• Genes
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The physical development and phenotype of organisms can be thought of as a product of genes interacting with each other and with the environment, 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.

"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.

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.

A recent Science article gives a final number of 20,488, with perhaps 100 more yet to be discovered .

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.