300px, Genome sizes and corresponding composition of six major model organisms as pie charts. The increase in genome size correlates with the vast expansion of noncoding (i.e., intronic, intergenic, and interspersed repeat sequences) and repeat DNA (e.g., satellite, LINEs, short interspersed nuclear element (SINEs), DNA (
Alu sequence), in red) sequences in more complex multicellular organisms. This expansion is accompanied by an increase in the number of epigenetic mechanisms (particularly repressive) that regulate the genome. Expansion of the genome also correlates with an increase in size and complexity of transcription units, except for plants. P =
Promoter DNA element.
The hereditary material i.e.
DNA (deoxyribonucleic acid) of an organism is composed of a sequence of four
nucleotides in a specific pattern, which encodes information as a function of their order. Genomic organization refers to the linear order of DNA elements and their division into
chromosomes. "Genome organization" can also refer to the 3D
structure of chromosomes and the positioning of DNA sequences within the nucleus.
Description
Organisms have a vast array of ways in which their respective
genomes are organized. A comparison of the genomic organization of six major model organisms shows size expansion with the increase of
complexity
Complexity characterises the behaviour of a system or model whose components interaction, interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence.
The term is generall ...
of the organism. There is a more than the 300-fold difference between the genome sizes of
yeast and
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, but only a modest 4- to 5-fold increase in overall
gene number (see the figure on the right). However, the ratio of coding to noncoding and repetitive sequences is indicative of the complexity of the genome: The largely "open" genomes of
unicellular fungi have relatively little noncoding
DNA compared with the highly
heterochromatic genomes of
multicellular organisms.
In particular, mammals have accumulated considerable repetitive elements and noncoding regions, which account for the majority of their DNA sequences (52% non-coding and 44%
repetitive DNA). Only 1.2% of the mammalian genome thus encodes for
protein function. This massive expansion of repetitive and noncoding sequences in multicellular organisms is most likely due to the incorporation of invasive elements, such as DNA
transposons,
retrotransposons, and other repetitive elements. The expansion of repetitive elements (such as
Alu sequences) has even infiltrated the
transcriptional
Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules calle ...
units of the mammalian genome. This results in transcription units that are frequently much larger (30–200 kb), commonly containing multiple promoters and DNA repeats within untranslated
intron
An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...
s.
The vast expansion of the genome with noncoding and repetitive DNA in higher
eukaryote
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
s implies more extensive
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
silencing mechanisms. Studies of the genomic organization are thought to be the future of genomic
medicine, which will provide the opportunity for personalized
prognoses in clinics.
[West M., et al., Embracing the complexity of genomic data for personalized medicine Genome Res. (2006)16:559-66]
See also
*
Genome comparison
Comparative genomics is a field of biological research in which the genomic features of different organisms are compared. The genomic features may include the DNA sequence, genes, gene order, regulatory sequences, and other genomic structural ...
*
Genome project
*
List of sequenced eukaryotic genomes
*
Molecular evolution
References
{{Medicine
Genomics
