Cell Signaling and Cancer
Answer to questions
Cell Signaling Mechanisms
At the cell surface:
1. Provide two examples of chemical signals that might start the signaling pathway.
Examples of chemical signals include hormones, growth factors, neurotransmitter, and cytokines
2. Describe what happens when the signal interacts with its cell surface receptor?
a. it causes a second protein receptor to move into place and bind with the signal.
b. the interaction of these two proteins with the signal causes the proteins to change shape.
c. this shape change is propagated through the cell membrane to the cytoplasm.
3. The cell is exposed to many different chemical signals at any given time. What ensure that the proper signal binds to initiate the desired cell response?
The key to the mechanism shown here is the “perfect match” between the signal and the receptor. The relationship between these two molecules is like a lock and key. Chemical signals whose shapes do not match the receptor found cannot initiate the proper signal pathway.
Beneath the cell membrane:
1. Describe how the part of the receptor on the inside of the cell responds to the binding of chemical signal.
The binding of the chemical signal to the portion of the receptor on the outside of the cell causes the portion of each protein found on the inside of the cell to intertwine with each other. The result of this interaction is an activation* of the region of the protein found on the inside of the cell.
*Although not mentioned in the animation this “activation” occurs through the addition of phosphate groups to certain amino acid residues found on the protein. The addition of the phosphate is an autocatalytic reaction, with each protein subunit catalyzing the addition of phosphates to the other.
2. Once the internal portion of the receptor has been activation, how is the signal relayed through the cell?
Once activated, the receptor interacts with cytoplasmic proteins. These then relay the chemical message received through the cytoplasm
A bevy of interactions: Questions to Answer
1. Place the following labels on the diagram below
a. Protein Receptor
b. Ras
c. Raf
2. Keeping in mind that you are looking at cell signaling mechanisms to understand how cells control growth and division, hypothesize why a mutation in the gene for the Ras protein might lead to cancer.
As a key player in the signal pathway, any changes to Ras can alter the normal functioning of the pathway. If, for example, the mutation causes the Ras protein to always be in the “on” position, then it would constantly be activating Raf proteins, leading to constant cell division, a key hallmark of cancer.
To the nucleus: Questions to Answer
1. What is a kinase? What role does it play in the cell signaling pathway?
A kinase is a protein, specifically an enzyme, which activates other proteins by adding a phosphate group. The source of the phosphate group is ATP. Kinases serve as relay molecules in the cell signaling pathway. Once activated, they activate other proteins, hence transmitting the message through the cell.
2. Since all cell signaling pathways end with a cellular response, explain why it is important that the cellular proteins that have been activated by the binding of the signal make their way to the nucleus. Hint: What role does the nucleus play in cell function? What processes take place there that might be involved in a cellular response to a chemical signal?
Cellular responses to a chemical signal can take on many forms. However, many involve a change in the expression pattern of various genes. A particular cellular response might involve “turning on” some genes, while reducing the expression of others. This sort of regulation involves the interaction of proteins and DNA. The proteins activated in the signal pathway have the capability of interacting with DNA to regulate the expression of certain genes. This altered expression as a result of the binding of the chemical signal results in a specific cellular response to the chemical signal.
Inside the nucleus: Questions to Answer
1. After viewing the animation, edit your response to the question in “To the nucleus.” Did you identify the correct process occurring in the nucleus that begins the cell’s response the signal?
Gene expression involves the processes of transcription and translation. These events involve converting the genetic information in the DNA molecule into an RNA molecule, then reading the RNA molecule in order to produce a protein. Every cell has a unique profile of proteins present. Which proteins are present is a function of which genes in the cell are made available for transcription.
2. What is the role of Fos and Jun?
Fos and Jun are proteins found in the nucleus. They are activated when a protein from the cytoplasm, activated during the signal pathway, enters the nucleus. Fos and Jun, once activated, intertwine and then bind to DNA at specific sites. The DNA is then uncoiled by other proteins and the gene is made available for transcription.
Making the protein and Completing the protein: Questions to Answer
1. Distinguish between the processes of transcription and translation.
Transcription is the process of synthesizing an RNA molecule from a DNA template. Translation is the process of protein synthesis. Translation involves converting the information carried by the RNA molecule into the appropriate amino acid sequence that will fold into an active protein
2. What is the function of the Golgi apparatus?
The Golgi apparatus is the cellular organelle responsible for modifying and packaging proteins.
3. Assume the protein being synthesized in this animation is a growth factor. What is the fate of this protein? In other words, once it is packaged by the Golgi, what will it do and how will it do it?
This growth factor is synthesized by the cell in response to the signal that originally interacted with the cell receptor. Once packaged by the Golgi, the protein will be transported to the cell membrane. It will move across the membrane and be released outside the cell. At this point, this growth factor is now ready to act as a chemical signal. It can bind to neighboring cells or it own cell and initiate a signal pathway that leads to cell growth. The mechanism used to carry out is message is the pathway that has just been described in the animation:
Binding of signal to receptor à Change in shape of protein receptor à Activation of protein receptor as a result of shape change à Activation of intracellular proteins as a result of interaction with activated receptor à Movement of activated intracellular proteins to the nucleus à Activation of nuclear proteins à Transcription of specific genes à Translation of mRNA into protein