Genome



In biology, the genome of a polyploid organism most often refers to the
intact DNA sequence of one set of chromosomes--that is, to 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 invidual cells may 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.

The Human Genome Project was organized to map and to sequence the human
genome. Other genome projects include mouse, rice, the plant Arabidopsis,
the puffer fish, bacterial like E. coli, etc. On the heels of this genome
sequencing, many researchers are examining the protein products of the
identified gene sequences. These gene products as a group are sometimes
called a "proteome."

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 should also
make clear that genome like gene refers to no particular DNA sequence, but
to a 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 may
learn something about cheetahs and "cheetah-ness" from a single example of either.

Typical genome sizes

        Organism        Genome size (base pairs)
 Phage λ         5×104

 E. coli                4×106

 Yeast                  2×107

 C. elegans             8×107

 Drosophila melanogaster2×108

 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.

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