Until just a few days ago, the only correct answer to this question would have been #3. The mitochondria of a cell are well recognized as the powerhouses of the cell. They are the location where energy-rich nutrients such as carbohydrates and fats are brought in and "burned"in the presence of oxygen to produce the energy (in the form of ATP) to power our cells. It is one of the earliest lessons of any introductory biology course.
However, in a development that is sure to change the way we look at cell biology, it now appears that answer #4 also may be correct. A group of researchers at the National Institutes of Health reported in the July 21, 2009 issue of PNAS that the fusion of the the mitochondria in a cell influences the buildup of a protein called cyclin. This in turn, acts as a control mechanism for the cell to bypass an important checkpoint (G1/S), allowing the cell to divide.
Cyclin is a form of protein "clock" within a cell. Its job is relatively simple, as cyclin levels accumulate in the cell, it overcomes a series of thresholds that tell the cell to undergo important activities. One of these activities is the cell cycle. The cell cycle is a type of cell "day". At specific points in the cycle the cell undergoes DNA replication and specific activities to prepare for cell division.
These activities are regulated by special proteins called checkpoint proteins. You should be very interested in these checkpoint proteins - their job is to ensure that the cell does not divide unless it needs to, since unrestricted cell growth is called cancer.
There are two specific classes of these proteins, proto-oncogenes and tumor supressor genes. Until earlier this week, they were the primary focus of most research dedicated to understanding the cell cycle.
What the NIH team reported is that just prior to one of these checkpoints (G1/S) is that the mitochondria form an unusual tubular network in the cell. The images from their paper show a network of mitochodnria that is reminiscent of a map of a subway line in a major city. The authors then show that the presence of this network is correlated to a increase in a form of cyclin that is known to be a major key in overcoming the G1/S checkpoint, allowing the cell to duplicate its DNA in preparation for cell division.
Why should we be interested in this? First of all, because it demonstrates that by looking in unconventional areas, we can find new clues as to how cell function. Until earlier this week, no one would have answered #4 in the above quiz. But more importantly, scientists who study cancer genetics and biochemistry can now look for relationships between these mitochondria and unregulated cell growth, which should, eventually, lead to new insights on how to battle some forms of cancer.
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Article: Kasturi Mitra, Christian Wunder, Badrinath Roysam, Gang Lin, Jennifer Lippincott-Schwartz, 'A hyperfused mitochondrial state achieved at G1–S regulates cyclin E buildup and entry into S phase', Published online before print July 15, 2009, doi: 10.1073/pnas.0904875106
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