In biology, a gene is a sequence of DNA or RNA that encodes a functioning molecule. During gene expression, DNA is first copied to RNA. RNA can function directly or become an intermediate template for proteins that perform functions. Transmission of genes to hereditary organisms is the basis of inherited phenotypic traits. These genes form different DNA sequences called genotypes. Genotypes together with environmental and developmental factors determine what the phenotypes are. Most biological properties are under the influence of polygenes (many different genes) as well as gene-environment interactions. Some genetic traits are immediately visible, such as eye color or number of limbs, and some are not, such as blood type, risk for certain diseases, or the thousands of basic biochemical processes that make up life.
Genes can obtain mutations in sequence, leading to different variants, known as alleles, in the population. These alleles encode a slightly different version of the protein, which causes different phenotypic properties. The use of the term "having a gene" (eg, "good genes," "hair color genes") usually refers to containing different alleles of the same shared genes. Genes evolved because of natural selection or the strongest survival of alleles.
Gene concepts continue to be refined when new phenomena are discovered. For example, the regulator region of a gene can be away from its encoding region, and the encoding region can be split into several exons. Some viruses store their genomes in RNA, not DNA and some gene products are functional non-coding RNA. Therefore, the broad and modern definition of gene work is any discrete locus of the inherited genome sequence that affects the properties of the organism by being expressed as a functional product or by the regulation of gene expression.
The term genes was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1905. It was inspired by the ancient Greek: ?????, gonos , it means offspring and procreation.
Video Gene
History
Invention of discrete inheritance unit
The existence of discrete inherited units was first suggested by Gregor Mendel (1822-1884). From 1857 to 1864, in Brno (Czech Republic), he studied the pattern of inheritance in 8,000 edible pea plants, tracking different traits from parent to offspring. He describes this mathematically as 2 n Ã, a combination where n is the number of distinct characteristics in the original pea. Although he does not use the term genes , he describes the results in terms of a separate inheritance unit that gives rise to observable physical characteristics. This description describes Wilhelm Johannsen's distinction between genotypes (the genetic material of an organism) and phenotype (the visible features of the organism). Mendel was also the first to demonstrate the independent variety, the difference between dominant and recessive traits, the difference between heterozygotes and homozygous, and the inherited phenomenon of disconnection.
Prior to Mendel's work, the dominant heredity theory was one of the heritage of mixing, which suggested that each parent provide fluid for the process of fertilization and that the characteristics of the parents are mixed and mixed to produce offspring. Charles Darwin developed the theory of inheritance he called pangenesis, from the Greek pan ("all, whole") and genesis ("born")/genos ("origin"). Darwin used the term
Mendel's work was largely unknown after his 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. In particular, in 1889, Hugo de Vries published his book Intracellular Pangenesis, where he postulates that different characters have individual hereditary carriers and inheritance of certain properties in the organisms coming in particles. De Vries calls these units "pangenes" ( Pangens in German), after Darwin's pangenesis theory of 1868.
Sixteen years later, in 1905 Wilhelm Johannsen introduced the terms 'gene' and William Bateson that 'genetics' while Eduard Strasburger, among others, still used the term 'pangene' for the fundamental physical and functional unit of heredity.
DNA discovery
Progress in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid (DNA) proved to be a molecular repository of genetic information with experiments in the 1940s and 1950s. The structure of DNA studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography, which led James D. Watson and Francis Crick to publish the model of double-stranded DNA molecules paired with the nucleotide base indicate an interesting hypothesis for genetic replication mechanisms.
In the early 1950s the prevailing view was that genes in a chromosome act like discrete entities, inseparable by recombination and arranged like beads on a rope. Benzer experiments using a defective mutant in the rII region of T4 bacteriophage (1955-1959) show that individual genes have a simple linear structure and tend to be equivalent to the linear portion of DNA.
Collectively, this body of research establishes the central dogma of molecular biology, which states that proteins are translated from RNA, which is transcribed from DNA. This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses. Modern studies of genetics at the DNA level are known as molecular genetics.
In 1972, Walter Fiers and his team at the University of Ghent were the first to determine the sequence of genes: the genes for the protein coat Bacteriophage MS2. The subsequent development of the DNA-stopping link in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool. The automatic version of the Sanger method is used in the early phases of the Human Genome Project.
