Deoxyribonucleic acid (DNA) is a molecule that contains the
instructions for the development and operation of living organisms.
Each organism has one complete DNA code, known as a genome, and a copy
of this genome is found in the nucleus of that organism's cells. Within
each cell the DNA is organised into different chromosomes, in the case
of humans 23 pairs of chromosomes.
DNA contains the information necessary for the production of the
operating system and building blocks of the body: RNA and proteins.
Specific sections of the genome that code for functions or
characteristics of the body are known as genes. Areas of the genome
that are not genes are called the 'non-coding regions' or sometimes
referred to as 'junk DNA'. These regions are now known to have many
regulatory functions, for example they may switch another gene on or
off.
DNA is a polymer of individual units known as nucleotides or 'bases'. These are arranged in a 'double helix' structure where pairs of nucleotides cross two deoxyribose sugar backbones, forming a structure like a twisted ladder. The pairs of nucleotides are referred to as 'base pairs'. There are four nucleotides: Cytosine, Adenine, Guanine and Thymine, often written as C, A, G and T. A always pairs with T and C with G.
To understand the size of DNA in comparison to other cell structures, visit this excellent resource from the University of Utah Genetic Science Learning Centre.
DNA produces proteins through a pair of
processes called transcription and translation. Firstly transcription:
here, enzymes read the DNA molecule and produce an intermediary
molecule called messenger RNA, or mRNA. Each base in the DNA is
reflected by a complementary base in the mRNA, maintaining the DNA code
sequence.
Then, translation: mRNA enters a part of the cell called a ribosome,
which is designed to produce the actual proteins. Here the mRNA code
will be translated in groups of three units. A specific sequence of
three nucleotides will code for one amino acid. There are twenty
standard amino acids. Combinations of these twenty amino acids result
in many different proteins, the building blocks of a living organism.
The human genome consists of 3.2 billion base pairs, organised into 23
lengths known as chromosomes. Each human inherits one set of
chromosomes from each parent, so the total genome, known as diploid,
actually contains 6.4 billion base pairs. Between any two humans there
will be differences in about 0.5% of these base pairs. Sometimes these
variations account for the normal differences between humans, for
example height or eye colour. However sometimes these genetic
differences are a factor in disease. For example, a single nucleotide
difference in a gene would code for a different amino acid, and
therefore cause an error in the protein, potentially causing the
development of disease.
Studies of the human genome are among the most significant scientific
advances of recent years. For more information click here to understanding the genome