|MadSci Network: Cell Biology|
You have asked an interesting question. It is hard to answer this, because I don't know what material we are allowed to start with, and I'm not sure what the intent of your question really is. Human sperm and eggs are complex, highly differentiated cell types that arise from other human cells. Because they only arise from other living cells, many people in the history of biological science have wondered whether there is anything special about living things that distinguishes them from nonliving things. This question can be framed more precisely by asking whether there are any processes outside the realm of chemistry and physics that explain what is going on in living organisms. A very limited aspect of this question was addressed in the nineteenth century, when it was thought that "organic" chemical compounds were those that could only be synthesized by living things, while inorganic compounds could be formed from chemical reactions that did not involve living cells. In 1828, Friedrich Wohler synthesized urea accidentally while attempting to make ammonium cyanate. We now use the term "organic chemistry" to refer to the study of carbon-containing compounds. Hundreds of thousands of organic compounds have been synthesized without the use of living cells or enzymes, showing that there is nothing about chemical synthesis in living cells that distinguishes it from chemical synthesis in glassware. We have learned a great deal about biology in the last half of the twentieth century. The mechanism of inheritance has been solved down to the molecular details. It is a reasonable summary to say that everything that a cell can do is ultimately dictated by the genetic material within the cell. We are very close to completing the sequencing of the human genome. This will allow us in principle to specify all of the proteins that can be synthesized by a human cell. If we understood the function of all of the proteins that are encoded by the genome, we could understand everything that a cell could do. Are there any processes that happen in living cells that would not be immediately apparent from looking at all of the genes? Many people who have thought carefully about this problem take difficult examples of human behavior to try to challenge the idea of physics and chemistry explaining living things. If you enjoy literature, poetry, or art, you probably don't see them as being entirely explained by physics and chemistry. Yet an artist's thoughts, besides being the consequence of his or her life experiences, are the product of chemical and electrical activity in his or her brain, all of which can ultimately be explained by chemistry and physics. This is not to say that we can synthesize art in a test tube, just that even though many scientists enjoy art, they do not regard it as having a supernatural or nonmaterial origin. The consensus in the life sciences now is that there are no new principles in biology that are not ultimately explained by chemistry or physics. There are many interesting stories in biology, but they rest on chemistry and physics. This is a sort of digression that really is an attempt to address your question. It is possible to chemically synthesize DNA. The longer the molecule, the more difficult and expensive the synthesis, but the entire human genome could in principle be synthesized once a complete sequence is known. This is not enough to make a cell, though, because the protein products of some genes regulate the activity of other genes. Differentiated cells express a subset of regulatory genes that result in the expression of a restricted subset of genes. This is just one example of a complex set of gene products that must be present in the cell. The entire array of gene products necessary for gene expression must also be present in the right amounts. Some of these are complex macromolecular assemblies, such as ribosomes. All of the enzymes necessary for metabolism must also be in place. In addition, the cell has internal membranes that divide it into compartments, such as the nucleus. We can chemically synthesize phospholipids, and even form them into membranes, but the real nuclear membrane has "pores" made of proteins that are encoded by genes. In your question, are we allowed to have genetically-engineered bacteria synthesize the proteins, or do we have to do that chemically? In addition, the cell has other complex subcomponents like mitochondria, which even have their own genome. To make a human cell, we would have to build these as well. The point of this is that we might in principle be able to synthesize each of the components of a human cell, and even put one together, with enough knowledge and technical skill. It may seem that accomplishing this would be a demonstration of the completeness of our knowledge, but each individual piece of knowledge about human cells can be demonstrated and verified in experiments that require less than building a human cell from scratch. So there is no scientific reason to attempt to construct a human cell by chemical synthesis. The answer to your question therefore is that it might be theoretically possible to build a human cell from scratch using only chemical synthesis, but that this is an extremely difficult technical feat that has no scientific purpose. For some good background reading on the structure and function of cells, please see "The Molecular Biology of the Cell" at: http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowTOC&rid=cell.TOC For definitions of terms in genetics, please see: http://www.informatics.jax.org/userdocs/glossary.shtml Thanks again for an interesting question. Yours, Paul Szauter Mouse Genome Informatics
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