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Why does every cell in our body contain DNA?
Category: Biology
Published: August 22, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Not every cell in the human body contains DNA bundled in a cell nucleus. Specifically, mature red blood cells and cornified cells in the skin, hair, and nails contain no nucleus.
As part of the maturation process, human red blood cells destroy their cell nuclei. They do this in order to carry as much oxygen as possible and still stay small enough to fit through narrow blood capillaries, thereby maximizing the oxygen delivery. In fact, humans have some of the smallest red blood cells of all vertebrates, thanks in part to the destruction of the nucleus. Most mammals have red blood cells without nuclei, while all other types of vertebrates do have nuclei in their red blood cells. However, all red blood cells, including human, must start with DNA, as DNA contains the code that tells each cell how to construct itself in the first place. Human red blood cells simply destroy their nucleus once it is no longer needed as part of the maturation process. A ring of actin within a maturing red blood cell pinches and splits the cell into two parts: one part with the DNA and one part without. Red blood cell enucleation is therefore a special type of cell division. Macrophages then come along and gobble up the parts containing DNA, leaving only the red blood cell parts that don't have DNA. Note that there is much more in blood than red blood cells. As a result, a blood sample does contain DNA due to the presence of other kinds of cells.
Cornified cells in the skin, hair, and nails also contain no cell nucleus. Like red blood cells, these cells start out with cell nuclei in order to develop properly, but then destroy their nuclei as part of the cornification process. They do this in order to maximize the space in the cell filled with the structural protein keratin. Keratin is a strong protein that gives hair, skin, finger nails, and toe nails their toughness. Cells that undergo cornification experience a form of programmed, controlled cell death in order to achieve their strength. The cell nucleus and other internal parts of the cell are destroyed and their space is filled by keratin. Once cornification is complete, these cells are dead and carry out no biochemical processes. But dead does not mean useless. Cornified cells fulfill their end purpose of giving structural strength and warmth to surrounding tissues despite being dead. The fact that cornified cells are dead means that you can cut your hair, clip your nails, and rub off the outer layer of skin without causing any damage or killing cells. The lack of nuclear DNA in cornified cells means that forensic biologists can rarely extract DNA from hair clippings in order to help determine the culprit.
Aside from red blood cells and cornified cells, all other cells in the human body contain nuclear DNA. Also, all cells start with nuclear DNA. The reason for this is that DNA contains the basic code that tells each cell how to grow, function, and reproduce.