History of Cytogenetics

After his origin, man began to think about the things around him – the earth, the sky, the mountains, the rivers, the animals, the plants and about himself. Whatever “knowledge” he could gather, he transferred it to his progeny/off-spring, and thus there was an accumulation of “knowledge” about various things.

Since long, man has been thinking about “life” and still it is not possible to fully understand the mystery. Various thinkers were born on the earth and various religions flourished which gave various concepts of the “origin of life”.

Living organisms are made up of the same atoms and elements as nonliving things. But what thing/force makes them (atoms, elements, molecules) to function as “living”/”life” is a question still unsolved. One of the most important properties of life is “continuity”. It is known that life originates from the life already existing.

After the invention of microscope in the 17th century, the internal structure of plants and animals could be known. In 1665, Robert Hooke showed that the cork was made of numerous units like those in honey comb, and he named these units as cells. It was the beginning of the science of cytology which seeks to understand the structural organisation of cells.

Leeuwenhoek observed bacteria and protozoa with the help of microscope and thus micro-organisms could be known. Through the work of numerous investigators, different parts of the cell and life processes came to our knowledge.

Following the rediscovery of Mendel’s laws of inheritance in 1900, the science of Genetics began to take shape; Genetics sought to understand the basis and the inheritance of variation present in the organisms.

The concepts of cytology and genetics were combined together to form the science of Cytogenetics. This fusion came about with realization that “units” or “factors” (genes) that govern different characters were in fact situated in the chromosomes.

Nucleic acid from cells was isolated by Miescher in 1869, but only in 1944, Avery and his associates showed that DNA (deoxyribonucleic acid) is the genetic material. The structure, replication, function and other aspects of nucleic acids were subsequently studied and discovered by different workers.

Elucidation of “genetic code” was achieved during the 1960’s. Gene was artificially synthesized for the first time in 1975 by Khorana and coworkers.

The science of Cytogenetics is advancing at a tremendous rate, and it covers all the aspects of life-improvement of plants, resistance to various diseases and pests in plants, improvement of animals, cure of genetic disorders in human, production of rare and essential enzymes and other substances.

It is hoped that the molecular cytogenetics will do much for the welfare of human beings. But, to appreciate the route this science has covered, the chronology of various important developments is presented here.

1665:

Robert Hooke (1635-1703) observed the section of cork by a primitive type of microscope and found that it was made up of small hollow units like honey comb. He termed the structural unit as cell. In those days microscopes could magnify the objects up to 100-200 times.

1672:

Nehemiah Grew (1628-1711) published microscopic anatomy of plants and advanced a theory that the stamen corresponds to male, while pistil corresponds to female reproductive organ.

1674:

Anthony van Leeuwenhoek (1632-1723) improved the microscope by the technique of grinding the lens. He reported the studies on free cells such as protozoa and bacteria and thus opened the door to a new world of microorganisms. He also studied blood cells.

1694:

Rudolf Jacob Camerarius (1665-1721) showed the presence of sex in plants. He demonstrated that in maize, seeds are not produced unless pollen is applied to the pistil. He concluded that pollen is the male element, while pistil is the female element.

1761-1766:

Joseph Gottlieb Kolreuter (1733-1806) published the information on his work of plant hybridization. He made hybrids between different varieties of tobacco and between some plant genera. By using reciprocal crosses, he showed the equal contribution of male and female parents to their offspring.

1809:

Jean Baptiste Lamarck (1744-1829) proposed a theory of evolution known as theory of inheritance of acquired characters.

Briefly, the main points of his theory are :

1. Variation in an individual is brought about by:

(a) conscious effort,

(b) reaction to environment, and

(c) use and disuse of the organ.

2. Heredity carries forward the changes that are acquired during the life time of the individual.

1828:

Robert Brown (1773-1858) studied plant cells and discovered nucleus in Tradescantia. He described it as a central feature of living cells.

1835:

Hugo von Mohl (1805-1872) described cell division and showed that cells arise through partition walls formed between preexisting cells.

1838-39:

Matthias Jacob Schleiden (1804-1881) and Theodor Schwann (1810-1882) put forth the cell theory on the basis of accumulated experimental results.

The cell theory states that:

1. The cell is the smallest structural element of a multicellular organism and as a unit, it is itself an elementary organism.

2. In a multicellular organism, every cell has a specific function to perform, and represents a working unit.

3. A cell can only be produced from another cell by cell division.

1852-1853:

Rodolf Ludwig Carl Yirchow (1821-1902) gave the theory of cell lineage.

1859:

Charles Darwin (1809-1882) published the Origin of Species that contains the careful observations in plants and animals made by him. He put forward a theory of evolution called the theory of natural selection.

