The Human DNA Structure

The Human DNA (Deoxyribonucleic acid) consists of more than 3 billion base pairs of 4 different nucleic acids (nucleotides) that make up the genetic code of a human. This equals about 750 Megabytes of data that holds all instructions needed for the development of a complete living organism. It is therefore often referred to as a "blueprint" to construct all cells, organs, skin, hair, nails and other components (such as proteins and RNA) necessary for creating a functioning organism like our body. This genetic information is stored in segments of the DNA called "genes". This is the main purpose of DNA - the long-term storage of genetic information. The DNA contained in Human Embryonic Stem Cells can let them grow into every part of the human body such as heart, liver, kidney, brain, ears, eyes, bones or skin. As such, these cells are currently subject of intensive scientific research. The results could provide many new possibilities for therapeutically treatments for a whole range of diseases that are currently deemed incurable.


The DNA resides in the nucleus of each cell in the human body. Inside the nucleus of each body cell, there are 46 Chromosomes in humans, which is 2 complete identically sets of 22 chromosomes (autosome chromosomes) and 2 additional gender specific chromosomes (sex chromosomes) - either 2 X chromosomes (XX) for a female (woman) or 1 X and 1 Y chromosome (XY) for a male (man). Therefore humans are considered diploid as each body cell contains 2 homologous copies of each chromosome, normally one from the mother and one from the father. However, human haploid gametes (sperm and egg) only contain 23 chromosomes - haploid cells and organisms only have 1 copy of chromosomes. These 46 chromosomes are nothing else than an organized structure of the coiled human DNA and certain proteins.

The shape of DNA describes a long spiral, comparable to a twisted rope ladder or a spiral staircase with a fixed diameter. Also called a "double helix", the spiral forming the human DNA molecule is chemically made up of 2 strands of a sugar-phosphate backbone, running antiparallel. This backbone (skeleton) consists of a phosphate group (Phosphoric Acid, H3PO4) and a sugar group (Deoxyribose, C5H10O4) forming phosphodiester bonds between them, resulting in the Phosphate-Deoxyribose backbone. In between those 2 twisted strands of DNA there are complementary purine-pyrimidine base pairs holding the two strands. There are only 2 possible base pairs combinations:


Adenine (C5H5N5) and Thymine (C5H6N2O2)
Guanine (C5H5N5O) and Cytosine (C4H5N3O)


Those base pairs forming 2 (Adenine-Thymine) or 3 (Guanine-Cytosine) hydrogen bonds between each other. They are connected to the sugar molecule of the 2 backbone strands. The complex of a sugar-phosphate molecule together with a base molecule forms a "nucleotide". The chemical formula of Adenine, Thymine, Guanine and Cytosine (their molecular formula) is very similar and explains the binding process. The DNA structure model or double helix model of human DNA as we know it today (twisted ladder model or spiral staircase) was discovered in 1953 by James D. Watson and Francis Crick.
Human DNA Sequence
The human DNA sequence is unique for every individual person, even though it is nearly 99.9% identical. It's the tiny portion of only 0.1% of DNA that differentiates us from every other human and that contributes to our individual differences. These small variations in the human genome such as "Single Nucleotide Polymorphisms" (SNPs) and the "Variable Number Tandem Repeat" (VNTR) allow further analysis using DNA fingerprinting (DNA profiling) techniques. The results of this kind of human DNA analysis are being used for ancestry testing, paternity testing or especially for forensic criminal investigations. The later shows the increasingly important role of genetics in investigating crimes such as rape or murder; hence human DNA can act as evidence in court cases.