Modern synthesis and successors
The theories developed in the early twentieth century to integrate Mendel's genetics with Darwinian evolution are called modern synthesis, a term introduced by Julian Huxley.
Evolutionary biologists later modified this concept, like the genetic-centered view of evolution from George C. Williams. He proposed the concept of gene evolution as a unit of natural selection by definition: "which separates and recombines with considerable frequency." In this view, the molecular genes transcribe as a unit, and the evolution genes inherit as a unit. Related ideas that emphasize the centrality of genes in evolution popularized by Richard Dawkins.
Maps Gene
Molecular basis
DNA
Most living organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of chains made of four types of nucleotide subunits, each consisting of five carbon sugars (2'-deoxyribose), phosphate group, and one of four adenine bases, cytosine, guanine, and thymine.
Two DNA chains rotate to each other to form a double helix of DNA with the backbone of sugar phosphate revolving around the outside, and the inward-bases with the adenine base pair become thymine and guanine into cytosine. The peculiarity of base pairs occurs because adenine and thymine are parallel to form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. Two strands in a double helix shall complement each other, in the corresponding sequence of bases so that the adenine of one strand is paired with the other strand thymines, and so on.
Due to the chemical composition of the pentose residue from the base, the DNA strands have a direction. One end of the DNA polymer contains an open hydroxyl group on deoxyribose; this is known as the 3'Ã, the tip of the molecule. The other end contains an open phosphate group; this is the 5 'tip. Two double-helix strands walked in opposite directions. Synthesis of nucleic acids, including DNA replication and transcription occurs in the direction of 5 '-> 3', since new nucleotides are added by dehydration reaction using 3 '' hydroxyl exposed as a nucleophile.
The expression of the gene encoded in DNA begins by transcribing the gene into RNA, a second type of nucleic acid very similar to DNA, but its monomer contains ribose sugar rather than deoxyribose. RNA also contains the basic uracil in place of thymine. RNA molecules are less stable than DNA and are usually single-stranded. The genes that encode proteins consist of a series of three nucleotide sequences called codons, which act as "words" in "genetic" languages. The genetic code determines correspondence during the translation of proteins between codons and amino acids. The genetic code is almost identical for all known organisms.
Chromosomes
The total gene complement in an organism or cell is known as its genome, which can be stored on one or more chromosomes. A chromosome consists of a single and very long DNA helix in which thousands of genes are encoded. The region of the chromosome where a particular gene is located is called its locus. Each locus contains one gene allele; However, members of the population may have different alleles at the loci, each with slightly different gene sequences.
The majority of eukaryotic genes are stored on a set of large linear chromosomes. Chromosomes are packed in nuclei in complexes with a storage protein called histones to form a unit called a nucleosome. DNA that is packed and condensed in this way is called chromatin. The manner in which DNA is stored in histones, as well as chemical modifications of the histone itself, governs whether a particular region of DNA is accessible for gene expression. In addition to genes, eukaryotic chromosomes contain sequences that are involved in ensuring that DNA is copied without degradation of the end region and sorted into child cells during cell division: replication of origin, telomeres and centromere. Original replication is the sequence region in which DNA replication begins to make two copies of the chromosome. Telomeres are a long sequence of repetitive sequences that limit the ends of linear chromosomes and prevent the degradation of encoding and regulatory areas during DNA replication. Telomere length decreases every time the genome is replicated and has been involved in the aging process. The centromere is needed to bind the spindle fibers to separate the chromatids into the child's cells during cell division.
Prokaryotes (bacteria and archaea) usually store their genomes on one large, circular chromosome. Similarly, some eukaryotic organelles contain residual circular chromosomes with a small number of genes. Prokaryotes sometimes supplement their chromosomes with the addition of a small circle of DNA called plasmids, which usually encode only a few genes and can be transferred between individuals. For example, genes for antibiotic resistance are usually encoded in bacterial plasmids and can be passed between individual cells, even from different species, via horizontal gene transfer.
