In the fields of molecular biology and genetics, a genome is the genetic material of an organism. It consists of DNA (or RNA in RNA viruses). The genome includes both the genes (the coding regions) and the noncoding DNA,[1] as well as mitochondrial DNA[2] and chloroplast DNA. The study of the genome is called genomics.

UCSC human chromosome colours
An image of the 46 chromosomes making up the diploid genome of a human male. (The mitochondrial chromosome is not shown.)

Origin of term

The term genome was created in 1920 by Hans Winkler,[3] professor of botany at the University of Hamburg, Germany. The Oxford Dictionary suggests the name is a blend of the words gene and chromosome.[4] However, see omics for a more thorough discussion. A few related -ome words already existed, such as biome and rhizome, forming a vocabulary into which genome fits systematically.[5]

Sequencing and mapping

A genome sequence is the complete list of the nucleotides (A, C, G, and T for DNA genomes) that make up all the chromosomes of an individual or a species. Within a species, the vast majority of nucleotides are identical between individuals, but sequencing multiple individuals is necessary to understand the genetic diversity.

Part of DNA sequence prototypification of complete genome of virus 5418 nucleotides
Part of DNA sequence - prototypification of complete genome of virus

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, Fred Sanger completed the first DNA-genome sequence: Phage Φ-X174, of 5386 base pairs.[6] The first complete genome sequences among all three 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 sequences of the 16 chromosomes of budding yeast Saccharomyces cerevisiae published as the result of a European-led effort begun in the mid-1980s. The first genome sequence for an archaeon, Methanococcus jannaschii, was completed in 1996, again by The Institute for Genomic Research.

The development of new technologies has made genome sequencing dramatically cheaper and easier, 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.[7] Among the thousands of completed genome sequencing projects include those for rice, a mouse, the plant Arabidopsis thaliana, the puffer fish, and the bacteria E. coli. In December 2013, scientists first sequenced 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.[8][9]

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.[10]

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 Human Genome Project was organized to 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.[11][12]

Reference genome sequences and maps continue to be updated, removing errors and clarifying regions of high allelic complexity.[13] The decreasing cost of genomic mapping has permitted genealogical sites to offer it as a service,[14] to the extent that one may submit one's genome to crowdsourced scientific endeavours such as DNA.LAND at the New York Genome Center,[15] an example both of the economies of scale and of citizen science.[16]

Viral genomes

Viral genomes can be composed of either RNA or DNA. The genomes of RNA viruses can be either single-stranded or double-stranded RNA, and may contain one or more separate RNA molecules. DNA viruses can have either single-stranded or double-stranded genomes. Most DNA virus genomes are composed of a single, linear molecule of DNA, but some are made up of a circular DNA molecule.[17]

Prokaryotic genomes

Prokaryotes and eukaryotes have DNA genomes. Archaea have a single circular chromosome.[18] Most bacteria also have a single circular chromosome; however, some bacterial species have linear chromosomes[19] or multiple chromosomes.[20] If the DNA is replicated faster than the bacterial cells divide, multiple copies of the chromosome can be present in a single cell, and if the cells divide faster than the DNA can be replicated, multiple replication of the chromosome is initiated before the division occurs, allowing daughter cells to inherit complete genomes and already partially replicated chromosomes. Most prokaryotes have very little repetitive DNA in their genomes.[21] However, some symbiotic bacteria (e.g. Serratia symbiotica) have reduced genomes and a high fraction of pseudogenes: only ~40% of their DNA encodes proteins.[22][23]

Some bacteria have auxiliary genetic material, also part of their genome, which is carried in plasmids. For this, the word genome should not be used as a synonym of chromosome.

Eukaryotic genomes

Eukaryotic genomes are composed of one or more linear DNA chromosomes. The number of chromosomes varies widely from Jack jumper ants and an asexual nemotode,[24] which each have only one pair, to a fern species that has 720 pairs.[25] A typical human cell has two copies of each of 22 autosomes, one inherited from each parent, plus two sex chromosomes, making it diploid. Gametes, such as ova, sperm, spores, and pollen, are haploid, meaning they carry only one copy of each chromosome.

In addition to the chromosomes in the nucleus, organelles such as the chloroplasts and mitochondria have their own DNA. Mitochondria are sometimes said to have their own genome often referred to as the "mitochondrial genome". The DNA found within the chloroplast may be referred to as the "plastome". Like the bacteria they originated from, mitochondria and chloroplasts have a circular chromosome.

