Chromosome 13 is one of the 23 pairs of chromosomes in humans. People have two copies of this chromosome. Chromosome 13 spans about 114 million base pairs and represents between 3.5 and 4% of the total DNA in cells. The following are some of the gene count estimates of human chromosome 13; because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes; the following is a partial list of genes on human chromosome 13. For complete list, see the link in the infobox on the right; the following diseases and disorders are some of those related to genes on chromosome 13: The following conditions are caused by changes in the structure or number of copies of chromosome 13: Retinoblastoma: A small percentage of retinoblastoma cases are caused by deletions in the region of chromosome 13 containing the RB1 gene.
Children with these chromosomal deletions may have mental retardation, slow growth, characteristic facial features. Researchers have not determined which other genes are located in the deleted region, but a loss of several genes is responsible for these developmental problems. Trisomy 13: Trisomy 13 occurs when each cell in the body has three copies of chromosome 13 instead of the usual two copies. Trisomy 13 can result from an extra copy of chromosome 13 in only some of the body's cells. In a small percentage of cases, trisomy 13 is caused by a rearrangement of chromosomal material between chromosome 13 and another chromosome; as a result, a person has the two usual copies of chromosome 13, plus extra material from chromosome 13 attached to another chromosome. These cases are called translocation trisomy 13. Extra material from chromosome 13 disrupts the course of normal development, causing the characteristic signs and symptoms of trisomy 13. Researchers are not yet certain how this extra genetic material leads to the features of the disorder, which include abnormal cerebral functions, a small cranium, non functional eyes and heart defects.
Other chromosomal conditions: Partial monosomy 13q is a rare chromosomal disorder that results when a piece of the long arm of chromosome 13 is missing. Infants born with partial monosomy 13q may exhibit low birth weight, malformations of the head and face, skeletal abnormalities, other physical abnormalities. Mental retardation is characteristic of this condition; the mortality rate during infancy is high among individuals born with this disorder. All cases of partial monosomy 13q occur randomly for no apparent reason. National Institutes of Health. "Chromosome 13". Genetics Home Reference. Retrieved 2017-05-06. "Chromosome 13". Human Genome Project Information Archive 1990–2003. Retrieved 2017-05-06
Chromosome 14 is one of the 23 pairs of chromosomes in humans. People have two copies of this chromosome. Chromosome 14 spans about 107 million base pairs and represents between 3 and 3.5% of the total DNA in cells. The centromere of chromosome 14 is positioned at position 17.2 Mbp. The following are some of the gene count estimates of human chromosome 14; because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes; the following is a partial list of genes on human chromosome 14. For complete list, see the link in the infobox on the right; the following diseases are some of those related to genes on chromosome 14: National Institutes of Health. "Chromosome 14". Genetics Home Reference. Retrieved 2017-05-06. "Chromosome 14".
Human Genome Project Information Archive 1990–2003. Retrieved 2017-05-06
Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome, with 48 million nucleotides representing about 1.5 percent of the total DNA in cells. Most people have two copies of chromosome 21, while those with three copies of chromosome 21 have Down syndrome called "trisomy 21". Researchers working on the Human Genome Project announced in May 2000 that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be sequenced, after chromosome 22; the following are some of the gene count estimates of human chromosome 21. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.
The following is a partial list of genes on human chromosome 21. For complete list, see the link in the infobox on the right; the following diseases are some of those related to genes on chromosome 21: Alzheimer's disease Amyotrophic lateral sclerosis Autoimmune polyendocrine syndrome Down syndrome Erondu–Cymet syndrome Holocarboxylase synthetase deficiency Homocystinuria Jervell and Lange-Nielsen syndrome Leukocyte adhesion deficiency Majewski osteodysplastic primordial dwarfism type II Nonsyndromic deafness Romano–Ward syndrome The following conditions are caused by changes in the structure or number of copies of chromosome 21: Cancers: Rearrangements of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemia has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia, acute myeloid leukemia, has been associated with a translocation between chromosomes 8 and 21.
