Human beings' inheritable traits are contained in their DNA, organized in 46 chromosomes (46 is the diploid number and the complete set of 46 chromosomes is also known as the human genome ). These 46 chromosomes are in the nucleus of every one of the cells of our body. Only the gametes - the ovum and the spermatozoon (plural: spermatozoa) - have 23 chromosomes (the haploid number).
The first event in the formation of an individual is precisely the encounter and fusion of these two highly-specialized cells: the mother's ovum and the father's spermatozoon. This event is known as conception, fecundation or fertilization. At the moment of conception, specific and exclusive changes are made, among them, is the completion of a new, unique and unrepeatable genome with the 46 chromosomes (i.e. 23 pairs, each pair containing one chromosome from the mother and one from the father) that correspond to the human species.
Biochemically, this process is complex. There is a certain chemical stimulus (a process known as chemotaxis ) that specifically guides the trajectory of the spermatozoa toward the fallopian tube in which ovulation occurs. Specific enzymes, the hyaluronidases , are contained in the acrosome (the "leading edge") of a spermatozoon, and have the role of penetrating the ovum's protective membranes (the corona radiata and the zona pellucida ). Only one spermatozoon will cross the membrane of the ovum, which then becomes impermeable from that instant, and only the head of the spermatozoon, which is where the genetic material is contained, crosses the membrane.
As soon as the head of a spermotozoon penetrates the ovum, the two nuclei fuse in a process known as syngamy, setting in motion a chain of activity. Two systems (that of the ovum and that of the spermatozoon) cease to function individually and constitute a new system (a new genome), which begins to operate as a "unit" called a zygote, or a one-celled embryo. When conception occurs, a process is set in motion that will not be stopped until the death of the individual.
Modern understanding of molecular biology enables us to know that every living being has a genome specific to its species; this is precisely what defines it as a member of one species and not another. But this genome differentiates beings even belonging to the same species. Referring concretely to the human genome, it is what makes us human - ending any doubt as to which species we belong. At the same time, the individuality of the genome makes each one of us unique and unrepeatable.
Thus, the biological nature of the product of human conception (the zygote) offers us the most interesting data to confirm its human nature.
From the very moment of conception, the existence of a new life commences - a life that is specifically human, with a unique and unrepeatable genetic code, identical to neither the mother nor the father. This human genome controls and irreversibly fixes its successive development. From this first cell, or zygote, nothing other than a person, and only this particular person could possibly result.
Why just this particular person?
I stated above that the process of conception is complex. But the complexities of human life begin long before conception, with the creation of the ovum and spermatozoon through meiosis. Recall that meiosis is the process of cellular division where the number of chromosomes is halved from 46 to the 23 found in the ovum and spermatozoon. Before the halving takes place, homologous chromosomes (the two chromosomes in a given pair) actually exchange genetic material. Between three and eight segments are exchanged in a process called crossing over . After crossing over, segregation (aka Mendal's First Law of Inheritance) takes place, where the newly-crossed cells randomly distribute genetic attributes among the two cells which are created upon meiotic division.
Because of crossing over and segregation, there are over one billion combinations for the human genome contained in each ovum or spermatozoon i.e. even before the combination of the two at conception. Thus, from the single-cell zygote stage, each individual is totally different from every other individual.
Today's knowledge of molecular biology permits us to verify that the unique and unrepeatable genome that is present at the very moment of conception is the same that we will find in each one of the cells of the embryo, the fetus, the child, the youth, the adult, the elderly person. Even during sicknesses, poor nutrition, accidents or any other mishap, the human genome remains exactly the same.
Upon fertilization, we have the most specialized of cells - so specialized that no other cell will ever have the same instructions for life possessed by the individual that is developing. During embryonic development, as cells divide and specialize, nothing new is learned by the DNA of the developing cells; rather many things are progressively forgotten . Thus, the information needed for this individual is written from the first cell: nothing will ever be added or changed. This is not a theory or a supposition, but rather is the information that the science of genetics has proven without a doubt. Conception establishes a new genotype - a new person - unique and unrepeatable, and activates him or her to develop.
In my next brief I will address the continuing development of the embryo and the autonomy of prenatal life.