Unlike prokaryotes, eukaryotes have exon-intron organization of protein coding genes and variable amounts of repetitive DNA. In mammals and plants, the majority of the genome is composed of repetitive DNA.[26]

Coding sequences

DNA sequences that carry the instructions to make proteins are coding sequences. The proportion of the genome occupied by coding sequences varies widely. A larger genome does not necessarily contain more genes, and the proportion of non-repetitive DNA decreases along with increasing genome size in complex eukaryotes.[26]

Simple eukaryotes such as C. elegans and fruit fly, have more non-repetitive DNA than repetitive DNA,[26][27] while the genomes of more complex eukaryotes tend to be composed largely of repetitive DNA. In some plants and amphibians, the proportion of repetitive DNA is more than 80%.[26] Similarly, only 2% of the human genome codes for proteins.

Components of the human genome
Composition of the human genome

Noncoding sequences

Noncoding sequences include introns, sequences for non-coding RNAs, regulatory regions, and repetitive DNA. Noncoding sequences make up 98% of the human genome. There are two categories of repetitive DNA in the genome: tandem repeats and interspersed repeats.[28]

Tandem repeats

Short, non-coding sequences that are repeated head-to-tail are called tandem repeats. Microsatellites consisting of 2-5 basepair repeats, while minisatellite repeats are 30-35 bp. Tandem repeats make up about 4% of the human genome and 9% of the fruit fly genome.[29] Tandom repeats can be functional. For example, telomeres are composed of the tandem repeat TTAGGG in mammals, and they play an important role in protecting the ends of the chromosome.

In other cases, expansions in the number of tandem repeats in exons or introns can cause disease.[30] For example, the human gene huntingtin typically contains 6–29 tandem repeats of the nucleotides CAG (encoding a polyglutamine tract). An expansion to over 36 repeats results in Huntington's disease, a neurodegenerative disease. Twenty human disorders are known to result from similar tandem repeat expansions in various genes. The mechanism by which proteins with expanded polygulatamine tracts cause death of neurons is not fully understood. One possibility is that the proteins fail to fold properly and avoid degradation, instead accumulating in aggregates that also sequester important transcription factors, thereby altering gene expression.[30]

Tandem repeats are usually caused by slippage during replication, unequal crossing-over and gene conversion.[31]

Transposable elements

Transposable elements (TEs) are sequences of DNA with a defined structure that are able to change their location in the genome.[29][21][32] TEs are categorized as either class I TEs, which replicate by a copy-and-paste mechanism, or class II TEs, which can be excised from the genome and inserted at a new location.

The movement of TEs is a driving force of genome evolution in eukaryotes because their insertion can disrupt gene functions, homologous recombination between TEs can produce duplications, and TE can shuffle exons and regulatory sequences to new locations.[33]

Retrotransposons can be transcribed into RNA, which are then duplicated at another site into the genome.[34] Retrotransposons can be divided into Long terminal repeats (LTRs) and Non-Long Terminal Repeats (Non-LTR).[33]

Long terminal repeats (LTRs) are derived from ancient retroviral infections, so they encode proteins related to retroviral proteins including gag (structural proteins of the virus), pol (reverse transcriptase and integrase), pro (protease), and in some cases env (envelope) genes.[32] These genes are flanked by long repeats at both 5' and 3' ends. 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.[35]

Non-long terminal repeats (Non-LTRs) are classified as 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.[36]

Long interspersed elements (LINEs) encode genes for reverse transcriptase and endonuclease, making them autonomous transposable elements. The human genome has around 500,000 LINEs, taking around 17% of the genome.[37]

Short interspersed elements (SINEs) are usually less than 500 base pairs and are non-autonomous, so they rely on the proteins encoded by LINEs for transposition.[38] The Alu element is the most common SINE found in primates. It is about 350 base pairs and occupies about 11% of the human genome with around 1,500,000 copies.[33]

DNA transposons encode a transposase enzyme between inverted terminal repeats. When expressed, the transposase recognizes the terminal inverted repeats that flank the transposon and catalyzes its excision and reinsertion in a new site.[29] This cut-and-paste mechanism typically reinserts transposons near their original location (within 100kb).[33] DNA transposons are found in bacteria and make up 3% of the human genome and 12% of the genome of the roundworm C. elegans.[33]

Genome size

Genome size vs protein count
Log-log plot of the total number of annotated proteins in genomes submitted to GenBank as a function of genome size.

