This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process (after the DNA is transcribed into RNA): only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality most genes are hundreds of times larger, and the relationships between Introns and exons can be highly complex. Genetics glossary A-F Adenine One of the four nucleotide bases in DNA or RNA; pairs with thymine in DNA or uracil in RNA. Allele One of multiple alternative forms of a single gene, each of which is a viable DNA sequence occupying a given position, or locus on a chromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes. Base pair A pair of nucleotide bases on complementary DNA or RNA strands organized in a double helix. Chromosome A molecular "package" for carrying DNA in cells, organized as two double-helical DNA molecules that encode many genes. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA. Cytosine One of the four nucleotide bases in DNA or RNA; pairs with guanine. DNA A polymeric molecule made of deoxyribonucleotides, hence then name deoxyribonucleic acid. Most often has the form of a "double helix", which consists of two paired DNA molecules and resembles a ladder that has been twisted. The "rungs" of the ladder are made of base pairs, or nucleotides with complementary hydrogen bonding patterns. G-Z Gene The unit of heredity in living organisms, typically encoded in a sequence of nucleotide monomers that make up a long strand of DNA, or deoxyribonucleic acid. A particular gene can have multiple different forms, or alleles, which are defined by different sequences of DNA. Gene expression The process in which the infomation encoded in a gene is converted into a form useful for the cell. The first step is transcription, which produces a messenger RNA molecule complementary to the DNA molecule on which a gene is encoded. For protein-coding genes, the second step is translation, in which the messenger RNA is read by the ribosome to produce a protein. Gene pool The sum of all the alleles shared by members of a single population. Genetics The field of biology that studies genes and their inheritance. Genome The total complement of genetic material contained in an organism or cell. Genotype The complement of alleles present in a particular individual's genome that give rise to the individual's phenotype. Guanine One of the four nucleotide bases in DNA or RNA; pairs with cytosine. Locus A location on a chromosome where a particular gene resides. Phenotype The observable physical or behavioral traits of an organism, largely determined by the organism's genotype. Protein A linear polymeric molecule made of of amino acids linked by peptide bonds. Proteins carry out the majority of chemical reactions that occur inside the cell. RNA A polymeric molecule made of ribonucleotides, hence the name ribonucleic acid, similar to but less stable than DNA. One type, messenger RNA, plays an important role in gene expression. Ribosomes are also made largely of RNA. Thymine One of the four nucleotide bases in DNA; pairs with adenine. In RNA, thymine is replaced with uracil. Transcription The first step in gene expression, in which a messenger RNA molecule complementary to particular gene encoded in DNA is synthesized by enzymes called RNA polymerases. To produce a functional protein, transcription is followed by translation. Translation The second step in gene expression, in which a messenger RNA molecule is read by the ribosome to produce a functional protein. Translation is always preceded by transcription. Uracil One of the four nucleotide bases in RNA; pairs with adenine. In DNA, uracil is replaced with thymine. A gene is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions.[1][2] The physical development and phenotype of organisms can be thought of as a product of genes interacting with each other and with the environment[3], and genes can be considered as units of inheritance. A concise definition of gene taking into account complex patterns of regulation and transcription, genic conservation and non-coding RNA genes, has been proposed by Gerstein et al.[4] "A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products". In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and a coding sequence, which determines what the gene produces. When a gene is active, the coding sequence is copied in a process called transcription, producing an RNA copy of the gene's information. This RNA can then direct the synthesis of proteins via the genetic code. However, RNAs can also be used directly, for example as part of the ribosome. These molecules resulting from gene expression, whether RNA or protein, are known as gene products. Most genes contain non-coding regions that do not code for the gene products, but regulate gene expression. The genes of eukaryotic organisms can contain non-coding regions called introns that are removed from the messenger RNA in a process known as splicing. The regions that actually encode the gene product, which can be much smaller than the introns, are known as exons. One single gene can lead to the synthesis of multiple proteins through the different arrangements of exons produced by alternative splicings. The total complement of genes in an organism or cell is known as its genome. The genome size of an organism is generally lower in prokaryotes such as bacteria and archaea have generally smaller genomes, both in number of base pairs and number of genes, than even single-celled eukaryotes, although there is no clear relationship between genome sizes and perceived complexity of eukaryotic organisms. One of the largest known genomes belongs to the single-celled amoeba Amoeba dubia, with over 670 billion base pairs, some 200 times larger than the human genome.[5] The estimated number of genes in the human genome has been repeatedly revised downward since the completion of the Human Genome Project; current estimates place the human genome at just under 3 billion base pairs and about 20,000?25,000 genes.[6]. A recent Science article gives a final number of 20,488, with perhaps 100 more yet to be discovered .[7] The gene density of a genome is a measure of the number of genes per million base pairs (called a megabase, Mb); prokaryotic genomes have much higher gene densities than eukaryotes. The gene density of the human genome is roughly 12?15 genes/Mb.[8]