While prokaryotic chromosomes are relatively dense genes, eukaryotes often contain areas of DNA that do not function clearly. Simple single cell eukaryotes have relatively small amounts of DNA, whereas the genomes of complex multicellular organisms, including humans, contain an absolute majority of DNA with no identified function. This DNA is often referred to as "junk DNA". However, more recent analyzes show that, although DNA coding proteins make up nearly 2% of the human genome, about 80% of the bases in the genome can be expressed, so the term "junk DNA" may be mistaken.
Structure and function
Structure
The gene structure consists of many elements whose sequence of actual protein coding is often only a small part. This includes the area of ​​undescribed DNA as well as the untranslated regions of RNA.
Flanking open reading, the genes contain the order of rules required for their expressions. First, genes need a promoter sequence. Promoters are recognized and bound by transcription factors and RNA polymerase to initiate transcription. Introduction usually occurs as a consensus sequence such as a TATA box. A gene can have more than one promoter, resulting in different messenger RNA (mRNA) in how far they extend at the 5 'end. Highly transcribed genes have a series of "strong" promoters that form strong associations with transcription factors, thus initiating transcription at high levels. Other genes have "weak" promoters that make up weak associations with transcription factors and initiate less frequent transcription. The eukaryotic promoter region is much more complex and difficult to identify than the prokaryotic promoter.
In addition, the gene can have a regulated area of ​​many upstream or downstream kilobases from the open reading frame that alter the expression. This acts by binding the transcription factor which then causes the sequential DNA so that the sequence of rules (and transcription factors are bound) becomes close to the RNA polymerase binding site. For example, enhancers enhance transcription by binding activator proteins which then help recruit RNA polymerase to the promoter; otherwise silencers bind repressor proteins and make the DNA less available for RNA polymerase.
The transcribed pre-mRNA contains an untranslated area at both ends that contains ribosome binding sites, terminators and start and stop codons. In addition, most open eukaryotic frames containing unspecified introns are deleted before the exons are translated. The sequence at the far end of the intron, dictates the splice site to produce a mature rodent mRNA that encodes a protein or RNA product.
Many prokaryotic genes are arranged into operons, with sequences of protein coding transcribed as one unit. The genes in the operon are transcribed as continuous messenger RNAs, called polycistronic mRNAs. The term cistron in this context is equivalent to genes. Transcription of operon mRNAs is often controlled by repressors that may occur in active or inactive states depending on the presence of specific specific metabolites. When active, the repressor binds the DNA sequence at the beginning of the operon, called the operator region, and represses the transcription of the operon; when the repressor is an inactive transcription of the operon may occur (see eg Lac operon). The gene operon product usually has a related function and is involved in the same network settings.
Functional definition
Determining precisely which part of the DNA sequence consists of genes is difficult. The regulatory region of genes such as enhancers does not have to be close to the sequence of encodings in linear molecules because mixed DNA can be looped to bring the gene and its regulatory region into proximity. Similarly, intron genes can be much larger than their exons. Regulatory areas can even be on completely different chromosomes and operate within trans: to allow regulatory areas on a single chromosome to come into contact with the target genes on other chromosomes.
Early work in molecular genetics suggests the concept that one gene makes one protein. This concept (originally called a single-gene hypothesis) emerged from an influential 1941 paper by George Beadle and Edward Tatum on experiments with the Neurospora crassa mushroom mutant. Norman Horowitz, an early colleague at the Neurospora study, recalled in 2004 that "these experiments found the science of so-called Beadle and Tatum biochemical genetics. Actually they proved to be an opening weapon in what is the molecular genetics and all the developments that have followed it. "The concept of one protein-gene has been perfected since the discovery of genes that can encode many proteins with alternative grafting and coding sequences split into short sections across the genome whose mRNAs are coupled with trans- splicing.
Broad operational definitions are sometimes used to encompass the complexities of this diverse phenomenon, in which genes are defined as the union of genome sequences that encode a potentially overlapping coherent set of functional products. This definition categorizes genes by their functional products (proteins or RNAs) rather than their specific DNA loci, with regulatory elements classified as gen-associated regions.
Gene expression
In all organisms, two steps are needed to read the information encoded in the gene's DNA and produce the protein it determines. First, the DNA genes are transcribed into messenger RNA (mRNA). Second, the mRNA is translated into protein. The RNA-coding gene still has to go through the first step, but not translated into the protein. The process of producing a biological functional molecule of either RNA or protein is called gene expression, and the resulting molecule is called the gene product.