This theory states that:

(a) Variation is constant in nature,

(b) Over production of offspring brings about struggle for existence,

(c) Natural selection operates by elimination of the unfit and survival of the fit, and

(d) Heredity continues the line of survivors.

Darwin was of the opinion that the hereditary materials from different body organs were transported by blood stream to the sex organs where they were assembled into gametes. These hereditary materials were called gemmules or pan-genes.

After fertilization, these gemmules separated out to different parts of the body during development. Thus Darwin attempted to give a physical basis to the Lamarck’s idea of inheritance of acquired characters. Darwin also recognized the spontaneous changes which he called sports.

1865:

Gregor Johann Mendel (1822-1884), an Austrian monk working as a teacher at the Augustinian Monastery at Briinn (now Berne in Czechoslovakia), started hybridization work in garden pea (Pisumsativum) in 1857 and presented his results entitled, Versuche uber Pflanzen Hybriden (= Experiment in Plant Hybridization) at the meeting of Briinn Society for the Study of Natural Science on 8th February and 8th March, 1865.

The paper was published in the proceedings of the Brunn Society for the Study of Natural Science in 1866. This paper became the basis of the modern science of genetics. Mendel considered that the characters are governed by certain units or factors which arc transmitted from parents to offspring through gametes.

He explained the inheritance on numerical basis, and the laws of inheritance he proposed are known as Mendel’s Laws which are:

(1) The Law of Segregation, and

(2) The Law of Independent Assortment.

But the findings of Mendel were left neglected for many years. In 1900, three scientists (Correns, Tschermak and de Vries) independently discovered the Mendel’s work.

Mendel also studied hybridization in the hawkweed (Hieracium) and communicated the results to the meeting of Brunn Society on 9th June, 1869. This paper entitled, Uber einige aus Kunstlicher Befruchtung gewonnen Hieracium Bastarde (= On Hieracium Hybrids Obtained by Artificial Fertilization) appeared in the proceedings in 1870.

However, the results on Hieracium differed from those on Pisum. The reason was the parthenogenetic behaviour of Hieracium, which was discovered some 30 years later.

1869:

Fredrick Miescher (1844-1895) reported the presence of nucleic acids in cells. He called it “nuclein.” He analysed the sperm head of Rhine winter salmon sperm and found nucleic acid and protamine.

1875:

Louis Pasteur (1822-1895) working with microorganisms, demonstrated that life arises only from preexisting life.

1875-76:

Wilhelm August Oscar Hertwig (1849-1922) studied reproduction in sea urchin and discovered that fertilization involves the union of sperm and egg.

1882:

Walter Flemming (1843-1915) described cell division in salamander and also in corneal epithelium of humans. He demonstrated that the chromosomes divide longitudinally during nuclear division. He coined the terms mitosis and Chromatin in 1882.

1883:

Francis Galton (1822-1911) developed the “concept of regression” which is a measurement of degree of resemblance or relatives. He also founded the study of human heredity. He coined the term eugenics which deals with the improvement of human genetic material.

Eugenics has been divided into two aspects:

(1) Negative eugenics (to decrease the frequency of harmful genes), and

(2) Positive eugenics (to increase the frequency of, beneficial genes).

1883:

Edouard van Beneden (1845-1910) studied spermatogenesis, oogenesis and fertilization in round worm (Ascarismegalocepliala). In 1883, he demonstrated that gametic chromosome number is half of that present in the body cells. He showed that the egg and sperm contribute equal cells. He showed that the egg and sperm contribute equal number of chromosomes during fertilization to form the zygote.

1883-1885:

August Weismann (1934-1914) put forth the germplasm theory. He supposed that very early in the development of the individual, the hereditary material was separated into germplasm from the rest of the body, the somatoplasm.

Any change affecting somatoplasm and not reaching the germplasm is not heritable. Thus he rejected the theory of inheritance of acquired characters. He made experiments using domesticated mice whose tails were cut off at young stage. After repeating the experiment up to 22 generations involving 1592 individuals, he found that no mice was born without tail.

1884:

Eduard Strasburger (1844-1912) described fertilization in angiosperms and demonstrated that the principles of fertilization described by Hertwig for animals was true for plants as well.

1886:

Ernst Abbe (1840-1908) produced the oil immersion objective lens which could increase the resolving power of the light microscope.

1887-1888:

Strasburger observed reduction of chromosome number in plants during gamete formation.

1890:

Rimpau produced triticale from a cross of wheat with rye.

1891:

Hermann Henking (1858-1942) observed that during spermatogenesis in the hemipteran insect Pyrrhocoris, one chromatin body goes to only one pole during anaphase II and thus half of the sperms carry this chromatin body, while half of sperms are lacking it. Henking termed this body as the “X- body” which was later called the X-chromosome or sex chromosome.

