Genes and DNA

A gene is a stretch of DNA found within the nucleus of a cell. Each gene encodes the structure of a protein that the cell may need to produce. According to current estimates, humans have between 20,000 and 25,000 genes with almost every cell in the human body containing two copies of each gene. One set is inherited from the father and the other from the mother. However, since each gene can produce more than one type of protein molecule, the total number of proteins produced in the human body is many times more than the number of genes.

Less than 2% of the human genome is made up of genes that code for protein molecules. Some short stretches of DNA, particularly those close to a gene, are important in regulating the expression level of the gene. These are termed ‘promoter regions’ and ‘regulatory elements’. The function of the rest of the genetic material within the nucleus is largely unknown. Many experts believe that these vast stretches of DNA may have no function at all and they are sometimes referred to as 'junk DNA'.

DNA is made of a string of four chemicals called ‘bases’ or ‘nucleotides'. These are adenine (A), cytosine (C), guanine (G), and thymine (T). The entire human genome contains approximately 3 billion bases, with a typical gene being around 2000 bases in length.

The order in which the bases are arranged along a gene instructs the cell to include a certain amino acid when it is making a new protein molecule. Each amino acid is signified by a string of three nucleotides, called a ‘triplet codon', such as AGG, TCG, and TTT. As there are twenty different amino acids used to build protein molecules, but 64 possible triplet codons, each amino acid can be indicated by more than one codon.

In addition to the DNA in the cell’s nucleus, DNA is also found within the mitochondria, the small bodies within the cell where energy is generated. Mitochondrial DNA is distinct from the DNA found within the nucleus of the cell and is inherited solely from the mother. The mitochondrial DNA contains around 40 genes that are involved in cellular respiration.

Genetic variation can exist on a number of levels, including changes in the number of chromosomes within a cell, through duplication or removal of long stretches of DNA, to replacement of one base with another, or removal of a single nucleotide. These changes in a single nucleotide, called single nucleotide polymorphisms (SNPs), have been the most intensively studied.

Genetic variation can lead to the cell being instructed to insert different amino acids in the growing protein molecule. This can result in the protein molecule having different chemical or biological properties, which can change the molecule’s function in the body. This, in turn, can influence the biological outcome or ‘phenotype’ of that gene.

Different versions of a gene are called ‘alleles’. Since most genes are present in two copies, it is possible for a person to have two copies of the same allele. This is referred to as being ‘homozygous’ for the gene. In contrast, where a person has inherited two different alleles of a particular gene, they are described as being ‘heterozygous’ for that gene. The final phenotype of a heterozygote is dependent upon which of the two alleles is dominant.

The commonest version of a gene is called ‘wild type’. This is the naturally occurring, normal, non-mutated version of a gene. If a gene does not replicate exactly and its genetic material is altered, it is said to have 'mutated'.

Mutations that are found in more than 1% of the population are called 'polymorphisms'. Common polymorphisms include the genes for sickle cell disease, G6PD, and thalessemia.