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All cells reproduce by dividing: one cell becomes two. When a cell divides, it divides roughly in half. The division of water and proteins between the dividing cell halves does not have to be exactly even. Instead, a roughly even distribution of the cellular material is acceptable. However, there is one important exception: the cell’s DNA, which is known to code the structure of individual proteins, and may contain other kinds of information. The DNA of a cell is like a single massive book. This book cannot be torn in half and roughly distributed between the two dividing cell halves. Instead, each new cell needs its own complete copy. Therefore, before a cell can divide, it must duplicate all its DNA, and each of the two new cells must receive a complete copy of the original DNA.
All multicellular organisms are made out of eucaryotic cells. Eucaryotic cells are characterized by having a well-defined cellular nucleus that contains all the cell’s DNA. Division for eucaryotic cells has three main steps. In the first step all the DNA is duplicated, and the chromosomes condense into clearly distinct and separate groupings of DNA. For a particular type of cell, such as a human cell, there are a fixed and unchanging number of condensed chromosomes formed; ordinary human cells always form 46 condensed chromosomes before dividing.
During the normal life of a cell, the chromosomes in the nucleus are sufficiently decondensed so that they are not easily seen as being separate from each other. During cell division, each condensed chromosome that forms—hereafter simply referred to as a chromosome—consists of two equal-length strands that are joined. The place where the two strands are joined is called a centromere. Each chromosome strand consists mostly of a long DNA molecule wrapped helically around specialized proteins called histones. For each chromosome, each of the two strands is a duplicate of the other, coming from the preceding duplication of DNA. For a human cell there are a total of 92 strands comprising 46 chromosomes. The 46 chromosomes comprise two copies of all the information coded in the cell’s DNA. One copy will go to one half of the dividing cell, and the other copy will go to the other half.
The second step of cell division is the actual distribution of the chromosomal DNA between the two halves of the cell. The membrane of the nucleus disintegrates, and simultaneously a spindle forms. The spindle is composed of microtubules, which are long, thin rods made of chained proteins. The spindle can have several thousand of these microtubules. Many of the microtubules extend from one half of the cell to the chromosomes, and a roughly equal number of microtubules extend from the opposite half of the cell to the chromosomes. Each chromosome’s centromere becomes attached to microtubules from both halves of the cell.
When the spindle is complete, and all the centromeres are attached to microtubules, the chromosomes are then aligned together. The alignment places all the centromeres in a plane that is oriented at a right angle to the spindle. The chromosomes are at their maximum contraction. All the DNA is tightly bound so that none will break off during the actual separation of each chromosome. The separation itself is caused by a shortening of the microtubules. In addition, in some cases the separation is caused by the two bundles of microtubules moving away from each other. The centromere, which held together the two strands of each chromosome, is pulled apart into two pieces. One piece of the centromere, attached to one chromosome strand, is pulled into one half of the cell. And the other centromere piece, attached to the other chromosome strand, is pulled into the opposite half of the cell. Thus, the DNA is equally divided between the two halves of the dividing cell.
The third step of cell division involves the construction of new membranes. Once the divided DNA has reached the two respective cell halves, a normal-looking nucleus forms in each cell half: at least some of the spindle’s microtubules first disintegrate, a new nuclear membrane assembles around the DNA, and the chromosomes become decondensed within the new nucleus. Once the two new nuclei are established, a new cell membrane is built in the middle of the cell, dividing the cell in two. Depending on the type of cell, the new cell membrane may be a shared membrane. Or the new cell membrane may be two separate cell membranes, with each membrane facing the other. Once the membranes are completed, and the two new cells are truly divided, the remains of the spindle disintegrate.