Eukaryotes are organisms whose cells are organized into complex structures enclosed within membranes. Most eukaryotic organisms, for example, human beings, contains billions of individual cells. Almost all of these cells contain, within each nucleus, the entire genome for that organism. This genome contains the organism’s complete hereditary information in the form of deoxyribonucleic acid (DNA), that encodes a complete blueprint for all activities and structures within the organism.
In the human body, the genome consists of 23 pairs of chromosomes. One of each of this pair is inherited from the mother and the other from the father. Each chromosome is made of chains of DNA. DNA consists of two polymers (large molecules of repeating subunits) made up of units called nucleotides, these molecules are wrapped around each other in a structure known as a double helix. Each nucleotide consists of a deoxyribose sugar, a phosphate group and one of the four nitrogen bases, guanine, adenine, thymine and cytosine. These bases, which are usually represented by their first letters, G, A, T and C, are where hereditary genetic information is actually encoded. It is worth noting that one of the two strands of the DNA double helix will suffice to describe this information; this is because of complementary base pairing, whereby an A on one strand always binds to a T on the other and a C always binds to a G. The molecular structure of DNA is shown in the animation below.
Genes are essentially segments of the DNA structure described above. Loosely speaking, a gene is a section of DNA that defines a single trait by encoding a particular pattern, about 27,000 of which exist in humans; more technically, a gene is a locatable region of genomic sequence, corresponding to a unit of inheritance.
The main purpose of genes is to act as a blueprint in the creation of proteins. Proteins are made of amino acids and are responsible for the structure and activity of an organism at a cellular level. They are created as follows; starting at the 5′ end (the leading end) of a gene and proceeding to the 3′ end (the tail end), the information contained in the gene is transcribed into a messenger ribonucleic acid (mRNA) strand. This process is performed by an enzyme called RNA polymerase. After transcription this mRNA molecule leaves the nucleus of the cell where it is transcribed into a protein in a process called translation. This is performed by ribosomes, which read the code carried by mRNA molecules from the cell nucleus and create proteins combining any of the 20 amino acids in the body into complex polypeptide chains. These proteins are the building blocks of the organism. This process of translating a gene into a functional product is known as gene expression.