Genome size is the total number of DNA base pairs in one copy of a haploid genome. In humans, the nuclear genome comprises approximately 3.2 billion nucleotides of DNA, divided into 24 linear molecules, the shortest 50 000 000 nucleotides in length and the longest 260 000 000 nucleotides, each contained in a different chromosome.[39] 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.[26][40] This phenomenon also indicates the mighty influence coming from repetitive DNA on the genomes.

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.[41][42]

Here is a table of some significant or representative genomes. See #See also for lists of sequenced genomes.

Organism type Organism Genome size
(base pairs)
Approx. no. of genes Note
Virus Porcine circovirus type 1 1,759 1.8kb Smallest viruses replicating autonomously in eukaryotic cells.[43]
Virus Bacteriophage MS2 3,569 3.5kb First sequenced RNA-genome[44]
Virus SV40 5,224 5.2kb [45]
Virus Phage Φ-X174 5,386 5.4kb First sequenced DNA-genome[46]
Virus HIV 9,749 9.7kb [47]
Virus Phage λ 48,502 48.5kb Often used as a vector for the cloning of recombinant DNA.

[48] [49] [50]

Virus Megavirus 1,259,197 1.3Mb Until 2013 the largest known viral genome.[51]
Virus Pandoravirus salinus 2,470,000 2.47Mb Largest known viral genome.[52]
Bacterium Nasuia deltocephalinicola (strain NAS-ALF) 112,091 112kb Smallest non-viral genome.[53]
Bacterium Carsonella ruddii 159,662 160kb
Bacterium Buchnera aphidicola 600,000 600kb [54]
Bacterium Wigglesworthia glossinidia 700,000 700Kb
Bacterium Haemophilus influenzae 1,830,000 1.8Mb First genome of a living organism sequenced, July 1995[55]
Bacterium Escherichia coli 4,600,000 4.6Mb 4288 [56]
Bacterium Solibacter usitatus (strain Ellin 6076) 9,970,000 10Mb [57]
Bacteriumcyanobacterium Prochlorococcus spp. (1.7 Mb) 1,700,000 1.7Mb 1884 Smallest known cyanobacterium genome[58][59]
Bacterium – cyanobacterium Nostoc punctiforme 9,000,000 9Mb 7432 7432 open reading frames[60]
Amoeboid Polychaos dubium ("Amoeba" dubia) 670,000,000,000 670Gb Largest known genome.[61] (Disputed)[62]
Eukaryotic organelle Human mitochondrion 16,569 16.6kb [63]
Plant Genlisea tuberosa 61,000,000 61Mb Smallest recorded flowering plant genome, 2014.[64]
Plant Arabidopsis thaliana 135,000,000[65] 135 Mb 27,655[66] First plant genome sequenced, December 2000.[67]
Plant Populus trichocarpa 480,000,000 480Mb 73013 First tree genome sequenced, September 2006[68]
Plant Fritillaria assyriaca 130,000,000,000 130Gb
Plant Paris japonica (Japanese-native, pale-petal) 150,000,000,000 150Gb Largest plant genome known[69]
Plant – moss Physcomitrella patens 480,000,000 480Mb First genome of a bryophyte sequenced, January 2008.[70]
Fungusyeast Saccharomyces cerevisiae 12,100,000 12.1Mb 6294 First eukaryotic genome sequenced, 1996[71]
Fungus Aspergillus nidulans 30,000,000 30Mb 9541 [72]
Nematode Pratylenchus coffeae 20,000,000 20Mb [73] Smallest animal genome known[74]
Nematode Caenorhabditis elegans 100,300,000 100Mb 19000 First multicellular animal genome sequenced, December 1998[75]
Insect Drosophila melanogaster (fruit fly) 175,000,000 175Mb 13600 Size variation based on strain (175-180Mb; standard y w strain is 175Mb)[76]
Insect Apis mellifera (honey bee) 236,000,000 236Mb 10157 [77]
Insect Bombyx mori (silk moth) 432,000,000 432Mb 14623 14,623 predicted genes[78]
Insect Solenopsis invicta (fire ant) 480,000,000 480Mb 16569 [79]
Mammal Mus musculus 2,700,000,000 2.7Gb 20210 [80]
Mammal Homo sapiens 3,289,000,000 3.3Gb 20000 Homo sapiens estimated genome size 3.2 billion bp[81]

