|Part of a series on|
|History and topics|
- Origin of term 1
- Overview 2
- Sequencing and mapping 3
Genome compositions 4
- Genome size 4.1
- Proportion of non-repetitive DNA 4.2
Proportion of repetitive DNA 4.3
- Tandem repeats 4.3.1
Interspersed repeats 4.3.2
- Retrotransposons 22.214.171.124
- DNA transposons 126.96.36.199
- Genome evolution 5
- See also 6
- References 7
- Further reading 8
- External links 9
Origin of term
The term was created in 1920 by Hans Winkler, professor of botany at the University of Hamburg, Germany. The Oxford English Dictionary suggests the name to be a blend of the words gene and chromosome. A few related -ome words already existed—such as biome, rhizome and, more recently, connectome—forming a vocabulary into which genome fits systematically.
Some organisms have multiple copies of mitochondrial genome" or the "chloroplast genome". Additionally, the genome can comprise non-chromosomal genetic elements such as viruses, plasmids, and transposable elements.
Both the number of base pairs and the number of genes vary widely from one species to another, and there is only a rough correlation between the two (an observation known as the C-value paradox). At present, the highest known number of genes is around 60,000, for the protozoan causing trichomoniasis (see List of sequenced eukaryotic genomes), almost three times as many as in the human genome.
An analogy to the human genome stored on DNA is that of instructions stored in a book:
- The book (genome) would contain 23 chapters (chromosomes);
- Each chapter contains 48 to 250 million letters (A,C,G,T) without spaces;
- Hence, the book contains over 3.2 billion letters total;
- The book fits into a cell nucleus the size of a pinpoint;
- At least one copy of the book (all 23 chapters) is contained in most cells of our body. The only exception in humans is found in mature red blood cells which become enucleated during development and therefore lack a genome.
Sequencing and mapping
In 1976, Walter Fiers at the University of Ghent (Belgium) was the first to establish the complete nucleotide sequence of a viral RNA-genome (Bacteriophage MS2). The next year, Phage Φ-X174, with only 5386 base pairs, became the first DNA-genome project to be completed, by Fred Sanger. The first complete genome sequences for representatives from all 3 domains of life were released within a short period during the mid-1990s. The first bacterial genome to be sequenced was that of Haemophilus influenzae, completed by a team at The Institute for Genomic Research in 1995. A few months later, the first eukaryotic genome was completed, with the 16 chromosomes of budding yeast Saccharomyces cerevisiae being released as the result of a European-led effort begun in the mid-1980s. Shortly afterward, in 1996, the first genome sequence for an archaeon, Methanococcus jannaschii, was completed, again by The Institute for Genomic Research.
The development of new technologies has made it dramatically easier and cheaper to do sequencing, and the number of complete genome sequences is growing rapidly. The US National Institutes of Health maintains one of several comprehensive databases of genomic information. Among the thousands of completed genome sequencing projects include those for mouse, rice, the plant Arabidopsis thaliana, the puffer fish, and bacteria like E. coli. In December 2013, scientists reported, for the first time, the entire genome of a Neanderthal, an extinct species of humans. The genome was extracted from the toe bone of a 130,000-year-old Neanderthal found in a Siberian cave.
New sequencing technologies, such as massive parallel sequencing have also opened up the prospect of personal genome sequencing as a diagnostic tool, as pioneered by Manteia Predictive Medicine. A major step toward that goal was the completion in 2007 of the full genome of James D. Watson, one of the co-discoverers of the structure of DNA.
Whereas a genome sequence lists the order of every DNA base in a genome, a genome map identifies the landmarks. A genome map is less detailed than a genome sequence and aids in navigating around the genome. The map and to sequence the human genome. A fundamental step in the project was the release of a detailed genomic map by Jean Weissenbach and his team at the Genoscope in Paris.
Genome composition is used to describe the make up of contents of a haploid genome, which should include genome size, proportions of non-repetitive DNA and repetitive DNA in details. By comparing the genome compositions between genomes, scientists can better understand the evolutionary history of a given genome.
When talking about genome composition, one should distinguish between
- UCSC Genome Browser - view the genome and annotations for more than 80 organisms.
