Genome

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For other uses, see Genome (disambiguation).

In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). This includes both the genes and the non-coding sequences. The term was coined in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany, as a portmanteau of the words gene and chromosome.

More precisely, the genome of an organism is a complete DNA sequence of one set of chromosomes; for example, one of the two sets that a diploid individual carries in every somatic cell. The term genome can be applied specifically to mean the complete set of nuclear DNA (i.e., the nuclear genome) but can also be applied to organelles that contain their own DNA, as with the mitochondrial genome or the chloroplast genome. When people say that the genome of a sexually reproducing species has been "sequenced," typically they are referring to a determination of the sequences of one set of autosomes and one of each type of sex chromosome, which together represent both of the possible sexes. Even in species that exist in only one sex, what is described as "a genome sequence" may be a composite from the chromosomes of various individuals. In general use, the phrase genetic makeup is sometimes used conversationally to mean the genome of a particular individual or organism. The study of the global properties of genomes of related organisms is usually referred to as genomics, which distinguishes it from genetics which generally studies the properties of single genes or groups of genes.

1 Types of genomes
2 Genomes and genetic variation
3 Minimal genomes
4 Genome projects
5 Comparison of different genome sizes
6 Genome evolution
7 Other Omics & Ome pages
8 Subfields of Genome
9 References
10 See also
11 External links

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Types of genomes

Most biological entities more complex than a virus sometimes or always carry additional genetic material besides that which resides in their chromosomes. In some contexts, such as sequencing the genome of a pathogenic microbe, "genome" is meant to include this auxiliary material, which is carried in plasmids. In such circumstances then, "genome" describes all of the genes and non-coding DNA that have the potential to be present.

In vertebrates such as humans, however, "genome" carries the typical connotation of only chromosomal DNA. So although human mitochondria contain genes, these genes are not considered part of the genome. In fact, mitochondria are sometimes said to have their own genome, often referred to as the "mitochondrial genome".

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Genomes and genetic variation

Note that a genome does not capture the genetic diversity or the genetic polymorphism of a species. For example, the human genome sequence in principle could be determined from just half the DNA of one cell from one individual. To learn what variations in DNA underlie particular traits or diseases requires comparisons across individuals. This point explains the common usage of "genome" (which parallels a common usage of "gene") to refer not to any particular DNA sequence, but to a whole family of sequences that share a biological context.

Although this concept may seem counter intuitive, it is the same concept that says there is no particular shape that is the shape of a cheetah. Cheetahs vary, and so do the sequences of their genomes. Yet both the individual animals and their sequences share commonalities, so one can learn something about cheetahs and "cheetah-ness" from a single example of either.

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Minimal genomes

Since genomes and their organisms are very complex, one research strategy is to reduce the number of genes in a genome to the bare minimum and still have the organism in question survive. There is experimental work being done on minimal genomes for single cell organisms as well as minimal genomes for multicellular organisms (see Developmental biology). The work is both in vivo and in silico. By understanding the functioning of minimal organisms one hopes to add complexity incrementally leading to the understanding of multicellular diseases such as Cancer.(see #References)

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Genome projects

Main article: Genome project

The Human Genome Project was organized to map and to sequence the human genome. Other genome projects include mouse, rice, the plant Arabidopsis thaliana, the puffer fish, bacteria like E. coli, etc. Many genomes have been sequenced by various genome projects. The cost of sequencing continues to drop, and it is possible that eventually an individual's genome could be sequenced for around several thousand dollars (US).

Compare: proteome

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Comparison of different genome sizes

For an updated list of sequenced genomes and their sizes, visit the Genomes OnLine Database (GOLD)

Organism	Genome size (base pairs)
Virus, Phage Φ-X174;	5386 - First sequenced genome
Virus, Phage λ	5×10⁴
Archaeum, Nanoarchaeum equitans	5×10⁵ - Smallest non-viral genome Dec, 2005
Bacterium, Buchnera aphidicola	6×10⁵
Bacterium, Wigglesworthia glossinidia	7×10⁵
Bacterium, Escherichia coli	4×10⁶
Amoeba, Amoeba dubia	67×10¹⁰ - Largest known genome Dec, 2005
Plant, Fritillary assyrica	13×10¹⁰
Fungus,Saccharomyces cerevisiae	2×10⁷
Nematode, Caenorhabditis elegans	8×10⁷
Insect, Drosophila melanogaster	2×10⁸
Mammal, Homo sapiens	3×10⁹

Note: The DNA from a single human cell has a length of ~1.8m.

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Genome evolution

Genomes are more than the sum of an organism's genes and have traits that may be measured and studied without reference to the details of any particular genes and their products. Researchers compare traits such as chromosome number (karyotype), genome size, gene order, codon usage bias, and GC-content to determine what mechanisms could have produced the great variety of genomes that exist today (for recent overviews, see Brown 2002; Saccone and Pesole 2003; Benfey and Protopapas 2004; Gibson and Muse 2004; Reese 2004; Gregory 2005).

Duplications play a major role in shaping the genome. Duplications may range from extension of short tandem repeats, to duplication of a cluster of genes, and all the way to duplications of entire chromosomes or even entire genomes. Such duplications are probably fundamental to the creation of genetic novelty.

Horizontal gene transfer is invoked to explain how there is often extreme similarity between small portions of the genomes of two organisms that are otherwise very distantly related. Horizontal gene transfer seems to be common among many microbes. Also, eukaryotic cells seem to have experienced a transfer of some genetic material from their chloroplast and mitochondrial genomes to their nuclear chromosomes.

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