Genome
In biology, the genome of a polyploid organism most often refers to the intact DNA of one set of chromosomes--that is, the genes and "non-coding" DNA arranged as they appear on the chromosomes of a typical individual. In some contexts, such as sequencing the genome of a pathogenic microbe, "genome" is meant to include not only chromosomal DNA but the optional genetic material some inviduals carry as plasmids. In such circumstances then, "genome" describes all of the genes and non-coding DNA that have the potential to be present in a cell. Applied to humans, "genome" refers only to chromosomal DNA, however. So even though human mitochondria contain genes, for example, these genes are not considered part of the genome. (In fact, mitochondria are sometimes said to have their own genome, as in the term "mitochondrial genome").
Most organisms more complex than a virus carry some genes outside their chromosomes; for example, bacteria carry plasmids, and eukaryotes have genes in their mitochondria (and chloroplasts, in plants).
The Human Genome Project aims to map and sequence the human genome. Other projects are sequencing the genomes of rice, Arabidopsis, etc.
Note that a genome does not capture the genetic diversity or polymorphism of a species. In principle, the sequence of the human genome 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.
Typical genome sizes
| Organism | Genome size (base pairs) |
|---|---|
| Phage λ | 5×104 |
| E. coli | 4×106 |
| Yeast | 2×107 |
| C. elegans | 8×107 |
| Drosophila melanogaster | 2×108 |
| Human | 3×109 |
Note : The DNA from a single human cell has a length of ~1.8m.
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, chromosome size, gene order, codon usage bias, and G-C content to determine what mechanisms could have produced the great variety of genomes that exist today.
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, 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.