DNA, genes, chromosomes and mutations
Last Reviewed 10/05/2016
Humans, like every other organism, are made up of cells. We all start off as just one cell at the time of fertilisation. As the single cell divides, all genes are copied so that every new cell possesses a full set of genetic material: the genome. The mechanism of copying the genome is quite remarkable considering that the human body contains approximately 10 trillion cells. The genome is made of a chemical called DNA or deoxyribonucleic acid and is organised in chromosomes, which contain genes.
What is DNA?
DNA is a long molecule that contains our unique genetic code. Like a recipe book, it holds the instructions for making all the proteins in our bodies. DNA is composed of two strands that wrap around each other to form a double helix shape, like a spiral staircase. Each strand of DNA is formed of four basic building blocks or ‘bases’: adenine (A), cytosine (C), guanine (G) and thymine (T). The order, or sequence, of these bases determines our unique genetic code and contains the instructions for producing molecules in our bodies. A single gene may be many thousands of bases long. The cell’s molecular machinery reads the DNA code three-letters at a time. Each three letter sequence codes for a particular amino acid: the building blocks of proteins. These proteins do the majority of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs. Examples of proteins include the keratin in hair, pigments that give eye colour and enzymes digesting food in the stomach.
What is a gene?
Genes are sections of DNA that contain the set of instructions to produce one specific molecule in your body, usually a protein. These proteins control how our body grows and works; they are also responsible for many of our characteristics, such as our eye colour, blood type or height.
The human genome contains around 20,687 protein-coding genes. Different genes or instructions are read at different times in different cells in response to the requirements of our bodies. Each cell contains two sets of genes, one from your mother and one from your father. For ease of storage and access, the genes are packaged up into 46 parcels called chromosomes.
What is a chromosome?
Chromosomes are bundles of tightly coiled DNA located within the nucleus of almost every cell in our body. Humans have 46 chromosomes in their somatic (non-reproductive) cells. We inherit one set of 23 chromosomes from our mother and one set of 23 chromosomes from our father. So we have two sets of 23 chromosomes or 23 pairs.
What is a mutation?
Over our lifetime, our DNA can undergo changes or mutations that result in differences in the DNA sequence and may affect the proteins that are made. Mutations can occur as a natural consequence of errors in DNA replication or as a result of exposure to environmental factors such as smoking, sunlight or radiation. Mutations can also be inherited when they are present in the germ cells, the sperm or the egg.
Often the cells in our bodies can recognise mutations and correct them. Mutations that are not repaired can disrupt normal gene activity and cause a genetic condition because the gene is not carrying the correct instructions to the body.
Mutations can also have a positive effect as they contribute to genetic variation within a species. For example, sickle cell anaemia is caused by a mutation in a gene that contains the instructions for producing a protein called haemoglobin, important for the transport of oxygen in our bodies. This mutation causes red blood cells to become abnormal, rigid and acquire a sickle shape. Although individuals that have this mutation have problems when these sickle-shaped red blood cells block the flow of blood causing joint pain and other symptoms, they are also protected against malaria infection.
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