With the exception of identical twins, all human beings are genetically unique. During conception, a paternal sperm (sperm) fuses with a maternal egg (ovum). The resulting embryo will have received approximately half of its genetic material from the father and half from the mother. Therefore, although all children are genetically unique, they usually exhibit a combination of characteristics inherited from both parents. The basic unit of inheritance in nature is the gene and genes are passed from generation to generation in a predictable way. Each gene stores information in the form of deoxyribonucleic acid (DNA).
Cells and DNA
The average adult human body is made up of approximately 50 trillion (50 million million) cells. Most of the DNA is found in the nuclei of these cells; a much smaller amount is found in the mitochondria: these are small bean-shaped organelles present in the cytoplasm of nucleated cells, which release the energy they need to survive.
The set of instructions, or genetic blueprint, that is used to build the proteins that provide structure in all living organisms is encoded in genes in the form of nucleic acids, the most common of which is DNA. This is true for the simplest viruses and bacteria to complex multicellular animals, including humans.
Composition and form
The building blocks of DNA are called nucleotides (VanPutte et al, 2017). Each nucleotide is built from:
Five-carbon sugar (deoxyribose);
In total, there are four nitrogenous bases, as follows:
DNA molecules take the characteristic shape of a double helix. It is useful to visualize them as a long staircase that has twisted into a spiral, with the sugar and phosphate groups forming the rails and the nitrogenous bases forming the rungs (Fig 1). DNA has an exceptional storage capacity (Table 1).