Gene

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Gene

A gene is the basic structural unit of inheritance in biological organisms. It is made up of a short segment of DNA and contains the necessary information to produce a specific protein. Each gene is separated from each other by non-coding sequences that serve other functions. Genes are strung together and tightly packed into structures called chromosomes. All the genes in an organism are located on chromosomes in the nucleus of most cells and represent the blueprint for instructions that make up an organism. For example, genes can determine physical characteristics in humans such as height, eye color , skin color, or any other trait. Genes are passed from one generation to the next through sex cells (the egg and the sperm) called gametes. Maternal and paternal genes combine at fertilization and each contribute to the observable features of the offspring, explaining why children often look like one or both parents.

Mutations, which are changes in the structure or sequence of DNA, can cause disease if it involves disruption in the specific sequence of a gene. For example, if a mutation disrupts a gene that encodes a protein responsible for controlling cell division , this loss of function might cause the cell and the cells that arise from it to continuously divide, producing cancer .

History

In 1909, Wilhelm Johannsen (1857–1927), a Danish biologist, first proposed the name gene as the term designating the basic unit of information that is inherited. In 1944, the Canadian bacteriologist Oswald T. Avery (1887–1955) and American scientists Colin M. Macleod (1909–1972) and Maclyn McCarty demonstrated that DNA is the material responsible for a process called transformation in bacteria , or the transfer of genetic information from one bacterium to another. These researchers had no idea how important their discoveries were until many years later when further studies demonstrated that DNA was the material responsible for the transfer of genetic information in most living organisms. James Watson, an American biochemist, and Francis Crick, a British scientist, presented in 1953 a model of DNA that resembles a twisted ladder. The sides of the ladder are composed of sugar-phosphate groups, and the rungs consist of paired nitrogenous bases. It was shown that there are bases in DNA and the arrangement of the four bases encodes the information held by genes. The DNA model explains how DNA replicates, or makes copies of itself. Later, the American biochemist Marshall W. Nirenberg, and others, used the model to work out the genetic code—the relationship between the arrangement of the DNA bases and the amino acids produced by the DNA sequences in each gene.

Gene expression

DNA is made up of four building blocks or nitrogenous bases; adenine (A), guanine (G), cytosine (C), and thymine (T). A, C, G, and T represents the DNA alphabet and different combinations of these letters mean something different. Every gene begins with a specific start sequence and ends with a stop sequence. Therefore, the specific sequences of these four bases determines whether the DNA codes for proteins (coding DNA), the specific protein it encodes, or whether it represents noncoding DNA that does not encode for protein. Non-coding DNA, also called junk DNA accounts for 97% of the genome and despite its name, it serves many purposes including the proper functioning of genes. Each gene can be converted or transcribed into a type of RNA called messenger RNA (mRNA). RNA is very similar to DNA except that instead of thymine as one of its four nitrogenous bases, uracil (U) is substituted. Gene expression can be controlled by proteins called transcription factors, enhancers, and silencers. These regulatory proteins influence whether proteins will be expressed by binding to specific sequences of DNA or through interactions with other DNA binding proteins.

Information is passed from the DNA molecule to the messenger RNA (mRNA) by the pairing of complementary bases in each of the two strands. The mRNA then carries the instructions from the DNA in the nucleus to a ribosome in the cytoplasm. A molecule called transfer RNA (tRNA) transports an amino acid that is designated to match a codon, or a three base pair sequence in the mRNA. Amino acids strung together form a particular protein. The proteins that are produced might, for example, be important for human growth and development or represent important enzymes in physiological pathways.

Knowing the sequence of every gene found in the human genome, made possible in part by The Human Genome Project , will allow scientists to better understand the cause of diseases such as cancer or cystic fibrosis and develop new ways to treat or cure these diseases by characterizing individual genes as well as gene-gene interactions. The rough draft sequence of the human genome was completed and published in February 2001 in both Nature and Science scientific journals and the final sequence is expected to be completed sometime during 2003.