Gene

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This article is about sequences of DNA or RNA that code for functional molecules. For other uses, see Gene (disambiguation).

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The image above contains clickable links
A chromosome and its
base pairs
encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

In biology, the word gene (Greek: γένος, génos;[1] generation,[2] or birth,[1] or gender) has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA, that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and non-coding genes.[3][4][5][6]

During gene expression, DNA is first copied into RNA. RNA can be directly functional or be the intermediate template for the synthesis of a protein.

The transmission of genes to an organism's

population of a given species. The genotype, along with environmental and developmental factors, ultimately determines the phenotype of the individual. Most biological traits occur under the combined influence of polygenes (a set of different genes) and gene–environment interactions. Some genetic traits are instantly visible, such as eye color or the number of limbs, others are not, such as blood type, the risk for specific diseases, or the thousands of basic biochemical processes that constitute life
.

A gene can acquire

phenotypical traits.[7] Genes evolve due to natural selection / survival of the fittest and genetic drift
of the alleles.

The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1909.[8] It is inspired by the Ancient Greek: γόνος, gonos, that means offspring and procreation.

Definitions

There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of these definitions fall into two categories, the Mendelian gene or the molecular gene.[3][9][10][11][12]

The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in The Selfish Gene.[13] More thorough discussions of this version of a gene can be found in the articles on Genetics and Gene-centered view of evolution.

The molecular gene definition is more commonly used across biochemistry, molecular biology, and most of genetics — the gene that is described in terms of DNA sequence.[3] There are many different definitions of this gene — some of which are misleading or incorrect.[9][14]

Very early work in the field that became

one gene makes one protein (originally 'one gene - one enzyme').[15][16] However, genes that produce repressor RNAs were proposed in the 1950s[17] and by the 1960s, textbooks were using molecular gene definitions that included those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes) as well as protein-coding genes.[18]

This idea of two kinds of genes is still part of the definition of a gene in most textbooks. For example,

"The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of the DNA helix that produces a functional RNA molecule constitutes a gene."[19]
"We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of a gene - surprisingly, there is no definition that is entirely satisfactory."[20]
"A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere."[21]

The important parts of such definitions are: (1) that a gene corresponds to a transcription unit; (2) that genes produce both mRNA and noncoding RNAs; and (3) regulatory sequences control gene expression but are not part of the gene itself. However, there's one other important part of the definition and it is emphasized in Kostas Kampourakis' book Making Sense of Genes.

"Therefore in this book I will consider genes as DNA sequences encoding information for functional products, be it proteins or RNA molecules. With 'encoding information,' I mean that the DNA sequence is used as a template for the production of an RNA molecule or a protein that performs some function.'[9]

The emphasis on function is essential because there are stretches of DNA that produce non-functional transcripts and they do not qualify as genes. These include obvious examples such as transcribed pseudogenes as well as less obvious examples such as junk RNA produced as noise due to transcription errors. In order to qualify as a true gene, by this definition, one has to prove that the transcript has a biological function.[9]

Early speculations on the size of a typical gene were based on high resolution genetic mapping and on the size of proteins and RNA molecules. A length of 1500 base pairs seemed reasonable at the time (1965).[18] This was based on the idea that the gene was the DNA that was directly responsible for production of the functional product. The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes, for example, are about 62,000 base pairs in length (transcribed region) and since there are about 20,000 of them they occupy about 35–40% of the mammalian genome (including the human genome).[22][23][24]

In spite of the fact that both protein-coding genes and noncoding genes have been known for more than 50 years, there are still a number of textbooks, websites, and scientific publications that define a gene as a DNA sequence that specifies a protein. In other words, the definition is restricted to protein-coding genes. Here is an example from a recent article in American Scientist.

... to truly assess the potential significance of de novo genes, we relied on a strict definition of the word "gene" with which nearly every expert can agree. First, in order for a nucleotide sequence to be considered a true gene, an open reading frame (ORF) must be present. The ORF can be thought of as the "gene itself"; it begins with a starting mark common for every gene and ends with one of three possible finish line signals. One of the key enzymes in this process, the RNA polymerase, zips along the strand of DNA like a train on a monorail, transcribing it into its messenger RNA form. This point brings us to our second important criterion: A true gene is one that is both transcribed and translated. That is, a true gene is first used as a template to make transient messenger RNA, which is then translated into a protein.[25]

This restricted definition is so common that it has spawned many recent articles that criticize this "standard definition" and call for a new expanded definition that includes noncoding genes.

editorializing
]

Although some definitions can be more broadly applicable than others, the fundamental complexity of biology means that no definition of a gene can capture all aspects perfectly. Not all genomes are DNA (e.g. RNA viruses),[29] bacterial operons are multiple protein-coding regions transcribed into single large mRNAs, alternative splicing enables a single genomic region to encode multiple district products and trans-splicing concatenates mRNAs from shorter coding sequence across the genome.[30][31][32] Since molecular definitions exclude elements such as introns, promotors and other regulatory regions, these are instead thought of as 'associated' with the gene and affect its function.