In a small percentage of cases, Down syndrome is caused by a rearrangement of chromosomal material between chromosome 21 and another chromosome. As a result, a person has the usual two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome; these cases are called translocation Down syndrome. Researchers believe that extra copies of genes on chromosome 21 disrupt the course of normal development, causing the characteristic features of Down syndrome and the increased risk of medical problems associated with this disorder. Other changes in the number or structure of chromosome 21 can have a variety of effects, including intellectual disability, delayed development, characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell and a circular structure called ring chromosome 21. A ring chromosome occurs. Duplication in Amyloid precursor protein locus on Chromosome 21 was found to cause early onset familial Alzheimer's disease in a French family set and a Dutch family set.
Compared to Alzheimer's caused by missense mutations in APP, the frequency of the Alzheimer's caused by APP duplications is significant. All patients that have an extra copy of APP gene due to the locus duplication show Alzheimer's with severe cerebral amyloid angiopathy. Antonarakis SE, Lyle R, Dermitzakis ET, Reymond A, Deutsch S. "Chromosome 21 and down syndrome: from genomics to pathophysiology". Nat Rev Genet. 5: 725–38. Doi:10.1038/nrg1448. PMID 15510164. Antonarakis SE, Lyle R, Deutsch S, Reymond A. "Chromosome 21: a small land of fascinating disorders with unknown pathophysiology". Int J Dev Biol. 46: 89–96. PMID 11902692. Antonarakis SE. "Chromosome 21: from sequence to applications". Curr Opin Genet Dev. 11: 241–6. Doi:10.1016/S0959-437X00185-4. PMID 11377958. Gilbert F. "Disease genes and chromosomes: disease maps of the human genome. Chromosome 21". Genet Test. 1: 301–6. Doi:10.1089/gte.1997.1.301. PMID 10464663. Hattori M, Fujiyama A, Taylor TD, Watanabe H, Yada T, Park HS, Toyoda A, Ishii K, Totoki Y, Choi DK, Groner Y, Soeda E, Ohki M, Takagi T, Sakaki Y, Taudien S, Blechschmidt K, Polley A, Menzel U, Delabar J, Kumpf K, Lehmann R, Patterson D, Reichwald K, Rump A, Schillhabel M, Schudy A, Zimmermann W, Rosenthal A, Kudoh J, Schibuya K, Kawasaki K, Asakawa S, Shintani A, Sasaki T, Nagamine K, Mitsuyama S, Antonarakis SE, Minoshima S, Shimizu N, Nordsiek G, Hornischer K, Brant P, Scharfe M, Schon O, Desario A, Reichelt J, Kauer G, Blocker H, Ramser J, Beck A, Klages S, Hennig S, Riesselmann L, Dagand E, Haaf T, Wehrmeyer S, Borzym K, Gardiner K, Nizetic D, Francis F, Lehrach H, Reinhardt R, Yaspo ML.
"The DNA sequence of human chromosome 21". Nature. 405: 311–9. Doi:10.1038/35012518. PMID 10830953. Sawinska M, Ladon D. "Mechanism and clinical significance of the reciprocal translocation t in the children suffering from acute lymphoblastic leukaemia". Leuk Res. 28: 35–42. Doi:10.1016/S0145-212600160-7. PMID 14630078. Rovelet-Lecrux A, Hannequin D, Raux G, Le Meur N, Laquerriere A, Vital A, Dumanchin C, Feuillette S, Brice A, Vercelletto M, Dubas F, Frebourg T, Campion D. "APP locus duplication causes autosomal dominant early-onset Alzheimer disease with cerebral amyloid angiopathy". Nature Genetics. 38: 24–6. Doi:10.1038/ng1718. PMID 16369530. Anita Rauch.
A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is used for the complete set of chromosomes in a species or in an individual organism and for a test that detects this complement or measures the number. Karyotypes describe the chromosome count of an organism and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, any other physical characteristics; the preparation and study of karyotypes is part of cytogenetics. The study of whole sets of chromosomes is sometimes known as karyology; the chromosomes are depicted in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size. The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. In the germ-line the chromosome number is n.p28 Thus, in humans 2n = 46.
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells haploid cells have single copies; the study of karyotypes is important for cell biology and genetics, the results may be used in evolutionary biology and medicine. Karyotypes can be used for many purposes. Chromosomes were first observed in plant cells by Carl Wilhelm von Nägeli in 1842, their behavior in animal cells was described by Walther Flemming, the discoverer of mitosis, in 1882. The name was coined by another German anatomist, Heinrich von Waldeyer in 1888, it is New Latin from Ancient Greek κάρυον karyon, "kernel", "seed", or "nucleus", τύπος typos, "general form"). The next stage took place after the development of genetics in the early 20th century, when it was appreciated that chromosomes were the carrier of genes. Lev Delaunay in 1922 seems to have been the first person to define the karyotype as the phenotypic appearance of the somatic chromosomes, in contrast to their genic contents.
