands of copies. These signal molecules work in combinations to regulate the behavior of the cell.

Answer 19

If deprived of survival signals, most cells undergo a form of cell suicide known as apoptosis

Question 20

A Cell’s Response to a Signal Can Be Fast or Slow

Answer 20

For example, 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.

Question 21

Extracellular signal molecules generally fall into two classes

Answer 21

➢The first and largest class consists of molecules that are too large or too hydrophilic to cross the plasma membrane of the target cell.
➢The second, and smaller, class of signals consists of molecules that are small enough or hydrophobic enough to pass through the plasma membrane .

Question 22

Some small hydrophobic hormones bind to intracellular receptors that act as transcription regulators

Answer 22

Although these signal molecules differ in their chemical structures and functions, they all act by binding to intracellular receptor proteins that act as transcription regulators. Their receptors are not identical, but they are related, belonging to the nuclear receptor superfamily.

Question 23

The steroid hormone cortisol acts by activating a transcription regulator

Answer 23

➢Cortisol is one of the hormones produced by the adrenal glands in response to stress.
➢Whereas the receptors for cortisol and some other steroid hormones are located in the cytosol, those for other steroid hormones and for thyroid hormones are already bound to DNA in the nucleus even in the absence of hormone.

Question 24

Some Dissolved Gases Cross the Plasma Membrane and Activate Intracellular Enzymes Directly

Answer 24

The neurotransmitter acetylcholine causes the blood vessel to dilate by binding to receptors on the surface of the endothelial cells, stimulating the cells to make and release NO. NO is synthesized from the amino acid arginine and diffuses readily from its site of synthesis into adjacent smooth muscle cells, where it regulates the activity of specific proteins, causing the muscle cells to relax. One key target protein that can be activated by NO in smooth muscle cells is guanylyl cyclase, which catalyzes the production of cyclic GMP from GTP.

Question 25

Cell-Surface Receptors Fall into Three Main Classes

Answer 25

All cell-surface receptor proteins bind to an extracellular signal molecule and transduce its message into one or more intracellular signaling molecules that alter the cell’s behavior. Most of these receptors belong to one of three large classes, which differ in the transduction mechanism they use.

LOOK AT GOODNOTES