An even broader operational definition is sometimes used to encompass the complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products.[33] This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as gene-associated regions.[33]

History

Main article: History of genetics

Discovery of discrete inherited units

Photograph of Gregor Mendel
Gregor Mendel

The existence of discrete inheritable units was first suggested by

homozygote
, and the phenomenon of discontinuous inheritance.

Prior to Mendel's work, the dominant theory of heredity was one of

gemmule
to describe hypothetical particles that would mix during reproduction.

Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries, Carl Correns, and Erich von Tschermak, who (claimed to have) reached similar conclusions in their own research.[38] Specifically, in 1889, Hugo de Vries published his book Intracellular Pangenesis,[39] in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" (Pangens in German), after Darwin's 1868 pangenesis theory.

Twenty years later, in 1909, Wilhelm Johannsen introduced the term 'gene'[8] and in 1906, William Bateson, that of 'genetics'[40][33] while Eduard Strasburger, amongst others, still used the term 'pangene' for the fundamental physical and functional unit of heredity.[39]: Translator's preface, viii 

Discovery of DNA

Advances in understanding genes and inheritance continued throughout the 20th century.

nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication.[43][44]

In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA.[45][46]

Collectively, this body of research established the

reverse transcription in retroviruses. The modern study of genetics at the level of DNA is known as molecular genetics
.

In 1972, Walter Fiers and his team were the first to determine the sequence of a gene: that of Bacteriophage MS2 coat protein.[47] The subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool.[48] An automated version of the Sanger method was used in early phases of the Human Genome Project.[49]

Modern synthesis and its successors

Main article: Modern synthesis (20th century)

The theories developed in the early 20th century to integrate

Darwinian evolution are called the modern synthesis, a term introduced by Julian Huxley.[50]

This view of evolution was emphasized by

George C. Williams' gene-centric view of evolution. He proposed that the Mendelian gene is a unit of natural selection with the definition: "that which segregates and recombines with appreciable frequency."[51]: 24  Related ideas emphasizing the centrality of Mendelian genes and the importance of natural selection in evolution were popularized by Richard Dawkins.[13][52]

The development of the neutral theory of evolution in the late 1960s led to the recognition that random genetic drift is a major player in evolution and that neutral theory should be the null hypothesis of molecular evolution.[53] This led to the construction of phylogenetic trees and the development of the molecular clock, which is the basis of all dating techniques using DNA sequences. These techniques are not confined to molecular gene sequences but can be used on all DNA segments in the genome.

Molecular basis

Main article: DNA
base-pairing A to T and C to G with hydrogen bonds
.

DNA

The vast majority of organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar (2-deoxyribose), a phosphate group, and one of the four bases adenine, cytosine, guanine, and thymine.[54]: 2.1 

Two chains of DNA twist around each other to form a DNA

base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be complementary, with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on.[54]
: 4.1 

Due to the chemical composition of the

transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile.[55]
: 27.2 

The

protein translation between codons and amino acids. The genetic code is nearly the same for all known organisms.[54]
: 4.1 

Chromosomes

Micrographic karyogram of human male, showing 23 pairs of chromosomes. The largest chromosomes are around 10 times the size of the smallest.[56]
Schematic karyogram of a human, with annotated bands and sub-bands. It shows dark and white regions on G banding. It shows 22 homologous chromosomes, both the male (XY) and female (XX) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (at bottom left).
Further information: Karyotype

The total complement of genes in an organism or cell is known as its genome, which may be stored on one or more chromosomes. A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded.[54]: 4.2  The region of the chromosome at which a particular gene is located is called its locus. Each locus contains one allele of a gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence.

The majority of

spindle fibres to separate sister chromatids into daughter cells during cell division.[54]
: 18.2 

antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer.[58]

Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas the genomes of complex multicellular organisms, including humans, contain an absolute majority of DNA without an identified function.[59] This DNA has often been referred to as "junk DNA". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the human genome, about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer.[30]

Structure and function

Structure

The image above contains clickable links
The structure of a
3' untranslated regions (blue) regulate translation into the final protein product.[60]
Prokaryote gene structure diagram
Operator
5'UTR
3'UTR
Transcription
RBS
RBS
Protein coding region
Protein coding region
mRNA
Translation
The image above contains clickable links
The structure of a
transcription of the gene into an mRNA. The mRNA untranslated regions (blue) regulate translation into the final protein products.[60]

The structure of a protein-coding gene consists of many elements of which the actual protein coding sequence is often only a small part. These include introns and untranslated regions of the mature mRNA. Noncoding genes can also contain introns that are removed during processing to produce the mature functional RNA.

