A human cell contains 23 pairs of Chromosomes or 46 chromosomes (somatic or body cells). Twenty three of them come from the father through a sperm and 23 from the mother through an ovum. That's why only gametes (sperms and ova) have half the number of chromosomes (23) that when unite together give a single cell with 46 chromosomes, called a Zygote or a fertilized ovum, that divide and divide giving rise to an embryo that grows into a full mature human being from one cell. 

The chromosomes are present in the nucleus of the cell and carry tens of thousands of genes that are responsible for our external features, internal body features, cellular function, susceptibility to certain diseases, or even cause diseases itself, etc.... That's why genes are called units of inheritance as every gene has a certain function that is transmitted from a generation to another through reproduction.

With the synthesis of sperms and ova in the testicles and ovaries, genes are distributed in a random fashion. So, different sperms in the same individual are NOT genetically identical 100% and the same thing is present in the ova. Thus the genetic material of any individual (genes or chromosomes), comes half from the father and half from the mother in a RANDOM fashion depending on which sperm has fertilized which ovum. Such genetic dissimilarities leads to the differences we observe between brothers and sisters and the whole human race at large.

Such biological variation is crucial for the continuity of the human race. If human responses to environmental conditions and diseases are the same, a single hazard, disease, or a microbe may be deadly to all of the human race leading to its extinction. But the differences in responses of every one of us to the same effector is protective to some and may be lethal to others, maintaining the propagation of individuals at a familial level and the human race at large.

Now the next step is to discuss the miraculous divine power put in those genes and how they control and master the growth of a single tiny cell, that you can just blow away by a gentle blow of your mouth, to grow into a mature individual who moves, sees, thinks, quarrels, and may be one day governs the whole world.

I look at it as a divine miraculous program that is present in the genes of sperms and ova that, again in a miraculous way, know how to unite to form a 46 chromosomes cell, how to move from the tube to reach its destination in the uterus (womb), how to fix itself in its walls through fingers which is called trophoblast, and how to get its blood supply from the mother to feed itself and respire.  It also then knows how to divide and divide into a multicellular structure, then arrange itself in what we call an inner cell mass and an outer cell mass that start to differentiate again in a miraculous way into different types of tissues (skin, muscle, bone, blood, cartilage, nerves, etc.),  Additionally, every group of tissues know how to blend together and to be organized to give highly specific organs like the eye, brain, and heart, in the same place with the same function in every individual, etc. This is something miraculous in itself. If you study embryology you cannot but feel God's hands and you feel very small and trivial in front of this divine power that is present in such a small "microscopic" piece of DNA.

The following illustrations show the travel of the Zygote through the tube and womb and its implantation deep in the wall of the womb; the second one shows division of the Zygote from a single cell into a multicellular structure that will makeup the various tissues and organs of the human embryo.

The next subject to be discussed is the forerunner of cloning, or what is called in plain English, test tube babies as a mean of treating infertile couples.

Called IVF & ET (In vitro* fertilization and embryo transfer), It is a process in which Gynecologists:
 

Only three embryos are normally transferred, creating a problem with the rest of the embryos.

What to do with them? 

Some freeze them to be used in the next cycle if pregnancy did not occur and others discard them.

There are thousands of frozen embryos in IVF labs and doctors don't know what to do with them.  This is a substrate for cloning research as we will see in a while, as the embryos are already there.

** in Western countries they may use donor sperms or eggs, something that is not practiced in Egypt.

Scientists became more familiar with the handling of the ova and sperms when they succeeded in "forcing" fertilization in some cases in which in-vitro fertilization failed to occur.

They are now able to cut the tail of the sperm to get its head that contains the 23 chromosomes and produce a small hole in the wall of the ovum and inject the head of the sperm inside it effecting fertilization. A process that is called ICSI or intra-cytoplasmic sperm injection.

In the following 2 figures the process of ICSI can be seen in a real lab (left) and in a diagrammatic representation (right)

These advances paved the way to more and more research in the area of fertilization and hence the era of cloning began.

Now to address the issue of cloning.

We have seen that both in natural conception and in IVF programs half of the genetic material comes from the father and the other half comes from the mother in a RANDOM manner. However, this is not the situation in cloning.

