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A gene is a short length of DNA found on a chromosome that codes for a particular characteristic or protein. Alleles are different forms of the same gene. For example, eye colour is...
- Overview
- Chemical structure of genes
- Gene transcription and translation
- Gene regulation
- Gene mutations
gene, unit of hereditary information that occupies a fixed position (locus) on a chromosome. Genes achieve their effects by directing the synthesis of proteins.
In eukaryotes (such as animals, plants, and fungi), genes are contained within the cell nucleus. The mitochondria (in animals) and the chloroplasts (in plants) also contain small subsets of genes distinct from the genes found in the nucleus. In prokaryotes (organisms lacking a distinct nucleus, such as bacteria), genes are contained in a single chromosome that is free-floating in the cell cytoplasm. Many bacteria also contain plasmids—extrachromosomal genetic elements with a small number of genes.
The number of genes in an organism’s genome (the entire set of chromosomes) varies significantly between species. For example, whereas the human genome contains an estimated 20,000 to 25,000 genes, the genome of the bacterium Escherichia coli O157:H7 houses precisely 5,416 genes. Arabidopsis thaliana—the first plant for which a complete genomic sequence was recovered—has roughly 25,500 genes; its genome is one of the smallest known to plants. Among extant independently replicating organisms, the bacterium Mycoplasma genitalium has the fewest number of genes, just 517.
A brief treatment of genes follows. For full treatment, see heredity.
Genes are composed of deoxyribonucleic acid (DNA), except in some viruses, which have genes consisting of a closely related compound called ribonucleic acid (RNA). A DNA molecule is composed of two chains of nucleotides that wind about each other to resemble a twisted ladder. The sides of the ladder are made up of sugars and phosphates, and the run...
The sequence of bases along a strand of DNA determines the genetic code. When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene will split. Through the process of transcription, a strand of RNA with bases complementary to those of the gene is created from the free nucleotides in the cell. (RNA has the base uracil [U] instead of thymine, so A and U form base pairs during RNA synthesis.) This single chain of RNA, called messenger RNA (mRNA), then passes to the organelles called ribosomes, where the process of translation, or protein synthesis, takes place. During translation, a second type of RNA, transfer RNA (tRNA), matches up the nucleotides on mRNA with specific amino acids. Each set of three nucleotides codes for one amino acid. The series of amino acids built according to the sequence of nucleotides forms a polypeptide chain; all proteins are made from one or more linked polypeptide chains.
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Experiments conducted in the 1940s indicated one gene being responsible for the assembly of one enzyme, or one polypeptide chain. This is known as the one gene–one enzyme hypothesis. However, since this discovery, it has been realized that not all genes encode an enzyme and that some enzymes are made up of several short polypeptides encoded by two or more genes.
Experiments have shown that many of the genes within the cells of organisms are inactive much or even all of the time. Thus, at any time, in both eukaryotes and prokaryotes, it seems that a gene can be switched on or off. The regulation of genes between eukaryotes and prokaryotes differs in important ways.
The process by which genes are activated and deactivated in bacteria is well characterized. Bacteria have three types of genes: structural, operator, and regulator. Structural genes code for the synthesis of specific polypeptides. Operator genes contain the code necessary to begin the process of transcribing the DNA message of one or more structural genes into mRNA. Thus, structural genes are linked to an operator gene in a functional unit called an operon. Ultimately, the activity of the operon is controlled by a regulator gene, which produces a small protein molecule called a repressor. The repressor binds to the operator gene and prevents it from initiating the synthesis of the protein called for by the operon. The presence or absence of certain repressor molecules determines whether the operon is off or on. As mentioned, this model applies to bacteria.
Mutations occur when the number or order of bases in a gene is disrupted. Nucleotides can be deleted, doubled, rearranged, or replaced, each alteration having a particular effect. Mutation generally has little or no effect, but, when it does alter an organism, the change may be lethal or cause disease. A beneficial mutation will rise in frequency within a population until it becomes the norm.
For more information on the influence of genetic mutations in humans and other organisms, see human genetic disease and evolution.
- The Editors of Encyclopaedia Britannica
Jan 11, 2024 · Genes are made up of DNA, which contains instructions to produce molecules called proteins. These proteins are responsible for characteristics including eye color, blood type, and height.
- Valencia Higuera
- A gene is a basic unit of inheritance passed on from male and female parents to their children. Genes contain DNA, which is made up of sequences th...
- Genes are the building blocks of life. They contain information for making specific molecules and proteins that allow human cells to function and t...
- The Human Genome Project has determined that humans have an estimated 30,000 genes.
May 21, 2024 · A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins, which are needed for the body to function. However, many genes do not code for proteins, instead they help control other genes.
Oct 27, 2020 · A gene is an extremely specific sequence of nucleotide monomers that has the ability to completely or partially control the expression of one or more traits in every type of living organism. Genes are formed from deoxyribonucleic acid (DNA) and, in the case of some viruses, ribonucleic acid (RNA) polymers.
Aug 22, 2024 · What Are Genes Made Of? Genes are composed of DNA and encoded for a specific purpose. How genes are encoded will determine how you look or how your body functions. Every person has two copies of a gene, each inherited from a parent.
Feb 8, 2019 · To make organs and tissues, the cells use the instructions on their DNA to build tiny machines. They control reactions between chemicals in the cell that eventually produce organs and tissues. The tiny machines are proteins. When a cell reads a gene’s instructions, we call it gene expression.