All genes are associated with

mRNA) that differ in how far they extend in the 5' end.[61] Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at a high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently.[54]: 7.2  Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters.[54]
: 7.3 

Additionally, genes can have regulatory regions many kilobases upstream or downstream of the gene that alter expression. These act by

silencers bind repressor proteins and make the DNA less available for RNA polymerase.[63]

The mature messenger RNA produced from protein-coding genes contains

mature mRNA, which encodes the protein or RNA product.[65] Although the general mechanisms defining locations of human genes are known, identification of the exact factors regulating these cellular processes is an area of active research. For example, known sequence features in the 3'-UTR can only explain half of all human gene ends.[66]

Many noncoding genes in eukaryotes have different transcription termination mechanisms and they do not have poly(A) tails.

Many prokaryotic genes are organized into

transcription of the operon; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon). The products of operon genes typically have related functions and are involved in the same regulatory network.[54]
: 7.3 

Complexity

Though many genes have simple structures, as with much of biology, others can be quite complex or represent unusual edge-cases. Eukaryotic genes often have introns are often much larger than their exons,[70][71] and those introns can even have other genes nested inside them.[72] Associated enhancers may be many kilobase away, or even on entirely different chromosomes operating via physical contact between two chromosomes.[73][74] A single gene can encode multiple different functional products by alternative splicing, and conversely gene may be split across chromosomes but those transcripts are concatenated back together into a functional sequence by trans-splicing.[75] It is also possible for overlapping genes to share some of their DNA sequence, either on opposite strands or the same strand (in a different reading frame, or even the same reading frame).[76]

Gene expression

Main article: Gene expression

In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it specifies. First, the gene's DNA is

translated to protein.[54]: 6.2  RNA-coding genes must still go through the first step, but are not translated into protein.[77] The process of producing a biologically functional molecule of either RNA or protein is called gene expression, and the resulting molecule is called a gene product
.

Genetic code

codons. Each three-nucleotide codon corresponds to an amino acid
when translated to protein.

The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the

codons, each correspond to a specific amino acid.[54]: 6  The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4[78] (see Crick, Brenner et al. experiment
).

Additionally, a "start codon", and three "stop codons" indicate the beginning and end of the protein coding region. There are 64 possible codons (four possible nucleotides at each of three positions, hence 43 possible codons) and only 20 standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms.[79]

Transcription

gene regulation is the blocking or sequestering the promoter region, either by tight binding by repressor molecules that physically block the polymerase or by organizing the DNA so that the promoter region is not accessible.[54]
: 7 

In

splicing of introns which are sequences in the transcribed region that do not encode a protein. Alternative splicing mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins. This is a major form of regulation in eukaryotic cells and also occurs in some prokaryotes.[54]: 7.5 [80]

Translation

mRNA intermediate, then translated to a functional protein. RNA-coding genes are transcribed to a functional non-coding RNA (PDB: 3BSE, 1OBB, 3TRA
​).

three-dimensional structure before they can carry out their cellular functions.[54]
: 3 

Regulation

E. coli (lac operon) was the first such mechanism to be described in 1961.[81]

RNA genes

A typical protein-coding gene is first copied into RNA as an intermediate in the manufacture of the final protein product.[54]: 6.1  In other cases, the RNA molecules are the actual functional products, as in the synthesis of ribosomal RNA and transfer RNA. Some RNAs known as ribozymes are capable of enzymatic function, while others such as microRNAs and riboswitches have regulatory roles. The DNA sequences from which such RNAs are transcribed are known as non-coding RNA genes.[77]

Some

reverse transcription of their genome
from RNA into DNA before their proteins can be synthesized.