- Build a DNA Molecule
- Some comparative genome sizes
- DNA Interactive: The History of DNA Science
- DNA From The Beginning
- All About The Human Genome Project—from Genome.gov
- Animal genome size database
- Plant genome size database
- GOLD:Genomes OnLine Database
- The Genome News Network
- NCBI Entrez Genome Project database
- NCBI Genome Primer
- GeneCards—an integrated database of human genes
- BBC News - Final genome 'chapter' published
- IMG (The Integrated Microbial Genomes system)—for genome analysis by the DOE-JGI
- GeKnome Technologies Next-Gen Sequencing Data Analysis—next-generation sequencing data analysis for Illumina and 454 Service from GeKnome Technologies.
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- Saccone, Cecilia; Pesole, Graziano (2003). Handbook of Comparative Genomics. Chichester: John Wiley & Sons.
- Werner, E. (2003). "In silico multicellular systems biology and minimal genomes". Drug Discov Today 8 (24): 1121–1127.
- Ridley, M. (2006). Genome. New York, NY: Harper Perennial. ISBN 0-06-019497-9
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- ScienceShot: Biggest Genome Ever, comments: "The measurement for Amoeba dubia and other protozoa which have been reported to have very large genomes were made in the 1960s using a rough biochemical approach which is now considered to be an unreliable method for accurate genome size determinations."
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- Molecular epidemiology
- Molecular pathological epidemiology
- Molecular pathology
- Precision medicine
- Whole genome sequencing
Duplications play a major role in shaping the genome. Duplication may range from extension of short tandem repeats, to duplication of a cluster of genes, and all the way to duplication of entire chromosomes or even entire genomes. Such duplications are probably fundamental to the creation of genetic novelty.
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).
DNA transposons generally move by "cut and paste" in the genome, but duplication has also been observed. Class 2 TEs do not use RNA as intermediate and are popular in bacteria, in metazoan it has also been found.
- Short interspersed elements (SINEs)
- are usually less than 500 base pairs and need to co-opt with the LINEs machinery to function as nonautonomous retrotransposons. The Alu element is the most common SINEs found in primates, it has a length of about 350 base pairs and takes about 11% of the human genome with around 1,500,000 copies.
- Long interspersed elements (LINEs)
- are able to encode two Open Reading Frames (ORFs) to generate transcriptase and endonuclease, which are essential in retrotransposition. The human genome has around 500,000 LINEs, taking around 17% of the genome.
- Non-Long Terminal Repeats (Non-LTRs)
- can be divided into long interspersed elements (LINEs), short interspersed elements (SINEs) and Penelope-like elements. In Dictyostelium discoideum, there is another DIRS-like elements belong to Non-LTRs. Non-LTRs are widely spread in eukaryotic genomes.
- Long Terminal Repeats (LTRs)
- similar to retroviruses, which have both gag and pol genes to make cDNA from RNA and proteins to insert into genome, but LTRs can only act within the cell as they lack the env gene in retroviruses. It has been reported that LTRs consist of the largest fraction in most plant genome and might account for the huge variation in genome size.
Retrotransposons can be transcribed into RNA, which are then duplicated at another site into the genome. Retrotransposons can be divided into Long terminal repeats (LTRs) and Non-Long Terminal Repeats (Non-LTR).
Interspersed repeats mainly come from transposable elements (TEs), but they also include some protein coding gene families and pseudogenes. Transposable elements are able to integrate into the genome at another site within the cell. It is believed that TEs are an important driving force on genome evolution of higher eukaryotes. TEs can be classified into two categories, Class 1 (retrotransposons) and Class 2 (DNA transposons).
Tandem repeats are usually caused by slippage during replication, unequal crossing-over and gene conversion, satellite DNA and microsatellites are forms of tandem repeats in the genome. Although tandem repeats count for a significant proportion in genome, the largest proportion in mammalian is the other type, interspersed repeats.
The proportion of repetitive DNA is calculated by using length of repetitive DNA divide by genome size. There are two categories of repetitive DNA in genome: tandem repeats and interspersed repeats.
