Some readers who do not have much interest in the science of genes can skip most of this post and move on to newer one, although it is not really difficult for the keen reader
The blue prints for all known life forms are contained within a single cell. After all, all life including human one starts from a single cell. Modern genetic science does not fully understand the workings of this single cell yet. At the core of a cell lies the genetic code that is the blue print for a life form. To gain an elementary knowledge of what this genetic code is, consider two single rungs of a ladder as shown in figure 1 with one of the four alphabets A, T, C or G sitting on the rungs. These two rungs are the only two basic elements that can now be joined up in various manners to construct the entire ladder or ladders that forms the basic code of all life forms. In the genetic code the rungs are made off a certain chemical compound and the four alphabets four different chemical compounds found in nature. It is not necessary to know what these compounds are exactly at the moment in order to gain an understanding of the genetic code.
Figure1: The beginning of the code of life
Imagine that you have an unlimited number of such rung bits as shown in figure 1 to play with. The first step in constructing the genetic code is to join the two rungs in various ways so as to form a three-rung ladder. Three of these are shown in figure 2.
A------T A------T A------T
T------A C------G A------T
A------T G------C C------G
Figure 2: Some three-rung ladders
The three rung ladders of figure 2 have been constructed by joining the two single rungs of figure 1 in different ways. In one an identical rung is stacked upon another. In another the rung has been turned around before joining. Several other types of three rungs can be constructed similarly. If you attempt that, then you will find that 64 different types of ladders may be constructed in this manner. Now let us try and make your ladder longer by joining up several such ladders together. One is shown in figure 3.
Figure three is a sample of a snippet from a DNA. An infinite number of such chains of varying length can be built up by combining the three ladder rungs of the type shown in figure 2. If the sixty-four three ladder rungs are regarded as alphabets, then their joining up in various ways may be regarded as words and sentences. If there are enough words you have a book. Just as any language has punctuation marks and certain rules as to how words may be joined, there are certain rules that have to be obeyed before we can get a real life code. Since real DNA is made up of flexible material it can be twisted and folded. Different types carry the code for different species of life including bacteria, animals and humans but all across the spectrum of life the coding language is the same except for some rare exceptions only. Two DNA strands form a helical spiral, winding around a helix axis in a right-handed spiral. The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a spiral staircase. The bases of the individual nucleotides (A, T, C or G) are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.
Figure 3: A longer ladder
A dotted line is shown passing through the middle of the DNA strand in figure 3. The rungs of the ladder can break right down the middle to create two bits of single stranded DNA. Since we know that A and T come in Pairs as well as C and G it is possible to reconstruct the ladders with new material so that we have two identical ladders instead of one. This is how individual cells divide and multiply all the while preserving the original code. Each new cell is an exact copy of the parent cell.
Just as books have chapters, the DNA codebook of different species of life divides the full DNA into chapters too. These chapters are analogous to chromosomes. Human DNA is divided up into forty-six chromosomes, twenty-three provided by the father and twenty-three by the mother. Each chromosome is like an immense ladder that may have a length larger than ten centimeters when stretched out. The forty-six chromosomes (forty six ladders) however fit into a cell that is about 0.000002 meters wide only. Each pair of chapters are on the same topic so to speak, one written by the mother and another by the father, so that when a real human is constructed on the basis of the instructions in the code one may choose the instructions from either chapter in a pair. One can mix matters in different ways here. That is why new babies are not precisely like their fathers or mothers but somewhere in between. There is a special chapter though, the one containing instructions on male sexual characteristics. Only the father provides this. Mothers can provide instructions for producing female babies and not males. A father may provide instructions for either. Thus we see how a combination of a few chemical components can be combined in various ways to encode instructions for the innumerable different type of life forms that can exist on your planet and beyond in the universe.
Genes that decide heredity consist of DNA base pairs of the type just described. Genes vary in size from a few hundred base pairs (A rung of the ladder) to more than two million base pairs. The exact number of genes in humans is not yet known. The human Genome project estimates that humans have about 25000 genes. The total number of base pairs in a human cell is estimated to be in millions. Every cell of the human body contains the same genes and the human body is made up of some ten trillion cells. RNA is similar to DNA. Some of the chemical components of DNA when replaced by other similar ones produce RNA. However the genetic code of most organisms is stored as DNA because the latter is more robust. However RNA is required for growth and cell production and it is produced in the cells by using the DNA as a template.
Life arrives on a planet from the cosmos in a bacterial form. It develops further by evolution, primarily assisted by viruses and plasmids. Bacteria return the new evolutionary information generated in the planet to the cosmos from time to time. Your own genetic code is nothing more than inumerable such bits of code added by viruses to the genetic code of an amoeba in small gradual steps, so as not to shock your mummy, in a manner of speaking.