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The dividing of eucaryotic cells is impressive in its precision and complexity. However, there is a special kind of cell division used to make the sex cells of most higher organisms, including man. This special division process is more complex than ordinary cell division. For organisms that use this process, each ordinary cell (ordinary in the sense of not being a sex cell) has half its total DNA from the organism’s mother, and the other half from the organism’s father. Thus, within the cell are two collections of DNA. One collection originated from the mother, and the other collection originated from the father. Instead of this DNA from the two origins being mixed, the separateness of the two collections is maintained within the cell. When the condensed chromosomes form during ordinary cell division, half the chromosomes contain all the DNA that was passed by the mother, and the other half contain all the DNA that was passed by the father. In any particular chromosome, all the DNA came from only one parent, either the mother or the father.
Regarding genetic inheritance, particulate inheritance requires that each inheritable characteristic be represented by an even number of genes. Genes are specific sections of an organism’s DNA. For any given characteristic encoded in the DNA, half the genes come from the mother, and the other half come from the father. For example, if the mother’s DNA contribution has a gene for making hemoglobin, then there is a gene for making hemoglobin in the father’s DNA contribution. The actual detail of the two hemoglobin genes may differ, but for every gene in the mother’s contribution, there is a corresponding gene in the father’s contribution. Thus, the DNA from the mother is always a rough copy of the DNA from the father, and vice versa. The only difference is in the detail of the individual genes.
Sex cells are made four-at-a-time from an original cell. The original cell divides once, and then the two newly formed cells each divide, producing the final four sex cells. The first step for the original cell is a single duplication of all its DNA. Then, ultimately, this DNA is evenly distributed among each resultant sex cell, giving each sex cell only half the DNA possessed by an ordinary nondividing cell. Then, when the male sex cell combines with the female sex cell, the then-fertilized egg has the normal amount of DNA for a nondividing cell.
The whole purpose of sexual reproduction is to provide a controlled variability of an organism’s characteristics, for those characteristics that are represented in that organism’s DNA. Differences between individuals of the same species give natural selection something to work with—allowing, within the limits of that variability, an optimization of that species to its environment. To help accomplish this variability, there is a mixed selection in the sex cell of the DNA that came from the two parents. However, the DNA that goes into a particular sex cell cannot be a random selection from all the available DNA. Instead, the DNA in the sex cell must be complete, in the sense that each characteristic specified by that organism’s DNA is specified in that sex cell, and the number of genes used to specify each such characteristic is only half the number of genes present for that characteristic in ordinary nondividing cells. Also, the order of the genes on the DNA must remain the same as it was originally—conforming to the DNA format for that species.
The mixing of DNA that satisfies the above constraints is partially accomplished by randomly choosing from the four strands of each functionally equivalent pair of chromosomes. Recall that a condensed chromosome consists of two identical strands joined by a centromere. For each chromosome that originated from the mother, there is a corresponding chromosome with the same genes that originated from the father. These two chromosomes together are a functionally equivalent pair. One of the chromosomes from each functionally equivalent pair of chromosomes is split between two of the sex cells. And the other chromosome from that pair is split between the other two sex cells. In addition to this mixing method, it would improve the overall variability if at least some corresponding sequences of genes on different chromosomes are exchanged with each other. And this exchange method is in fact used. Thus, a random exchanging of corresponding sequences of genes within a functionally equivalent pair of chromosomes, followed by a random choosing of a chromosome strand from each functionally equivalent pair of chromosomes, provides good overall variability, and preserves the DNA format for that species.
Following are the details of how the sex cells get their DNA: The original cell, as already stated, duplicates all its DNA. The same number of condensed chromosomes are formed as during ordinary cell division. However, these chromosomes are much longer and thinner than chromosomes formed during ordinary cell division. These chromosomes are stretched out, so as to make the exchanging of sequences of genes easier.
Once these condensed, stretched-out chromosomes are formed, each chromosome, in effect, seeks out the other functionally equivalent chromosome and lines up with it, so that corresponding sequences of genes are directly across from each other. Then, on average, for each functionally equivalent pair of chromosomes, several random exchanges of corresponding sequences of genes take place.
After the exchanging is done, the next step has the paired chromosomes move away somewhat from each other. However, they remain connected in one or more places. Also, the chromosomes themselves undergo contraction, losing their stretched-out long-and-thin appearance. As the chromosomes contract, the nuclear membrane disintegrates, and a spindle forms. Each connected pair of contracted chromosomes lines up so that one centromere is closer to one end of the spindle, and the other centromere is closer to the opposite end of the spindle. The microtubules from each end of the spindle attach to those centromeres that are closer to that end. The two chromosomes of each connected pair are then pulled apart, moving into opposite halves of the cell. It is random as to which chromosome of each functionally equivalent pair goes to which cell half. Thus, each cell half gets one chromosome from each pair of what was originally mother and father chromosomes, but which have since undergone random exchanges of corresponding sequences of genes.
After the chromosomes have been divided into the two cell halves, there is a delay, the duration of which depends on the particular species. During this delay—which may or may not involve the forming of nuclei and the construction of a dividing cell membrane—the chromosomes remain unchanged. After the delay, the final step begins. New spindles form—in each cell half if there was no cell membrane constructed during the delay; or in each of the two new cells if a cell membrane was constructed—and the final step divides each chromosome at its centromere. The chromosomes line up, the microtubules attach to the centromeres, and the two strands of each chromosome are pulled apart in opposite directions. Four new nuclear membranes form. The chromosomes become decondensed within each new nucleus. The in-between cell membranes form, and the spindles disintegrate. There are now four sex cells, and each sex cell contains a well-varied blend of that organism’s genetic inheritance which originated from its two parents.
 The exception to this rule, and the exception to the rules that follow, are genes and chromosomes that are sex-specific, such as the X and Y chromosomes in man. There is no further mention of this complicating factor.
 In female sex cells, four cells are made from an original cell, but only one of these four cells is a viable egg (this viable egg has most of the original cell’s cytoplasm). The other three cells are not viable eggs and they disintegrate. There is no further mention of this complicating factor.
 The idea of natural selection is that differences between individuals translate into differences in their ability to survive and reproduce. If a species has a pool of variable characteristics, then those characteristics that make individuals of that species less likely to survive and reproduce, tend to disappear from that species. Conversely, those characteristics that make individuals of that species more likely to survive and reproduce, tend to become common in that species.
A species is characterized by the ability of its members to interbreed. It may appear that if one had a perfect design for a particular species, then that species would have no need for sexual reproduction. However, the environment could change and thereby invalidate parts of any fixed design. In contrast, the mechanism of sexual reproduction allows a species to change as its environment changes.