Initial sequencing and analysis of the human genome[82]

Mammal Pan paniscus 3,286,640,000 3.3Gb 20000 Bonobo - estimated genome size 3.29 billion bp[83]
Bird Gallus gallus 1,043,000,000 1.0Gb 20000 [84]
Fish Tetraodon nigroviridis (type of puffer fish) 385,000,000 390Mb Smallest vertebrate genome known estimated to be 340 Mb[85][86] – 385 Mb.[87]
Fish Protopterus aethiopicus (marbled lungfish) 130,000,000,000 130Gb Largest vertebrate genome known

Genomic alterations

All the cells of an organism originate from a single cell, so they are expected to have identical genomes; however, in some cases, differences arise. Both the process of copying DNA during cell division and exposure to environmental mutagens can result in mutations in somatic cells. In some cases, such mutations lead to cancer because they cause cells to divide more quickly and invade surrounding tissues.[88] In certain lymphocytes in the human immune system, V(D)J recombination generates different genomic sequences such that each cell produces a unique antibody or T cell receptors.

During meiosis, diploid cells divide twice to produce haploid germ cells. During this process, recombination results in a reshuffling of the genetic material from homologous chromosomes so each gamete has a unique genome.

Genome-wide reprogramming

Genome-wide reprogramming in mouse primordial germ cells involves epigenetic imprint erasure leading to totipotency. Reprogramming is facilitated by active DNA demethylation, a process that entails the DNA base excision repair pathway.[89] This pathway is employed in the erasure of CpG methylation (5mC) in primordial germ cells. The erasure of 5mC occurs via its conversion to 5-hydroxymethylcytosine (5hmC) driven by high levels of the ten-eleven dioxygenase enzymes TET1 and TET2.[90]

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 karyotype (chromosome number), 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. 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.

Horizontal gene transfer is invoked to explain how there is often an 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. Recent empirical data suggest an important role of viruses and sub-viral RNA-networks to represent a main driving role to generate genetic novelty and natural genome editing.

In fiction

Works of science fiction illustrate concerns about the availability of genome sequences.

Michael Crichton's 1990 novel Jurassic Park and the subsequent film tell the story of a billionaire who creates a theme park of cloned dinosaurs on a remote island, with disastrous outcomes. A geneticist extracts dinosaur DNA from the blood of ancient mosquitoes and fills in the gaps with DNA from modern species to create several species of dinosaurs. A chaos theorist is asked to give his expert opinion on the safety of engineering an ecosystem with the dinosaurs, and he repeatedly warns that the outcomes of the project will be unpredictable and ultimately uncontrollable. These warnings about the perils of using genomic information are a major theme of the book.

The 1997 film Gattaca is set in a futurist society where genomes of children are engineered to contain the most ideal combination of their parents' traits, and metrics such as risk of heart disease and predicted life expectancy are documented for each person based on their genome. People conceived outside of the eugenics program, known as "In-Valids" suffer discrimination and are relegated to menial occupations. The protagonist of the film is an In-Valid who works to defy the supposed genetic odds and achieve his dream of working as a space navigator. The film warns against a future where genomic information fuels prejudice and extreme class differences between those who can and can't afford genetically engineered children.[91]

See also


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Further reading

  • Benfey P, Protopapas AD (2004). Essentials of Genomics. Prentice Hall.
  • Brown TA (2002). Genomes 2. Oxford: Bios Scientific Publishers. ISBN 978-1-85996-029-5.
  • Gibson G, Muse SV (2004). A Primer of Genome Science (Second ed.). Sunderland, Mass: Sinauer Assoc. ISBN 978-0-87893-234-4.
  • Gregory TR (2005). The Evolution of the Genome. Elsevier. ISBN 978-0-12-301463-4.
  • Reece RJ (2004). Analysis of Genes and Genomes. Chichester: John Wiley & Sons. ISBN 978-0-470-84379-6.
  • Saccone C, Pesole G (2003). Handbook of Comparative Genomics. Chichester: John Wiley & Sons. ISBN 978-0-471-39128-9.
  • Werner E (December 2003). "In silico multicellular systems biology and minimal genomes". Drug Discovery Today. 8 (24): 1121–27. doi:10.1016/S1359-6446(03)02918-0. PMID 14678738.

