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The spark of life: a biological paradox.
It has been my joy recently to participate in teaching a science overview course as part of the University of North Carolina's senior learning program in Asheville. Reviewing material for my part of the course (biology), I was struck by the lack of attention given in the teaching of biology to "maternal inheritance", meaning the part of an offspring's inherited characteristics that comes uniquely from the mother. I don't mean the mother's chromosomes, which she contributes to the offspring in the same measure as the father, but rather extra-chromosomal material in the mother's egg that will strongly influence the developing offspring, and that in fact provides the offspring with the "spark of life".
Here is briefly how it works, for those whose basic biology class has receded into the murky past: In humans, just as in all higher animals and plants, sexual reproduction involves the merger of an egg (ovum) from the female parent – which carries chromosomes with a copy all her genes – and a sperm from the male, similarly carrying all his genes. When implantation of the sperm in the egg occurs, the chromosome sets of the parents merge to give the fertilized egg a full complement of both parents' genes. The fertilized egg (called a "zygote") begins a remarkable period of cell division and differentiation that results in development of an individual of the next generation.
This is the classical story of genetic inheritance that everyone learned about in high school. It's often called "Mendelian genetics" after Georg Mendel, the Czech monk who discovered the basic laws of inheritance in the 1860s. In this story, the gene is king. The genes are the units (30,000 or so in humans) on the chromosomes that determine (or "code for") heritable traits, and they're made of the now-well-known substance "DNA".
DNA and genes have gained fame as the "crucible of life" and similar appellations. But is that really where the spark of life is to be found? The DNA and genes of a developing zygote carry the information that determines – to a degree – most of the physical characteristics of the new life. But the DNA doesn't actually do anything. It's a passive template – much like a book or a blueprint. And like a book or a blueprint, it needs a reader. The one and only reader of DNA is an enzyme – a protein – called RNA synthetase. It reads the DNA and starts the process where numerous other enzymes translate the genetic information and build the needed metabolic products. In the absence of this enzyme in the nucleus of the fertilized egg, the genes will never be read, and nothing will happen. The DNA will just lie there, like a dusty book on a shelf.
And here's the paradox: Reading the DNA and turning it into metabolic products – in other words, living – requires the presence of a series of enzymes. But wait ... these enzymes are coded for by the DNA. In other words, only by reading the DNA can these enzymes be made, yet these enzymes are required to read the DNA in order to make themselves! It's a "catch-22". It's the "chicken-or-egg" paradox.enzymes Which came first?
And here's the solution to the paradox: In the history of life on Earth, the genes and the proteins needed to decode them undoubtedly evolved together as a system. But how about the new life, the fetus developing from a fertilized egg? How does it get started, finding itself in a "catch-22" fix? The answer lies in the difference between the sperm and the egg. While the sperm is tiny, barely larger than a virus, and carries essentially nothing but the father's genes, the mother's egg is enormous by comparison. It is large because it carries everything the developing zygote will need. The mother's egg provides the offspring with the "chemical starter kit" of life, the activity that is life itself. Although we haven't yet identified exactly what constitutes the "spark of life" that is transferred from generation to generation and kick-starts life anew, we can be sure that it is found in the kit that the mother bequeaths to the child. The "spark of life" lies in the system made up of the constituents of the mother's egg: the mitochondria that provide the energy for enzymes to do their work, the enzymes themselves that are ready to read and decode the new DNA and carry out its blueprint, and all the cell organelles that enable and participate in the activity of the cell. This system constitutes life, and thus the spark that vitalizes a new life comes from the mother alone. Of course the male has the useful role of adding genetic variability to the offspring, and he serves as a catalyst in the reproductive process, but the spark, the essence of life – of our lives – is our mothers' essence, flowing uninterrupted in us, and if we are female, through us to the next generation. So...
Honor thy mother!
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