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Lecture 25

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McGill University
Biology (Sci)
BIOL 300
Siegfried Hekimi

th BIOL 300 November 5 2012 Lecture 25 Dr. Schock We will now begin talking about signals which can tell the cell which genes need to be active and which genes need to be silenced. These experiments focus more on a protein-chemistry view, whereas the previous section focused on the mRNA and DNA aspect of the mechanisms. In the simplest form, a signal transduction involves converting a signal from one form to another • E.g. a radio receives some kind of electromagnetic radiation, and this is somehow translated into sound which is the output of the radio • What happens during this translation is actually quite complex, despite the simplicity of the concept The cell works very much like this: • There is some extracellular signal which is translated by a series of proteins into an intracellular response • These pathways are usually depicted as linear pathways because in reality, we know very little of about these pathways, as they are probably much more complex than this • This differentiates engineering, which takes more of a bottom up approach, to scientists research, which have a top-down approach (where we look at the overall picture first and then try to find out the individual mechanisms) Signal transduction in usually depicted as a linear pathway involving the following steps • The sending cell would synthesis a compound known as a ligand (or signal) which can be a protein or a small chemical and then release it into the extracellular environment • Usually, between these two steps, the ligand is usually modified or processed in one way • The ligand is transported to the receiving cell at which point it will bind its receptor, initiating some sort of cascade in the receiving cell • There are various amplification, modification and translation steps happening inside the cell upon binding of ligand • Amplification is very common • You then have some sort of output from the receiving cell: • Altered gene expression • Direct modification of the cell (e.g. phosphorylation, physical changes to proteins, etc.) • Also, you have termination of the response • E.g. by removal of the ligand (degradation) • Ligands often have certain half-lives after which it begins to degrade 1 th BIOL 300 November 5 2012 Lecture 25 Dr. Schock • Effector proteins could also shut down the receptor in a feedback loop Signal transduction is an ancient process; it can be found in every type of cell (from bacteria to humans and plants) • In this example, we can see yeast mating: • A yeast of a certain mating type can secrete an a+α pheromone which can attract yeast cells of the opposite mating type • The yeast will then come together through signal transduction mechanisms to exchange genetic information • In bacteria, exchange of DNA can be carried out signal transduction, often involved in regulation population density of a bacterial colony Signal transduction can be divided into certain “classes” depending on various factors One way you can distinguish a signal transduction mechanism through the speed of the response: • A fast response involves changes in activity of existing proteins, called a physiological response • All our sensory or perception mechanisms (visions, sight, smell, etc.) are physiological transduction mechanisms which need to happen very quickly • Changes in the amount of protein through modification of transcription or RNA modification, known as slow, or “developmental” transduction mechanisms • This can be seen in developments from the fertilization of an egg to an adult, in which various genes are turned on and off sequentially • During pregnancy, the female body is “remodelled” to make in more suitable for the growth of an embryo uring this mechanism • Another example would be altitude adaptation (learning to live at high altitudes); you are initially very short of breath because of the lower pressure, and over time there is a transcriptional response to produce more RBCs to deal with this • Other mechanisms include alcohol adaptation, or any drug tolerance development Another kind of class in which we can divide signal transduction mechanism is by the different ranges of signaling:
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