The subsequent history of the concept can be followed in the works of C. D. Darlington and Michael JD White. Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain? In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism. Painter in 1922 was not certain whether the diploid of humans was 46 or 48, at first favoring 46, but revised his opinion from 46 to 48, he insisted on humans having an XX/XY system. Considering the techniques of the time, these results were remarkable. In textbooks, the number of human chromosomes remained at 48 for over thirty years. New techniques were needed to correct this error. Joe Hin Tjio working in Albert Levan's lab was responsible for finding the approach: Using cells in tissue culture Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes Arresting mitosis in metaphase by a solution of colchicine Squashing the preparation on the slide forcing the chromosomes into a single plane Cutting up a photomicrograph and arranging the result into an indisputable karyogram.
The work took place in 1955, was published in 1956. The karyotype of humans includes only 46 chromosomes; the great apes have 48 chromosomes. Human chromosome 2 is now known to be a result of an end-to-end fusion of two ancestral ape chromosomes; the study of karyotypes is made possible by staining. A suitable dye, such as Giemsa, is applied after cells have been arrested during cell division by a solution of colchicine in metaphase or prometaphase when most condensed. In order for the Giemsa stain to adhere all chromosomal proteins must be digested and removed. For humans, white blood cells are used most because they are induced to divide and grow in tissue culture. Sometimes observations may be made on non-dividing cells; the sex of an unborn fetus can be determined by observation of interphase cells. Six different characteristics of karyotypes are observed and compared: Differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family.
For example, the legumes Lotus tenuis and Vicia faba each have six pairs of chromosomes, yet V. faba chromosomes are many times larger. These differences reflect different amounts of DNA duplication. Differences in the position of centromeres; these differences came about through translocations. Differences in relative size of chromosomes; these differences arose from segmental interchange of unequal lengths. Differences in basic number of chromosomes; these differences could have resulted from successive unequal translocations which removed all the essential genetic material from a chromosome, permitting its loss without penalty to the organism or through fusion. Humans have one pair fewer chromosomes than the great apes. Human chromosome 2 appears to have resulted from the fusion of two ancestral chromosomes, many of the genes of those two original chromosomes have been translocated to other chromosomes. Differences in number and position of satellites. Satellites are small bodies attached to a chromosome by a thin thread.
Differences in degree and distribution of heterochromatic regions. Het
Biologically, a child is a human being between the stages of birth and puberty, or between the developmental period of infancy and puberty. The legal definition of child refers to a minor, otherwise known as a person younger than the age of majority. Child may describe a relationship with a parent or, metaphorically, an authority figure, or signify group membership in a clan, tribe, or religion. Biologically, a child is a person between birth and puberty, or the period of human development from infancy to puberty; the term child may refer to anyone below the age of majority or some other age limit. The United Nations Convention on the Rights of the Child defines child as "a human being below the age of 18 years unless under the law applicable to the child, majority is attained earlier"; this is ratified by 192 of 194 member countries. The term child may refer to someone below another defined age limit unconnected to the age of majority. In Singapore, for example, a child is defined as someone under the age of 14 under the "Children and Young Persons Act" whereas the age of majority is 21.
In U. S. Immigration Law, a child refers to anyone, under the age of 21; some English definitions of the word child include the fetus. In many cultures, a child is considered an adult after undergoing a rite of passage, which may or may not correspond to the time of puberty. Children have fewer rights than adults and are classed as unable to make serious decisions, must always be under the care of a responsible adult or child custody, whether their parents divorce or not. Recognition of childhood as a state different from adulthood began to emerge in the 16th and 17th centuries. Society began to relate to the child not as a miniature adult but as a person of a lower level of maturity needing adult protection and nurturing; this change can be traced in paintings: In the Middle Ages, children were portrayed in art as miniature adults with no childlike characteristics. In the 16th century, images of children began to acquire a distinct childlike appearance. From the late 17th century onwards, children were shown playing with toys and literature for children began to develop at this time.
