The following is my provisional definition of a gene. I’m sure it needs a great deal of work! I am particularly unsure of some of the generalities (or lack there of) across the full diversity of life. (I tend to gravitate in my thinking towards multicelled organisms, then usually the furry or feathered kind).

This is part of an interesting process whereyby various blogger of science (BOS) are trying to generate brief, “basic” informational units for people to refer to. Go here to see PZ Myer’s definition, and a lively and informative discussion. Larry Moran is planning on putting one here in the near future, but for now you may find this discussion of the Central Dogma relevant to the question “what is a gene?”

You can also find a definition by putting “gene definition” in Google. But you won’t get anytyig as good as you will be getting from this growing bloggus nexus.

The Definition:

A gene is a unit of information, in nature stored as subunits of a DNA or RNA molecule. Most of the information specifies the order in which amino acids are attached to synthesize a protein (“protein synthesis” or “gene expression”). Some of the information regulates the process of protein synthesis. The encoded information to synthesize a protein is processed through two steps: Transcription, in which a messenger RNA molecule is formed, and Translation in which the messenger RNA molecule specifies the order of assembly of the amino acids.

In nature, a the sequence of DNA or RNA in which the gene is stored is replicated and passed on from parent to daughter cells during mitosis, to gametes (sex cells) during meiosis, or to zygotes during fusion of gametes.

Additional information is encoded in DNA that is sometimes referred to as “genes” and sometimes not. This includes information that serves as a template for various RNA molecules that are in turn used in DNA transcription and translation.

The information in the DNA molecule is usually located, within a given species, at a particular location or set of locations, along a DNA molecule much longer than any one gene. From time to time a particular gene may move from one location to another, duplicate, or become deactivated (and thus no longer serves as information). In eukaryotes, the DNA sequence that corresponds to the gene is usually discontinuous, with incomplete segments of gene-specifying information (exons) separated by other DNA (introns). The process of transcription involves creating a messenger RNA molecule that represents the information encoded in exons in the proper order, excluding information encoded in introns.

For every aspect of this definition there are exceptions. Sometimes genes pass from one individual to another via means other than replication in mitosis or meiosis. There is not always a one to one correspondence between a particular “gene” in the DNA and a particular protein product … sequences are sometimes rearranged in different ways depending on the context in which transcription and translation occurs. Although introns are very common in eukaryotes, there are some with very few (possibly none). A given segment of DNA may have more than one gene on it, in a complex and overlapping pattern. The information in a given DNA sequence may be changed during the life of an organism by the actions of a virus, or by permanent adaptive rearrangement of the DNA sequence in a specialized cell.

In nature, DNA molecules are typically stored in units called chromosomes with hundreds or thousands of genes. Eukaryotic and prokaryotic chromosomes, and mitochondrial and other plastid chromosomes do not all have the same structure. The specific molecules used to make up the informational sequence are called base pairs, which are organized in sets of three (a “codon”) and their relationship to amino acid sequences and other expressed products is important and somewhat complex. The pattern of chromosomal storage can be complex with a given “individual” having a haploid complement of genes (one copy of each gene), a diploid complement (two copies of each gene) or any of several other patterns. The number of sets of genes in a cell is called “ploidy.” Most multicelled organisms undergo generational alteration of ploidy between lower and higher amounts (i.e., alternating haploid and diploid, with the former being sex cells, the latter being multicelled diploid forms).

With the exception of the RNA template sequences, genes code for proteins. More than one protein may be associated with a particular gene depending on the context of “gene expression” (protein synthesis), and many functional proteins require the information from more than one gene. Expression always happens in cells. The diversity of cell types is closely linked to the pattern of gene expression in each cell. Most cells contain the entire genome (total compliment of genetic information) for a given individual, but only some genes are expressed in each cell type, and only under certain circumstances. That there is a complex pattern of expression is the reason for cells becoming different during development in multicelled organisms, and for cells being of different types and doing different things in those organisms.

The relationship between genetic information an the various – and varying – traits observed in whole organisms is only vaguely understood at this time.