Genetic code
The nucleotide sequence of the gene DNA determines the amino acid sequence of proteins through the genetic code. The set of three nucleotides, known as codons, each corresponds to a particular amino acid. The principle that three sequential series of DNA codes for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of the T4 bacteriophage (see Crick, Brenner et al.).
In addition, "start codon", and three "stop codons" indicate the beginning and end of the protein coding region. There are 64 possible codons (four possible nucleotides in each of three positions, 4 3 Ã, possible codons) and only 20 standard amino acids; then the code is redundant and some codons can determine the same amino acid. Correspondence between codons and amino acids is almost universal among all known living organisms.
Transcription
Transcription produces a single-stranded RNA molecule known as a messenger RNA, whose nucleotide sequence is complementary to the transcribed DNA. MRNA acts as an intermediate between the DNA gene and the final protein product. DNA genes are used as templates to produce complementary mRNAs. The MRNA fits the sequence of the gene DNA coding strand as it is synthesized as a complement to the mold strand. Transcription is done by an enzyme called RNA polymerase, which reads a mold strand in the 3 'to 5' direction and synthesizes RNA from 5 'to 3'. To begin transcription, the first polymerase recognizes and binds the promoter region of the gene. Thus, the main mechanism of gene regulation is to block or seize the promoter region, either by tight binding by repressor molecules that physically block the polymerase, or by regulating the DNA so that the promoter region is inaccessible.
In prokaryotes, transcription occurs in the cytoplasm; for very long transcripts, the translation can start at the end of RNA 5 while tip 3 is still transcribed. In eukaryotes, transcription occurs in the nucleus, where cell DNA is stored. The RNA molecule produced by the polymerase is known as the primary transcript and undergoes post-transcription modification before it is exported to the cytoplasm for translation. One of the modifications made is the intron connection which is a sequence in the transcription region that does not encode the protein. Alternative splicing mechanisms can produce mature transcripts of the same genes having different sequences and thus coding for different proteins. This is the main form of regulation in eukaryotic cells and also occurs in some prokaryotes.
Translation
Translation is the process by which mature mRNA molecules are used as templates to synthesize new proteins. Translation is done by ribosomes, a large complex of RNA and proteins responsible for conducting chemical reactions to add new amino acids to the growing polypeptide chain with the formation of peptide bonds. The genetic code reads three nucleotides at a time, in units called codons, by interaction with a special RNA molecule called RNA transfer (tRNA). Each tRNA has three unpaired bases known as anticodons that complement the codons read in mRNA. The TRNA is also bound covalently with the amino acids determined by complementary codons. When the tRNA binds its complementary codon on the mRNA strand, the ribosome attaches the amino acid charge to the new polypeptide chain, which is synthesized from the amino terminal to the carboxyl end. During and after synthesis, most new proteins must fold into their active three-dimensional structure before they can perform their cellular functions.
Rule
The gene is regulated so that it is only expressed when the product is needed, since the expression refers to a limited resource. Cells regulate the expression of their genes depending on their external environment (eg available nutrients, temperature and other stresses), their internal environment (eg cell division cycle, metabolism, infection status), and their specific role if they are in multicellular organisms. Gene expression can be adjusted in any step: from transcriptional initiation, RNA processing, to post-translational protein modification. The setting of the lactose metabolism gene in E. coli ( lac operon) was the first mechanism described in 1961.
RNA gene
The typical protein coding gene was first copied to RNA as an intermediate in the manufacture of final protein products. In other cases, RNA molecules are actual functional products, as in the synthesis of ribosomal RNA and RNA transfer. Some RNAs known as ribozymes are capable of enzymatic functioning, and microRNAs have a regulatory role. The DNA sequence from which the RNA is transcribed is known as the non-coding RNA gene.
Some viruses store their entire genome in the form of RNA, and do not contain any DNA at all. Because they use RNA to store genes, their cellular hosts can synthesize their proteins as soon as they are infected and without delay awaiting transcription. On the other hand, retrovirus RNA, like HIV, requires their reverse transcription of genomes from RNA into DNA before their proteins can be synthesized. RNA-mediated epigenetic inheritance has also been observed in plants and is very rare in animals.