According to Professor Peter Jones of Cambridge university development of the brain continues long past legal definitions of adulthood so "to have a definition of when you move from childhood to adulthood looks absurd. It's a much more nuanced transition that takes place over three decades." Children go through stages of social development. Children learn through play and in most societies through formal schooling; as a child is growing they are learning. They learn how to prioritize their actions, their behavior is transcending. They learn how to learn new behavior. Children with ADHD and learning disabilities may need extra help to develop social skills; the impulsive characteristics of an ADHD child may lead to poor peer relationships. Children with poor attention spans may not tune into social cues in their environment, making it difficult for them to learn social skills through experience. Health issues affecting children are managed separately from those affecting adults, by pediatricians; the age at which children are considered responsible for their society-bound actions has changed over time, this is reflected in the way they are treated in courts of law.
In Roman times, children were regarded as not culpable for crimes, a position adopted by the Church. In the 19th century, children younger than seven years old were believed incapable of crime. Children from the age of seven forward were considered responsible for their actions. Therefore, they could face criminal charges, be sent to adult prison, be punished like adults by whipping, branding or hanging. Minimum employment age and marriage age vary; the age limit of voluntary/involuntary military service is disputed at the international level. During the early 17th century in England, about two-thirds of all children died before the age of four. During the Industrial Revolution, the life expectancy of children increased dramatically, and this has continued. Child mortality rates have fallen across the world. About 12.6 million under-five infants died worldwide in 1990, which declined to 6.6 million in 2012. The infant mortality rate dropped from 90 deaths per 1,000 live births in 1990, to 48 in 2012.
The highest average infant mortality rates are in sub-Saharan Africa, at 98 deaths per 1,000 live births – over double the world's average. Education, in the general sense, refers to the act or process of imparting or acquiring general knowledge, developing the powers of reasoning and judgment, preparing intellectually for mature life. Formal education most takes place through schooling. A right to education has been recognized by some governments. At the global level, Article 13 of the United Nations' 1966 International Covenant on Economic and Cultural Rights recognizes the right of everyone to an education. Education is compulsory in most places up to a certain age, but attendance at school may not be, with alternative options such as home-schooling or e-learning being recognized as valid forms of education in certain jurisdictions. Children in some countries are kept out of school, or attend only for short periods. Data from UNICEF indicate
A fusion gene is a hybrid gene formed from two separate genes. It can occur as a result of: interstitial deletion, or chromosomal inversion; the first fusion gene was described in cancer cells in the early 1980s. The finding was based on the discovery in 1960 by Peter Nowell and David Hungerford in Philadelphia of a small abnormal marker chromosome in patients with chronic myeloid leukemia—the first consistent chromosome abnormality detected in a human malignancy designated the Philadelphia chromosome. In 1973, Janet Rowley in Chicago showed that the Philadelphia chromosome had originated through a translocation between chromosomes 9 and 22, not through a simple deletion of chromosome 22 as was thought. Several investigators in the early 1980s showed that the Philadelphia chromosome translocation led to the formation of a new BCR/ABL1 fusion gene, composed of the 3' part of the ABL1 gene in the breakpoint on chromosome 9 and the 5' part of a gene called BCR in the breakpoint in chromosome 22.
In 1985 it was established that the fusion gene on chromosome 22 produced an abnormal chimeric BCR/ABL1 protein with the capacity to induce chronic myeloid leukemia. At present, scientists have identified 21,404 gene fusions; these genes have been found in all main subtypes of human neoplasia. The identification of these fusion genes play a prominent role in being a diagnostic and prognostic marker, it has been known for 30 years that the corresponding gene fusion plays an important role in tumorgenesis. Fusion genes can contribute to tumor formation because fusion genes can produce much more active abnormal protein than non-fusion genes. Fusion genes are oncogenes that cause cancer. In the case of TMPRSS2-ERG, by disrupting androgen receptor signaling and inhibiting AR expression by oncogenic ETS transcription factor, the fusion product regulates the prostate cancer. Most fusion genes are found from hematological cancers and prostate cancer. BCAM-AKT2 is a fusion gene, specific and unique to high-grade serous ovarian cancer.