External links


Bioinformatics (listen) is an interdisciplinary field that develops methods and software tools for understanding biological data. As an interdisciplinary field of science, bioinformatics combines biology, computer science, information engineering, mathematics and statistics to analyze and interpret biological data. Bioinformatics has been used for in silico analyses of biological queries using mathematical and statistical techniques.

Bioinformatics is both an umbrella term for the body of biological studies that use computer programming as part of their methodology, as well as a reference to specific analysis "pipelines" that are repeatedly used, particularly in the field of genomics. Common uses of bioinformatics include the identification of candidates genes and single nucleotide polymorphisms (SNPs). Often, such identification is made with the aim of better understanding the genetic basis of disease, unique adaptations, desirable properties (esp. in agricultural species), or differences between populations. In a less formal way, bioinformatics also tries to understand the organisational principles within nucleic acid and protein sequences, called proteomics.


A chromosome is a deoxyribonucleic acid (DNA) molecule with part or all of the genetic material (genome) of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle.Chromosomes are normally visible under a light microscope only when the cell is undergoing the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form). Before this happens, every chromosome is copied once (S phase), and the copy is joined to the original by a centromere, resulting either in an X-shaped structure (pictured to the right) if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends. The original chromosome and the copy are now called sister chromatids. During metaphase the X-shape structure is called a metaphase chromosome. In this highly condensed form chromosomes are easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.

Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

Craig Venter

John Craig Venter (born October 14, 1946) is an American biotechnologist, biochemist, geneticist, and businessman. He is known for leading the first draft sequence of the human genome and assembled the first team to transfect a cell with a synthetic chromosome. Venter founded Celera Genomics, The Institute for Genomic Research (TIGR) and the J. Craig Venter Institute (JCVI), where he currently serves as CEO. He was the co-founder of Human Longevity Inc. and Synthetic Genomics. He was listed on Time magazine's 2007 and 2008 Time 100 list of the most influential people in the world. In 2010, the British magazine New Statesman listed Craig Venter at 14th in the list of "The World's 50 Most Influential Figures 2010". He is a member of the USA Science and Engineering Festival's Advisory Board.

DNA sequencing

DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. The advent of rapid DNA sequencing methods has greatly accelerated biological and medical research and discovery.Knowledge of DNA sequences has become indispensable for basic biological research, and in numerous applied fields such as medical diagnosis, biotechnology, forensic biology, virology and biological systematics. The rapid speed of sequencing attained with modern DNA sequencing technology has been instrumental in the sequencing of complete DNA sequences, or genomes, of numerous types and species of life, including the human genome and other complete DNA sequences of many animal, plant, and microbial species.

The first DNA sequences were obtained in the early 1970s by academic researchers using laborious methods based on two-dimensional chromatography. Following the development of fluorescence-based sequencing methods with a DNA sequencer, DNA sequencing has become easier and orders of magnitude faster.

Ensembl genome database project

Ensembl genome database project is a joint scientific project between the European Bioinformatics Institute and the Wellcome Trust Sanger Institute, which was launched in 1999 in response to the imminent completion of the Human Genome Project. Ensembl aims to provide a centralized resource for geneticists, molecular biologists and other researchers studying the genomes of our own species and other vertebrates and model organisms. Ensembl is one of several well known genome browsers for the retrieval of genomic information.

Similar databases and browsers are found at NCBI and the University of California, Santa Cruz (UCSC).


In biology, a gene is a sequence of nucleotides in DNA or RNA that codes for a molecule that has a function. During gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate template for a protein that performs a function. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic trait. These genes make up different DNA sequences called genotypes. Genotypes along with environmental and developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes (many different genes) as well as gene–environment interactions. Some genetic traits are instantly visible, such as eye color or number of limbs, and some are not, such as blood type, risk for specific diseases, or the thousands of basic biochemical processes that constitute life.

Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a protein, which cause different phenotypical traits. Usage of the term "having a gene" (e.g., "good genes," "hair colour gene") typically refers to containing a different allele of the same, shared gene. Genes evolve due to natural selection / survival of the fittest and genetic drift of the alleles.