Inheritance
Organisms inherit their genes from their parents. Asexual organisms only inherit a full copy of their parent's genome. Sexual organisms have two copies of each chromosome because they inherit a complete set of each parent.
Mendelian Heritage
According to Mendelian inheritance, variations in organism phenotypes (observable physical and behavioral characteristics) are partly due to genotype variation (specific gene pools). Each gene defines a particular property with different gene sequences (alleles) that give rise to different phenotypes. Most eukaryotic organisms (such as Mendel's pea plants work) have two alleles for each trait, one inherited from each parent.
Alleles in the locus may be dominant or recessive; dominant alleles produce an appropriate phenotype when paired with another allele for the same properties, whereas the recessive allele gives rise to an appropriate phenotype only when paired with another copy of the same allele. If you know the genotype of the organism, you can determine which alleles are dominant and which are recessive. For example, if the allele determines the high stem in the dominant peanut plant above the allele that determines the short stem, then the bean plant that inherits one high allele from one parent and one short allele from another will also have a high stem. Mendel's work shows that alleles pair 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 a single gene (including a number of famous genetic disorders) it does not include the physical process of DNA replication and cell division.
DNA replication and cell division
The growth, development, and reproduction of an organism depend on cell division; a process in which a single cell is divided into two daughter cells that are usually identical. This requires first duplicating copies of each gene in the genome in a process called DNA replication. The copy is made by a special enzyme known as DNA polymerase, which "reads" a strand of double helical DNA, known as a mold strand, and synthesizes a new complementary strand. Because the DNA double helix is ​​held together by the base pair, the sequence of one strand actually determines its complement order; then only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is semiconservative; ie, the copy of the genome inherited by each child cell contains one original DNA and one newly synthesized DNA strand.
The rate of DNA replication in living cells was first measured as the rate of elongation of T4 fossa DNA in the infected phage E. coli and was found to be very rapid. During the period of exponential DNA increase at 37 ° C, the elongation rate is 749 nucleotides per second.
After DNA replication is complete, the cell must physically separate the two copies of the genome and divide it into two different membrane-bound cells. In prokaryotes (bacteria and archaea) this usually occurs through a relatively simple process called binary division, where each circular genome is attached to the cell membrane and separated into the child's cell as an evolved membrane to divide the cytoplasm into two parts bound to the membrane.. Binary cleavage is very rapid compared to the rate of cell division in eukaryotes. Eukaryotic cell division is a more complex process known as the cell cycle; DNA replication occurs during this cycle phase known as S phase, whereas the process of chromosomal separation and cytoplasmic breakdown occurs during the M. phase
Molecular relics
Duplication and transmission of genetic material from one cell generation to the next is the basis of molecular inheritance, and the relationship between classical and molecular gene images. Organisms inherit the characteristics of their parents because the hereditary cells contain a copy of the gene in their parent cell. In asexually reproducing organisms, the offspring will become genetic or artificial copies of the parent organism. In sexually reproducing organisms, a special form of cell division called meiosis produces a cell called a gamete or a haploid germ cell, or contains only one copy of each gene. Gametes produced by women are called eggs or ova, and those produced by men are called sperm. Two gametes fuse to form a diploid 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.
During the process of meiotic cell division, an event called genetic recombination or crossing-over can occasionally occur, in which the length of DNA on a single chromatid is exchanged for the length of DNA in the corresponding non-homologous. chromatid sisters. This can result in reassortment of connected alleles. Mendel's principle of independent assertion asserts that each of the two parent genes for each trait will independently sort into the gamete; which alleles are inherited by an organism for one trait unrelated to which allele is inherited for another trait. This actually only applies to genes that are not on the same chromosome, or are located very far from each other on the same chromosome. The two closer genes lie on the same chromosome, the closer the genes will be associated in the gametes and the more often they come together; very close genes are essentially never separated because it is highly unlikely that crossover points will occur between them. This is known as a genetic relationship. The main contribution to the idea of ​​a genetic relationship was made in the early 1900s by Edith Saunders, who was part of the Cambridge team of William Bateson. He did this by studying the inheritance of different traits in plant leaves such as the furry and wavy crops of plants, like the Matthiola, providing Saunders and excellent models for his interbreeding experiments. In his experiments, he found that the proportion of the expected progeny was different from what he expected. While he expects the phenoytpes of his cross hybridization in 9: 3: 3: 1 proportions, he finds that progeny appears in 9: 7 proportions, suggesting that the two factors interact in several ways to create this difference by masking the appearance of two other phenotypes. This leads him to the conclusion that some properties are related to each other because of their proximity to each other on chromosomes. This gives him a reason to determine the difference between independent allele and codependent.