Oncogenic fusion genes may lead to a gene product with a new or different function from the two fusion partners. Alternatively, a proto-oncogene is fused to a strong promoter, thereby the oncogenic function is set to function by an upregulation caused by the strong promoter of the upstream fusion partner; the latter is common in lymphomas, where oncogenes are juxtaposed to the promoters of the immunoglobulin genes. Oncogenic fusion transcripts may be caused by trans-splicing or read-through events. Since chromosomal translocations play such a significant role in neoplasia, a specialized database of chromosomal aberrations and gene fusions in cancer has been created; this database is called Mitelman Database of Chromosome Aberrations and Gene Fusions in Cancer. Presence of certain chromosomal aberrations and their resulting fusion genes is used within cancer diagnostics in order to set a precise diagnosis. Chromosome banding analysis, fluorescence in situ hybridization, reverse transcription polymerase chain reaction are common methods employed at diagnostic laboratories.
These methods all have their distinct shortcomings due to the complex nature of cancer genomes. Recent developments such as high-throughput sequencing and custom DNA microarrays bear promise of introduction of more efficient methods. Gene fusion plays a key role in the evolution of gene architecture. We can observe its effect. Duplication, sequence divergence, recombination are the major contributors at work in gene evolution; these events can produce new genes from existing parts. When gene fusion happens in non-coding sequence region, it can lead to the misregulation of the expression of a gene now under the control of the cis-regulatory sequence of another gene. If it happens in coding sequences, gene fusion cause the assembly of a new gene it allows the appearance of new functions by adding peptide modules into multi domain protein; the detecting methods to inventory gene fusion events on a large biological scale can provide insights about the multi modular architecture of proteins. In recent years, next generation sequencing technology has become available to screen known and novel gene fusion events on a genome wide scale.
However, the precondition for large scale detection is a paired-end sequencing of the cell's transcriptome. The direction of fusion gene detection is towards data analysis and visualization; some researchers developed a new tool called Transcriptome Viewer to directly visualize detected gene fusions on the transcript level. Biologists may deliberately create fusion genes for research purposes; the fusion of reporter genes to the regulatory elements of genes of interest allows researches to study gene expression. Reporter gene fusions can be used to measure activity levels of gene regulators, identify the regulatory sites of genes, identify various genes that are regulated in response to the same stimulus, artificially control the expression of desired genes in particular cells. For example, by creating a fusion gene of a protein of interest and green fluorescent protein, the protein of interest may be observed in cells or tissue using fluorescence microscopy; the protein synthesized when a fusion gene is expressed is called a fusion protein.
ETV6-NTRK3 gene fusion ChiTaRS 3.1: The Improved Database of Chimeric Ttanscripts and RNA-seq Data. ChiPPI: The Server Protein-Protein Interaction of Chimeric Proteins. ChimerDB 2.0
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 developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes as well as gene–environment interactions; some genetic traits are visible, such as eye color or number of limbs, 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 different versions of a protein, which cause different phenotypical traits.
Usage of the term "having a gene" 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. For example, regulatory regions of a gene can be far removed from its coding regions, 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, that means offspring and procreation; the existence of discrete inheritable units was first suggested by Gregor Mendel. From 1857 to 1864, in Brno, he studied inheritance patterns in 8000 common edible pea plants, tracking distinct traits from parent to offspring.
He described these mathematically as 2n combinations where n is the number of differing characteristics in the original peas. Although he did not use the term gene, he explained his results in terms of discrete inherited units that give rise to observable physical characteristics; this description prefigured Wilhelm Johannsen's distinction between phenotype. Mendel was the first to demonstrate independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance, which suggested that each parent contributed fluids to the fertilisation process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis, from Greek pan and genesis / genos. Darwin used the term gemmule to describe hypothetical particles. Mendel's work went unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries, Carl Correns, Erich von Tschermak, who reached similar conclusions in their own research.
In 1889, Hugo de Vries published his book Intracellular Pangenesis, 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", after Darwin's 1868 pangenesis theory. Sixteen years in 1905, Wilhelm Johannsen introduced the term'gene' and William Bateson that of'genetics' while Eduard Strasburger, amongst others, still used the term'pangene' for the fundamental physical and functional unit of heredity. Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s; the structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography, which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication.
In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities, indivisible by recombination and arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 showed that individual genes have a simple linear structure and are to be equivalent to a linear section of DNA. Collectively, this body of research established the central dogma of molecular biology, which states that proteins are translated from RNA, transcribed from DNA; this dogma has since been shown to have exceptions, such as 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; 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.
An automated version of the Sanger method was used in early phases of the