The spark of life: a biological paradox.

September 2006

Abstract:
A mother and a father equally contribute their genes to the next generation.  But genes are only genes.  The motive force of life – the stuff that activates the genes and starts the process of living – passes from the mother, and has passed from mother to child through all generations that have been.

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 oocyte (often imprecisely called an 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 oocyte occurs, the chromosome sets of the parents merge to give the fertilized oocyte (now called a "zygote") a full complement of both parents' genes. The 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 was earlier 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 double-helix shaped molecule "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 zygote (which is about to become an embryo), the genes would never be read, and nothing would happen.  The DNA would 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, the act of 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. DNA or enzymes ... which came first?

And here's the solution to the paradox:  In the history of life on Earth, the genes and the enzymes needed to decode them undoubtedly evolved together as a system.  But how about the new life, the embryo/fetus developing after fertilization?  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 oocyte is enormous by comparison.  It is large because it carries everything the developing embryo will need.  The mother's egg provides the offspring with the "chemical starter kit" of life, the activity that is life itself.  The "spark of life" that is transferred from generation to generation and kick-starts life anew 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 oocyte: the mitochondria that provide the energy for enzymes to do their work, the enzymes themselves that are ready to read and decode the new individual's 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!