Cloning research aims at two different goals:
 

First: Embryo splitting:

In the very early stages of embryonic development, the cells of the embryo are exactly the same and are pluripotent (a pluripotent cell is a cell that is able to divide into all other types of cells and tissues). 

This is already happening in nature with identical twin who were initially a single embryo that divided very early into 2 embryos (and sometimes three) before cellular differentiation or specialization.

The question is what can this type of cloning serve in humans? 

Obviously nothing but scientific research, or to satisfy a strange request from a couple to have an identical twin instead of a single baby, or perhaps one day some parents will ask to have a copy of their baby frozen as a spare in case something wrong happens to their child as some people believe that one advantage of cloning is to compensate someone for the loss of a child by cloning a replica of it.

Second: Nuclear transplantation:

In this type of cloning, there is no mother and father, there is no natural selection and there is no individuality (unique "genetic" criteria for every human being) as happens in natural conception. 

Explanation follows:

In this technique:
 

By this method, the clone (the baby) will be genetically identical to the owner of the adult cell, as all of its genetic material has been gained from this cell only, while the woman acts here as a receptacle just for the growth of the baby. She is not a genetic mother. She is a surrogate mother unless the adult cell itself was hers.

These techniques pose many hazards:
 

1- Dolly, the famous first mammalian clone, cost its cloner 277 embryos/baby sheep to have a successful clone. Other embryos either aborted, died, or were born malformed. But the announcements in the media was only for the one successful trial masking the miserable 276 failed, dead, or malformed embryos/sheep.  2- Dr. Wilmut,  who cloned Dolly, is against human cloning himself because he knows the problems that he faced with the other cloned embryos/sheep. 3- Dolly itself has a cellular age that is 6 years older than its actual age raising the possibility of premature aging of the clone. 4- The other, more recent technique, that was used to clone mice is only successful in less than 3% of the attempts, and also yields dead or malformed clones in most of the attempts. 5- The long term consequences of cloning on the cloned animals is not yet known or enough studied. Many of the cloned animals suffered diseases and some suffered morbid obesity. 6- Our understanding of cloning and how cells grow normally (or abnormally) is not yet complete enough to risk it in humans. 7- The effect of playing with the human genome at large is not yet fully understood and may get out of control, or yield a new trait that may threaten the whole human race. 8- Perhaps some lunatic scientists may use cloning to produce a clone of people to be used for a certain purpose or trying to improve the human race. The above are only some of the aspects that make many doctors refuse cloning on a scientific base rather than on an ethical or a religious base (that of course has a lot against reproductive cloning)

Third: Therapeutic cloning:

This type of cloning aims at creating embryos to get their pluripotent cells (the so called Embryonic Stem Cells) to experiment with. The rest of the embryo is then discarded.  Actually embryologists make use of the frozen embryos in IVF labs after getting the consent of the parents to use their "extra" embryos for scientific research. As I said, the pool is already there.

The aim of using this technique, if successful, would be as follows:

Scientists believe that they can treat certain diseases that are untreatable now with the use of stem cell cloning.

Suppose that someone develops a certain disease such as Alzheimer, Parkinsonism, kidney failure, liver failure, or diabetes. They think that the scenario can go as follows:
 

The idea here is to get a tissue or an organ that is immunologically identical to the person so it won't be rejected.

Stem cell research is currently in process, but is in its very early stages. However, the question is why don't geneticists and embryologists experiment, as they wish, with animal stem cells first and keep away from human stem cells and human embryos till they know for sure the whole story in animal research before they "play" with human embryos?

Finally, cloning literally means "to have a copy of something." Here it is meant to be a genetic copy of another individual. So, the first thing that can be criticized in cloning is that someone will give him/herself the right to decide the genetic makeup and traits (properties) of another human being. Someone can ask for a baby who is genetically identical to himself (to satisfy his self ego), or identical to someone else whom he thinks has good traits. It won't cost him/her more than a single cell, but will cost the cloned individual the chance to be a unique individual that has its own genetic makeup that for sure will be reflected on his behavior and whole career.

NB: This was meant to be a simplified, but scientific, explanation of the subject. I will not go into the many ethical and religious constraints on cloning as it differs between societies and individuals according to their background, set of ethics, values, and of course religious beliefs.