The gene is the fundamental unit of heredity. The word comes from the Greek γενεά or "genea" meaning "to generate".
A section of DNA that codes for the production of an RNA molecule. In most cases that RNA molecule codes for the production of a protein, but in some cases (most notably the transfer RNA (tRNA) and ribosomal RNA (rRNA) genes) the RNA molecule is the end product.
Gene expression refers to all the processes involved in converting genetic information from a DNA sequence, or protein. In prokaryotes, there are just two processes required: transcription and translation. In eukaryotes, there is an additional step RNA processing (splicing), which intervenes. The synthesis of a single-stranded RNA molecule using DNA as a template is referred to as transcription. The enzyme that catalyzes this reaction is known as RNA polymerase. Although the subunit structure and details of the process differ significantly in prokaryotes and eukaryotes, the chemical reaction is identical.
By 1900 scientists knew that cells were the building blocks of life, and that all living things start as a single fertilized cell which keeps dividing. Scientists identified tiny threads in the nucleus of the cells which were called chromosomes because they could be stained with colored dyes for microscopic study - from the Greek words for colored bodies. They could see that chromosomes came in pairs, and that human cells all contained 23 matching pairs. American biologist Walter Sutton knew Mendel's work on peas, and suggested that chromosomes held the secret of inheritance.
Another American biologist, Thomas Hunt Morgan, developed the idea that chromosomes were made up of linked groups of factors called genes. He experimented with red-eyed fruit flies and found that sometimes a white-eyed fly appeared. When he mated them, he found that as he expected there were three red-eyed flies to every white-eyed fly, but that all the white-eyed flies were male. He concluded that the gene for white eyes must be on a chromosome that was related to being male. Later workers found that this is why some hereditary diseases such as hemophilia and muscular dystrophy only show in males, though women can carry the gene for the disease without showing it.
Once Crick and Watson had unraveled the structure of DNA, the function of genes became clearer.
One strand of DNA contains many genes. DNA is made of four nucleotides: guanine, adenine, thymine and cytosine. G always pairs with C, and A with T. Different combinations of GC and AT join together in different orders along the strands of DNA coiled up along chromosomes to give cells instructions for making the different kind of proteins from which cells are made. These groups of instructions are called genes.
Our bodies are made up of about 100 trillion cells, each of which is responsible for a specific function. The thousands of different proteins in the cells work together like a machine to make the cell function as it should. If the genes are normal, the body part will function well. If a change (mutation) has happened in a gene, the protein may be faulty, as for example in sickle cell anemia where the instructions for making blood cells are abnormal.
In 2005 it was shown that genes were not the only way that genetic information could be inherited. Scientists working at Purdue University discovered that Arabidopsis plants could remember what genes they had in the past, and correct mistakes in their current genes. This non-mendelian inheritance probably involves a RNA cache of past messenger RNA transcripts.
Changes of a species from its normal state are called mutations. Sometimes they happen on their own, but they can also happen when a parent is exposed to something that will affect the way they reproduce. High doses of X-rays can produce mutations, and so can some chemicals. According to the theory of natural selection, most mutations are neutral, some are deleterious (harmful), and a very small proportion are beneficial such that they improve the individual's chance of survival and reproduction, thus passing on to the next generation. Some mutations can be both helpful and harmful. For example, people from some African countries carry a gene for an illness called sickle-cell anemia. This illness causes health problems, but people who have the gene for sickle-cell anemia are also less likely to catch malaria, a serious illness common in Africa.