Molecular evolution
Mutations
DNA replication is for the most part very accurate, but mistakes (mutations) do occur. The rate of errors in eukaryotic cells can reach 10 -8 per nucleotide per replication, whereas for some RNA viruses it can be as high as 10 -3 . This means that every generation, every human genome accumulates 1-2 new mutations. Minor mutations can be caused by DNA replication and due to DNA damage and include point mutations in which a single base is altered and a frameshift mutation in which a single base is inserted or removed. One of these mutations can change the gene by missense (change the codon to encode the different amino acids) or nonsense (premature stop codon). Larger mutations may be caused by recombination errors that cause chromosomal abnormalities including duplication, deletion, rearrangement or inversion of large parts of chromosomes. In addition, DNA repair mechanisms can introduce mutational defects when repairing physical damage to molecules. Improvements, even with mutations, are more important to survive than to return exact copies, such as when repairing multiple strand damage.
When several different alleles for genes are present in the species population it is called polymorphic. The most distinct alleles are functionally equivalent, but some alleles may give rise to different phenotypic properties. A gene allele is most commonly called wild type, and rare alleles are called mutants. The genetic variation in the relative frequency of different alleles in a population is due to natural selection and genetic drift. The wild-type allele is not necessarily the common ancestor of alleles, nor does it need to be fit.
Most mutations in genes are neutral, not affecting the phenotype of the organism (silent mutation). Some mutations do not alter the amino acid sequence because some codons encode the same amino acid (synonym mutation). Other mutations may be neutral if they cause changes in the amino acid sequence, but proteins still function similarly to new amino acids (eg conservative mutations). Many mutations, however, are destructive or even deadly, and are removed from the population by natural selection. Genetic disorders are the result of damaging mutations and can be caused by spontaneous mutations in affected individuals, or may be inherited. Finally, a small fraction of mutations are beneficial, improving the fitness of the organism and are essential for evolution, since the selection of their direction leads to adaptive evolution.
Homology sequence
Genes with the latest common ancestors, and thus common evolutionary ancestors, are known as homologists. These genes appear either from the duplication of genes in the genomes of organisms, where they are known as paralog genes, or are the result of gene divergence after a speciation event, where they are known as orthologic genes, and often perform similar or similar functions. in related organisms. It is often assumed that the function of the ortholog gene is more similar than that of a paralogical gene, although the difference is minimal.
The relationship between genes can be measured by comparing the alignment of their DNA sequences. The degree of sequence similarity between homologous genes is called a preserved sequence. Most of the changes in the gene sequence do not affect its function so that the gene accumulates mutations over time by neutral molecular evolution. In addition, any choice of genes will cause the sequence to deviate at different levels. Genes under stable selection are limited and change more slowly while genes under the direction selection change faster order. The sequence difference between genes can be used for phylogenetic analysis to study how the gene evolved and how the organism originated.
The origin of new genes
The most common source of new genes in eukaryotic lineages is the duplication of genes, which creates variations in the number of copies of genes present in the genome. The resulting gene (paralog) can then deviate consecutively and in function. The set of genes formed in this way form the gene family. Gene duplication and loss in a family are common and are a major source of evolutionary biodiversity. Sometimes, gene duplication may produce a copy of a malfunctioning gene, or a functional copy may be subject to a mutation resulting in a loss of function; Such nonfunctional genes are called pseudogenes.
The "Orphan" gene, whose sequence shows no similarity to the genes present, is less common than the duplicate genes. Estimates of the number of genes without homologues outside the human range from 18 to 60. The two main sources of the protein-coding orphan genes are gene duplication followed by a very rapid sequence change, so the original relationship is undetectable by sequence comparison, and de novo conversion of non-coding sequences previously a protein coding gene. Gen de novo is usually shorter and simpler in structure than eukaryotic genes, with little if any introns. During a long period of evolution, the birth of the de novo gene may be responsible for a significant fraction of the taxonomic gene family.