The concept of a gene continues to be refined as new phenomena are discovered. For example, regulatory regions of a gene can be far removed from its coding regions, and coding regions can be split into several exons. Some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs. Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism's traits by being expressed as a functional product or by regulation of gene expression.The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1909. It is inspired by the ancient Greek: γόνος, gonos, that means offspring and procreation.

Genome-wide association study

In genetics, a genome-wide association study (GWA study, or GWAS), also known as whole genome association study (WGA study, or WGAS), is an observational study of a genome-wide set of genetic variants in different individuals to see if any variant is associated with a trait. GWASs typically focus on associations between single-nucleotide polymorphisms (SNPs) and traits like major human diseases, but can equally be applied to any other genetic variants and any other organisms.

When applied to human data, GWA studies compare the DNA of participants having varying phenotypes for a particular trait or disease. These participants may be people with a disease (cases) and similar people without the disease (controls), or they may be people with different phenotypes for a particular trait, for example blood pressure. This approach is known as phenotype-first, in which the participants are classified first by their clinical manifestation(s), as opposed to genotype-first. Each person gives a sample of DNA, from which millions of genetic variants are read using SNP arrays. If one type of the variant (one allele) is more frequent in people with the disease, the variant is said to be associated with the disease. The associated SNPs are then considered to mark a region of the human genome that may influence the risk of disease.

GWA studies investigate the entire genome, in contrast to methods that specifically test a small number of pre-specified genetic regions. Hence, GWAS is a non-candidate-driven approach, in contrast to gene-specific candidate-driven studies. GWA studies identify SNPs and other variants in DNA associated with a disease, but they cannot on their own specify which genes are causal.The first successful GWAS published in 2002 studied myocardial infarction. This study design was then implemented in the landmark GWA 2005 study investigating patients with age-related macular degeneration, and found two SNPs with significantly altered allele frequency compared to healthy controls. As of 2017, over 3,000 human GWA studies have examined over 1,800 diseases and traits, and thousands of SNP associations have been found. In general, these associations are very weak, and may even be entirely meaningless except in the case of rare genetic diseases.


Genomics is an interdisciplinary field of biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes. In contrast to genetics, which refers to the study of individual genes and their roles in inheritance, genomics aims at the collective characterization and quantification of all of an organism genes, their interrelations and influence on the organism. Genes may direct the production of proteins with the assistance of enzymes and messenger molecules. In turn, proteins make up body structures such as organs and tissues as well as control chemical reactions and carry signals between cells. Genomics also involves the sequencing and analysis of genomes through uses of high throughput DNA sequencing and bioinformatics to assemble and analyze the function and structure of entire genomes. Advances in genomics have triggered a revolution in discovery-based research and systems biology to facilitate understanding of even the most complex biological systems such as the brain.The field also includes studies of intragenomic (within the genome) phenomena such as epistasis (effect of one gene on another), pleiotropy (one gene affecting more than one trait), heterosis (hybrid vigour), and other interactions between loci and alleles within the genome.

Human Genome Organisation

The Human Genome Organisation (HUGO) is an organization involved in the Human Genome Project, a project about mapping the human genome. HUGO was established in 1989 as an international organization, primarily to foster collaboration between genome scientists around the world. The HUGO Gene Nomenclature Committee (HGNC), sometimes referred to as "HUGO", is one of HUGO's most active committees and aims to assign a unique gene name and symbol to each human gene.

Human Genome Project

The Human Genome Project (HGP) was an international scientific research project with the goal of determining the sequence of nucleotide base pairs that make up human DNA, and of identifying and mapping all of the genes of the human genome from both a physical and a functional standpoint. It remains the world's largest collaborative biological project. After the idea was picked up in 1984 by the US government when the planning started, the project formally launched in 1990 and was declared complete on April 14, 2003. Funding came from the US government through the National Institutes of Health (NIH) as well as numerous other groups from around the world. A parallel project was conducted outside government by the Celera Corporation, or Celera Genomics, which was formally launched in 1998. Most of the government-sponsored sequencing was performed in twenty universities and research centers in the United States, the United Kingdom, Japan, France, Germany and China.The Human Genome Project originally aimed to map the nucleotides contained in a human haploid reference genome (more than three billion). The "genome" of any given individual is unique; mapping the "human genome" involved sequencing a small number of individuals and then assembling these together to get a complete sequence for each chromosome. Therefore, the finished human genome is a mosaic, not representing any one individual.