Inheritance

recessive to the blue allele. The probability of each outcome in the children's generation is one quarter, or 25 percent.
Main articles: Mendelian inheritance and Heredity

Organisms inherit their genes from their parents. Asexual organisms simply inherit a complete copy of their parent's genome. Sexual organisms have two copies of each chromosome because they inherit one complete set from each parent.[54]

: 1 

Mendelian inheritance

According to

alleles) giving rise to different phenotypes. Most eukaryotic organisms (such as the pea plants Mendel worked on) have two alleles for each trait, one inherited from each parent.[54]
: 20 

Alleles at a locus may be

recessive; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. If you know the genotypes of the organisms, you can determine which alleles are dominant and which are recessive. For example, if the allele specifying tall stems in pea plants is dominant over the allele specifying short stems, then pea plants that inherit one tall allele from one parent and one short allele from the other parent will also have tall stems. Mendel's work demonstrated that alleles assort independently in the production of gametes, or germ cells, ensuring variation in the next generation. Although Mendelian inheritance remains a good model for many traits determined by single genes (including a number of well-known genetic disorders) it does not include the physical processes of DNA replication and cell division.[85][86]

DNA replication and cell division

The growth, development, and reproduction of organisms relies on

daughter cells. This requires first making a duplicate copy of every gene in the genome in a process called DNA replication.[54]: 5.2  The copies are made by specialized enzymes known as DNA polymerases, which "reads" one strand of the double-helical DNA, known as the template strand, and synthesize a new complementary strand. Because the DNA double helix is held together by base pairing, the sequence of one strand completely specifies the sequence of its complement; hence only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is semiconservative; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA.[54]
: 5.2 

The rate of DNA replication in living cells was first measured as the rate of phage T4 DNA elongation in phage-infected E. coli and found to be impressively rapid.[87] During the period of exponential DNA increase at 37 °C, the rate of elongation was 749 nucleotides per second.

After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells.

M phase.[54]
: 18.1 

Molecular inheritance

The duplication and transmission of genetic material from one generation of cells to the next is the basis for molecular inheritance and the link between the classical and molecular pictures of genes. Organisms inherit the characteristics of their parents because the cells of the offspring contain copies of the genes in their parents' cells. In

fertilized egg, a single cell that has two sets of genes, with one copy of each gene from the mother and one from the father.[54]
: 20 

During the process of meiotic cell division, an event called genetic recombination or crossing-over can sometimes occur, in which a length of DNA on one chromatid is swapped with a length of DNA on the corresponding homologous non-sister chromatid. This can result in reassortment of otherwise linked alleles.[54]: 5.5  The Mendelian principle of independent assortment asserts that each of a parent's two genes for each trait will sort independently into gametes; which allele an organism inherits for one trait is unrelated to which allele it inherits for another trait. This is in fact only true for genes that do not reside on the same chromosome or are located very far from one another on the same chromosome. The closer two genes lie on the same chromosome, the more closely they will be associated in gametes and the more often they will appear together (known as genetic linkage).[88] Genes that are very close are essentially never separated because it is extremely unlikely that a crossover point will occur between them.[88]

Molecular evolution

Main article: Molecular evolution

Mutation

DNA replication is for the most part extremely accurate, however errors (

double-strand breaks.[54]
: 5.4 

When multiple different alleles for a gene are present in a species's population it is called polymorphic. Most different alleles are functionally equivalent, however some alleles can give rise to different phenotypic traits. A gene's most common allele is called the wild type, and rare alleles are called mutants. The genetic variation in relative frequencies of different alleles in a population is due to both natural selection and genetic drift.[94] The wild-type allele is not necessarily the ancestor of less common alleles, nor is it necessarily fitter.

Most mutations within genes are

beneficial, improving the organism's fitness and are extremely important for evolution, since their directional selection leads to adaptive evolution.[54]
: 7.6 

Sequence homology

The relationship between genes can be measured by comparing the sequences of their DNA. If the level of similarity exceeds a minimum value, one can conclude that the genes descend from a common ancestor; they are homologous.[95][96] Genes that are related by direct descent from a common ancestor are orthologous genes - they are usually found at the same locus in different species. Genes that are related as a result of a gene duplication event are parologous genes.[97][98]

It is often assumed that the functions of orthologous genes are more similar than those of paralogous genes, although the difference is minimal.[99][100]

Origins of new genes

Evolutionary fate of duplicate genes

The most common source of new genes in eukaryotic lineages is gene duplication, which creates copy number variation of an existing gene in the genome.[101][102] The resulting genes (paralogs) may then diverge in sequence and in function. Sets of genes formed in this way compose a gene family. Gene duplications and losses within a family are common and represent a major source of evolutionary biodiversity.[103] Sometimes, gene duplication may result in a nonfunctional copy of a gene, or a functional copy may be subject to mutations that result in loss of function; such nonfunctional genes are called pseudogenes.[54]: 7.6 

non-coding sequence, or gene duplication followed by such rapid sequence change that the original relationship becomes undetectable.[106] De novo genes are typically shorter and simpler in structure than most eukaryotic genes, with few if any introns.[101] Over long evolutionary time periods, de novo gene birth may be responsible for a significant fraction of taxonomically restricted gene families.[107]

alga genomes containing genes of bacterial origin.[110][111]