Horizontal gene transfer refers to the transfer of genetic material through mechanisms other than reproduction. This mechanism is a common source of new genes in prokaryotes, sometimes thought to contribute more to genetic variation than to gene duplication. This is a common way to spread antibiotic resistance, virulence, and adaptive metabolic function. Although horizontal gene transfer is uncommon in eukaryotes, examples may have been identified from the protist genome and algae containing bacterial genes.
Genome
The genome is the total genetic material of an organism and includes both genes and non-coding sequences.
Number of genes
The size of the genome, and the number of genes encoded vary between organisms. The smallest genome occurs in viruses, and viroids (which act as a single non-coding RNA gene). In contrast, plants can have a very large genome, with rice containing & gt; 46,000 protein-coding genes. The total number of genes coding proteins (proteome Earth) is estimated at 5 million sequences.
Although the number of DNA base pairs in the human genome has been known since the 1960s, estimates of the number of genes have changed over time as gene definitions, and methods for detecting them have been perfected. Preliminary theoretical predictions of the number of human genes as high as 2,000,000. Initial experimental steps showed there were genes up to 50,000-100,000 genes written (expressed sequence tags). Furthermore, sequencing in the Human Genome Project shows that many of these transcripts are alternative variants of the same genes, and the total number of protein-encoding genes is revised down to ~ 20,000 with 13 genes encoded in the mitochondrial genome. With the GENCODE annotation project, that estimate continues to fall to 19,000. The human genome, only 1-2% consists of protein-encoding genes, with the rest being non-coding DNA such as introns, retrotransposons, and non-coded RNA. Each multicellular organism has all the genes in every cell of its body but not every gene works in every cell.
Essential gene
The essential gene is a set of genes that are considered essential for the survival of the organism. This definition assumes the abundant availability of all relevant nutrients and the absence of environmental stress. Only a small part of the genes of the organism are important. In bacteria, about 250-400 genes are important for Escherichia coli and Bacillus subtilis , which is less than 10% of their genes. Half of these genes are orthologists in both organisms and are mostly involved in protein synthesis. In beginner yeast Saccharomyces cerevisiae the number of essential genes is slightly higher, in 1000 genes (~ 20% of their genes). Although this amount is more difficult to measure in higher eukaryotes, mice and humans are estimated to have about 2,000 essential genes (~ 10% of their genes). Synthetic organisms, Syn 3 , have a genome of at least 473 essential genes and quasi-essential genes (necessary for rapid growth), although 149 have unknown functions.
Essential genes include the Housekeeping gene (essential for basic cell functions) as well as genes expressed at different times in the development of organisms or life cycles. Housekeeping genes are used as experimental controls when analyzing gene expression, because they are constitutively expressed at a relatively constant level.
Genetic Nomenclature and Genome
The gene nomenclature has been established by the HUGO Gen Nomenography Committee (HGNC) for each known human gene in the form of an approved gene name and symbol (short form abbreviation), accessible via a database maintained by HGNC. Symbols are chosen to be unique, and each gene has only one symbol (though the approved symbols sometimes change). Symbols are preferably kept consistent with other members of the gene family and with homologs in other species, especially mice for their role as common model organisms.
Genetic engineering
Genetic engineering is a modification of the genomes of organisms through biotechnology. Since the 1970s, various techniques have been developed to specifically add, remove and edit genes within an organism. The recently developed genome engineering technique uses a nuclease enzyme that is engineered to make targeted DNA improvements in chromosomes to disrupt or edit genes when breakthroughs are improved. The term related synthetic biology is sometimes used to refer to the extensive genetic engineering of an organism.
Genetic engineering is now a regular research tool with model organisms. For example, genes are easily added to bacteria and lineages of knockout mice with the function of certain disrupted genes used to investigate gene function. Many organisms have been genetically modified for applications in agriculture, industrial biotechnology, and medicine.
For multicellular organisms, the embryo is usually engineered that grows into a genetically modified adult organism. However, cell genomes in adult organisms can be edited using gene therapy techniques to treat genetic diseases.
See also
References
External links
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