Human genome

The human genome is the complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the nuclear genome, and the mitochondrial genome. Human genomes include both protein-coding DNA genes and noncoding DNA. Haploid human genomes, which are contained in germ cells (the egg and sperm gamete cells created in the meiosis phase of sexual reproduction before fertilization creates a zygote) consist of three billion DNA base pairs, while diploid genomes (found in somatic cells) have twice the DNA content. While there are significant differences among the genomes of human individuals (on the order of 0.1%), these are considerably smaller than the differences between humans and their closest living relatives, the chimpanzees (approximately 4%) and bonobos.

The first human genome sequences were published in nearly complete draft form in February 2001 by the Human Genome Project and Celera Corporation. Completion of the Human Genome Project Sequence was published in 2004. The human genome was the first of all vertebrates to be completely sequenced. As of 2012, thousands of human genomes have been completely sequenced, and many more have been mapped at lower levels of resolution. This data is used worldwide in biomedical science, anthropology, forensics and other branches of science. There is a widely held expectation that genomic studies will lead to advances in the diagnosis and treatment of diseases, and to new insights in many fields of biology, including human evolution.

Although the sequence of the human genome has been (almost) completely determined by DNA sequencing, it is not yet fully understood. Most (though probably not all) genes have been identified by a combination of high throughput experimental and bioinformatics approaches, yet much work still needs to be done to further elucidate the biological functions of their protein and RNA products. Recent results suggest that most of the vast quantities of noncoding DNA within the genome have associated biochemical activities, including regulation of gene expression, organization of chromosome architecture, and signals controlling epigenetic inheritance.

There are an estimated 19,000-20,000 human protein-coding genes. The estimate of the number of human genes has been repeatedly revised down from initial predictions of 100,000 or more as genome sequence quality and gene finding methods have improved, and could continue to drop further. Protein-coding sequences account for only a very small fraction of the genome (approximately 1.5%), and the rest is associated with non-coding RNA molecules, regulatory DNA sequences, LINEs, SINEs, introns, and sequences for which as yet no function has been determined.In June 2016, scientists formally announced HGP-Write, a plan to synthesize the human genome.


KEGG (Kyoto Encyclopedia of Genes and Genomes) is a collection of databases dealing with genomes, biological pathways, diseases, drugs, and chemical substances. KEGG is utilized for bioinformatics research and education, including data analysis in genomics, metagenomics, metabolomics and other omics studies, modeling and simulation in systems biology, and translational research in drug development.

Mitochondrial DNA

Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, adenosine triphosphate (ATP). Mitochondrial DNA is only a small portion of the DNA in a eukaryotic cell; most of the DNA can be found in the cell nucleus and, in plants and algae, also in plastids such as chloroplasts.

In humans, the 16,569 base pairs of mitochondrial DNA encode for only 37 genes. Human mitochondrial DNA was the first significant part of the human genome to be sequenced. In most species, including humans, mtDNA is usually inherited solely from the mother. However, in exceptional cases, human babies sometimes inherit mtDNA from both their fathers and their mothers resulting in mtDNA heteroplasmy.Since animal mtDNA evolves faster than nuclear genetic markers, it represents a mainstay of phylogenetics and evolutionary biology. It also permits an examination of the relatedness of populations, and so has become important in anthropology and biogeography.

Mouse Genome Informatics

Mouse Genome Informatics (MGI) is a free, online database and bioinformatics resource hosted by The Jackson Laboratory, with funding by the National Human Genome Research Institute (NHGRI), the National Cancer Institute (NCI), and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). MGI provides access to data on the genetics, genomics and biology of the laboratory mouse to facilitate the study of human health and disease. The database integrates multiple projects, with the two largest contributions coming from the Mouse Genome Database and Gene Expression Database (GXD).The Mouse Genome Informatics resource is a collection of data, tools, and analyses created and tailored for use in the laboratory mouse, a widely used model organism. It is "the authoritative source of official names for mouse genes, alleles, and strains", which follow the guidelines established by the International Committee on Standardized Genetic Nomenclature for Mice. The history and focus of Jackson Laboratory research and production facilities generates tremendous knowledge and depth which researchers can mine to advance their research. A dedicated community of mouse researchers, worldwide enhances and contributes to the knowledge as well. This is an indispensable tool for any researcher using the mouse as a model organism for their research, and for researchers interested in genes that share homology with the mouse genes. Various mouse research support resources including animal collections and free colony management software are also available at the MGI site.