Genome

The

non-coding sequences.[112] Eukaryotic genes can be annotated using FINDER.[113]

Number of genes

fungi (yellow), bacteria (purple), and viruses (grey). An inset on the right shows the smaller genomes expanded 100-fold area-wise.[114][115][116][117][118][119][120][121]

The genome size, and the number of genes it encodes varies widely between organisms. The smallest genomes occur in viruses,[122] and viroids (which act as a single non-coding RNA gene).[123] Conversely, plants can have extremely large genomes,[124] with rice containing >46,000 protein-coding genes.[118] The total number of protein-coding genes (the Earth's proteome) is estimated to be 5 million sequences.[125]

Although the number of base-pairs of DNA in the human genome has been known since the 1950s, the estimated number of genes has changed over time as definitions of genes, and methods of detecting them have been refined. Initial theoretical predictions of the number of human genes in the 1960s and 1970s were based on mutation load estimates and the numbers of mRNAs and these estimates tended to be about 30,000 protein-coding genes.[126][127][128] During the 1990s there were guesstimates of up to 100,000 genes and early data on detection of mRNAs (expressed sequence tags) suggested more than the traditional value of 30,000 genes that had been reported in the textbooks during the 1980s.[129]

The initial draft sequences of the human genome confirmed the earlier predictions of about 30,000 protein-coding genes however that estimate has fallen to about 19,000 with the ongoing GENCODE annotation project .[130] The number of noncoding genes is not known with certainty but the latest estimates from Ensembl suggest 26,000 noncoding genes.[131]

Essential genes

Main article: Essential gene
Syn 3[132]

Essential genes are the set of genes thought to be critical for an organism's survival.

Syn 3, has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function.[132]

Essential genes include

analysing gene expression, since they are constitutively expressed
at a relatively constant level.

Genetic and genomic nomenclature

Gene nomenclature has been established by the HUGO Gene Nomenclature Committee (HGNC), a committee of the Human Genome Organisation, for each known human gene in the form of an approved gene name and symbol (short-form abbreviation), which can be accessed through a database maintained by HGNC. Symbols are chosen to be unique, and each gene has only one symbol (although approved symbols sometimes change). Symbols are preferably kept consistent with other members of a gene family and with homologs in other species, particularly the mouse due to its role as a common model organism.[141]

Genetic engineering

Comparison of conventional plant breeding with transgenic and cisgenic genetic modification
Main article: Genetic engineering

Genetic engineering is the modification of an organism's

genome engineering techniques use engineered nuclease enzymes to create targeted DNA repair in a chromosome to either disrupt or edit a gene when the break is repaired.[143][144][145][146] The related term synthetic biology is sometimes used to refer to extensive genetic engineering of an organism.[147]

Genetic engineering is now a routine research tool with

knockout mice with a specific gene's function disrupted are used to investigate that gene's function.[149][150] Many organisms have been genetically modified for applications in agriculture, industrial biotechnology, and medicine
.

For multicellular organisms, typically the embryo is engineered which grows into the adult genetically modified organism.[151] However, the genomes of cells in an adult organism can be edited using gene therapy techniques to treat genetic diseases.

See also

References

Citations

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Sources

Main textbook
Referenced chapters of Molecular Biology of the Cell
Glossary
Ch 1: Cells and genomes
1.1: The Universal Features of Cells on Earth
Ch 2: Cell Chemistry and Biosynthesis
2.1: The Chemical Components of a Cell
Ch 3: Proteins
Ch 4: DNA and Chromosomes
4.1: The Structure and Function of DNA
4.2: Chromosomal DNA and Its Packaging in the Chromatin Fiber
Ch 5: DNA Replication, Repair, and Recombination
5.2: DNA Replication Mechanisms
5.4: DNA Repair
5.5: General Recombination
Ch 6: How Cells Read the Genome: From DNA to Protein
6.1: DNA to RNA
6.2: RNA to Protein
Ch 7: Control of Gene Expression
7.1: An Overview of Gene Control
7.2: DNA-Binding Motifs in Gene Regulatory Proteins
7.3: How Genetic Switches Work
7.5: Posttranscriptional Controls
7.6: How Genomes Evolve
Ch 14: Energy Conversion: Mitochondria and Chloroplasts
14.4: The Genetic Systems of Mitochondria and Plastids
Ch 18: The Mechanics of Cell Division
18.1: An Overview of M Phase
18.2: Mitosis
Ch 20: Germ Cells and Fertilization
20.2: Meiosis

Further reading

External links

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