Proportion of repetitive DNA
It had been found that the proportion of non-repetitive DNA can vary a lot between species. Some E. coli as prokaryotes only have non-repetitive DNA, lower eukaryotes such as C. elegans and fruit fly, still possess more non-repetitive DNA than repetitive DNA. Higher eukaryotes tend to have more repetitive DNA than non-repetitive one. In some plants and amphibians, the proportion of non-repetitive DNA is no more than 20%, becoming a minority component.
The proportion of non-repetitive DNA is calculated by using length of non-repetitive DNA divided by genome size. Protein-coding genes and RNA-coding genes are generally non-repetitive DNA. Bigger genome does not mean more genes, and the proportion of non-repetitive DNA decreases along with the increase of genome size in higher eukaryotes.
Proportion of non-repetitive DNA
|Virus||Porcine circovirus type 1||1,759||1.8kb||Smallest viruses replicating autonomously in eukaryotic cells.|
|Virus||Bacteriophage MS2||3,569||3.5kb||First sequenced RNA-genome|
|Virus||Phage Φ-X174||5,386||5.4kb||First sequenced DNA-genome|
Often used as a vector for the cloning of recombinant DNA.
  
|Virus||Megavirus||1,259,197||1.3Mb||Until 2013 the largest known viral genome.|
|Virus||Pandoravirus salinus||2,470,000||2.47Mb||Largest known viral genome.|
|Bacterium||Haemophilus influenzae||1,830,000||1.8Mb||First genome of a living organism sequenced, July 1995|
|Bacterium||Nasuia deltocephalinicola (strain NAS-ALF)||112,091||112kb||Smallest non-viral genome.|
|Bacterium||Solibacter usitatus (strain Ellin 6076)||9,970,000||10Mb|||
|Amoeboid||Polychaos dubium ("Amoeba" dubia)||670,000,000,000||670Gb||Largest known genome. (Disputed)|
|Plant||Arabidopsis thaliana||157,000,000||157Mb||First plant genome sequenced, December 2000.|
|Plant||Genlisea margaretae||63,400,000||63Mb||Smallest recorded flowering plant genome, 2006.|
|Plant||Populus trichocarpa||480,000,000||480Mb||First tree genome sequenced, September 2006|
|Plant||Paris japonica (Japanese-native, pale-petal)||150,000,000,000||150Gb||Largest plant genome known|
|Moss||Physcomitrella patens||480,000,000||480Mb||First genome of a bryophyte sequenced, January 2008.|
|Yeast||Saccharomyces cerevisiae||12,100,000||12.1Mb||First eukaryotic genome sequenced, 1996|
|Nematode||Caenorhabditis elegans||100,300,000||100Mb||First multicellular animal genome sequenced, December 1998|
|Nematode||Pratylenchus coffeae||20,000,000||20Mb||Smallest animal genome known|
|Insect||Drosophila melanogaster (fruit fly)||130,000,000||130Mb|||
|Insect||Bombyx mori (silk moth)||432,000,000||432Mb||
14,623 predicted genes
|Insect||Apis mellifera (honey bee)||236,000,000||236Mb|
|Insect||Solenopsis invicta (fire ant)||480,000,000||480Mb|||
|Fish||Tetraodon nigroviridis (type of puffer fish)||385,000,000||390Mb||Smallest vertebrate genome known estimated to be 340 Mb - 385 Mb.|
Homo sapiens estimated genome size 3.2 billion bp
Initial sequencing and analysis of the human genome
|Fish||Protopterus aethiopicus (marbled lungfish)||130,000,000,000||130Gb||Largest vertebrate genome known|
Since genomes 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 multi-cellular organisms (see Developmental biology). The work is both in vivo and in silico.
Genome size is the total number of DNA base pairs in one copy of a haploid genome. The genome size is positively correlated with the morphological complexity among prokaryotes and lower eukaryotes; however, after mollusks and all the other higher eukaryotes above, this correlation is no longer effective. This phenomenon also indicates the mighty influence coming from repetitive DNA act on the genomes.
Most biological entities that are 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 information stored on this auxiliary material, which is carried in plasmids. In such circumstances then, "genome" describes all of the genes and information on non-coding DNA that have the potential to be present.