A retrovirus is a type of RNA virus that inserts a copy of its genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Such viruses are either single stranded RNA (e.g. HIV) or double stranded DNA (e.g. Hepatitis B virus) viruses.

Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus. The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus. It is difficult to detect the virus until it has infected the host. At that point, the infection will persist indefinitely.

In most viruses, DNA is transcribed into RNA, and then RNA is translated into protein. However, retroviruses function differently, as their RNA is reverse-transcribed into DNA, which is integrated into the host cell's genome (when it becomes a provirus), and then undergoes the usual transcription and translational processes to express the genes carried by the virus. The information contained in a retroviral gene is thus used to generate the corresponding protein via the sequence: RNA → DNA → RNA → polypeptide. This extends the fundamental process identified by Francis Crick (one gene-one peptide) in which the sequence is DNA → RNA → peptide (proteins are made of one or more polypeptide chains; for example, haemoglobin is a four-chain peptide).

Retroviruses are valuable research tools in molecular biology, and they have been used successfully in gene delivery systems.

Sequence homology

Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speciation event (orthologs), or a duplication event (paralogs), or else a horizontal (or lateral) gene transfer event (xenologs).Homology among DNA, RNA, or proteins is typically inferred from their nucleotide or amino acid sequence similarity. Significant similarity is strong evidence that two sequences are related by evolutionary changes from a common ancestral sequence. Alignments of multiple sequences are used to indicate which regions of each sequence are homologous.

UCSC Genome Browser

The UCSC Genome Browser is an on-line, and downloadable, genome browser hosted by the University of California, Santa Cruz (UCSC). It is an interactive website offering access to genome sequence data from a variety of vertebrate and invertebrate species and major model organisms, integrated with a large collection of aligned annotations. The Browser is a graphical viewer optimized to support fast interactive performance and is an open-source, web-based tool suite built on top of a MySQL database for rapid visualization, examination, and querying of the data at many levels. The Genome Browser Database, browsing tools, downloadable data files, and documentation can all be found on the UCSC Genome Bioinformatics website.


A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 virus species have been described in detail, although there are millions of types. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology.

While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles, also known as virions, consist of: (i) the genetic material made from either DNA or RNA, long molecules that carry genetic information; (ii) a protein coat, called the capsid, which surrounds and protects the genetic material; and in some cases (iii) an envelope of lipids that surrounds the protein coat. The shapes of these virus particles range from simple helical and icosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an optical microscope. The average virion is about one one-hundredth the size of the average bacterium.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity. Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, but lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life", and as replicators.Viruses spread in many ways; viruses in plants are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; viruses in animals can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route and are passed from person to person by contact, entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its "host range". This can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many.Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause AIDS and viral hepatitis, evade these immune responses and result in chronic infections. Several antiviral drugs have been developed.

Whole genome sequencing

Whole genome sequencing (also known as WGS, full genome sequencing, complete genome sequencing, or entire genome sequencing) is ostensibly the process of determining the complete DNA sequence of an organism's genome at a single time. This entails sequencing all of an organism's chromosomal DNA as well as DNA contained in the mitochondria and, for plants, in the chloroplast. In practice, genome sequences that are nearly complete are also called whole genome sequences.Whole genome sequencing has largely been used as a research tool, but is currently being introduced to clinics. In the future of personalized medicine, whole genome sequence data may be an important tool to guide therapeutic intervention. The tool of gene sequencing at SNP level is also used to pinpoint functional variants from association studies and improve the knowledge available to researchers interested in evolutionary biology, and hence may lay the foundation for predicting disease susceptibility and drug response.

Whole genome sequencing should not be confused with DNA profiling, which only determines the likelihood that genetic material came from a particular individual or group, and does not contain additional information on genetic relationships, origin or susceptibility to specific diseases. In addition, whole genome sequencing should not be confused with methods that sequence specific subsets of the genome - such methods include whole exome sequencing (1-2% of the genome) or SNP genotyping (<0.1% of the genome).

As of 2017 there were no complete genomes for any mammals, including humans. Between 4% to 9% of the human genome, mostly satellite DNA, had not been sequenced.

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