He also found that the egg fertilized by one type of sperm produced female, while the egg fertilized by the other type of sperm produced male insect. This mechanism is now called the XO mechanism of sex determination.

1892:

Theodor Boveri (1862-1915) studied meiosis in Ascaris and described synapsis of chromosomes. He also contributed to the formulation of the chromosome theory of inheritance.

1896:

Edmund Beecher Wilson (1856-1939) organised the cytological and embryological knowledge in his classical book “The Cell in Development and Inheritance.”

1900:

Rediscovery of Mendel’s laws and recognition of their significance by three scientists, Correns, Tschermak and de Vries occurred.

Carl Franz Joseph Correns (1864-1933), a German botanist, did hybridization experiments in maize, peas, beans and in some other plants and reported that his results were similar to those obtained by Mendel in 1865 in garden peas.

Erich von Tschermak (1871-1962), an Austrian botanist, studied the effects of crossing on vigour in peas and got similar results as Mendel had obtained. Hugo de Vries (1848-1935), a Dutch biologist, rediscovered Mendel’s laws through his hybridization experiments.

1901:

Hugo de Vries, during studies of evening primrose (Oenotheralamarckiana), observed sudden heritable variations in this plant and coined the term mutation for sudden heritable changes.

In 1901, he published the book “Die Mutationstheorie” (The Mutation Theory) in which he compiled the data on mutations. However, the mutations in Oenothera reported by de Vries are the changes in chromosome structure and number and not gene or point mutations.

1902:

C.E. McCiung observed X-chromosomes in many insects and suggested that these chromosomes are associated with sex determination.

1902:

William Bateson (1861-1926) coined the terms F₁, F₂, allelomorph, homozygote and heterozygote.

1902-1903:

Walter S. Sutton (1876-1916) studied reduction division and recognized parallelism between the behaviour of chromosomes during meiosis and the Mendelian segregation of genes. In 1903, he published his second, paper in which he elaborated the chromosome theory of heredity. He described that chromosome pairs (bivalents) segregate randomly giving rise to independent assortment of genes.

The work of Sutton led to the combination of cytology and genetics into a new discipline called cytogenetics. The “chromosome theory of inheritance” is also called the “Sutton-Boveri theory of chromosomal inheritance”. The parallelism between genes and chromosomes may be briefly Summarised as follows.

(1) During inheritance, the genes and the chromosomes both behave like individual units. Each pair of chromosomes can be seen to be different from other pair and each chromosome has individuality. Similarly each gene has an individuality.

(2) The inheritance pattern can be explained on the assumption that the genes in an individual occur in pairs (allelic pair) and that one member of each pair was contributed by one parent, while the other member of the allelic pair was provided by the other parent. Similarly, chromosomes in an individual also occur in pairs, each member of which has been derived from one of the two parents of the individual.

(3) Each gamete contains only one member of each pair of chromosomes. Similarly, each gamete contains one member of each pair of alleles of genes. In Mendel’s principle, segregation of one allele occurs from its pair and each allele enters in separate gamete. Similarly each chromosome separates from its homologous pair during meiosis and is included in a separate gamete.

1903:

W.L. Johannsen (1857-1927) coined the terms gene, genotype and phenotype. He studied the effects of selection for seed weight in Princess variety of bean (Phaseolus vulgaris), and observed that progenies derived from heavier seeds were characterized by greater mean weight than the progenies from the lighter seeds.

In 1903, he gave the concept of pure lines which is defined as “the progeny of a single self-fertilized homozygous individual.” Thus a pure line is a strain homozygous at all loci.

1904:

T.H. Montgomery assigned the term autosomes to the chromosomes which are alike in male and female, and other than sex chromosomes.

1905:

From the studies of male protenor (squash bug), E.B. Wilson proved the importance of X-chromosome in sex determination. He found that females of Protenor have 14 chromosomes, while males have only 13 chromosomes. In females, 7 bivalents are formed and all the eggs receive a haploid set of chromosomes. In males, 6 bivalents and one unpaired chromosome (univalent) were formed.

At anaphase I, the odd chromosome divided and each pole received 7 chromosomes. At the second meiotic division, the unpaired chromosome passed to one of the two daughter cells. Thus two types of spermatozoa were formed in equal numbers, 50% with 6 chromosomes and 50% with 7 chromosomes.

Fertilization of the egg (7 chromosomes) by a sperm with 7 chromosomes produced a female bug with 14 chromosomes, while fertilization of the egg with a sperm bearing 6 chromosomes produces a male (with 13 chromosomes). In organisms with male heterogamety, the sex chromosome limited to the male sex was termed Y chromosome by Wilson in 1909.

1906:

R.C. Punnet and W. Bateson reported the first case of linkage. The term genetics was coined by Bateson in this year. The first case of sex-linked inheritance was found by Doncaster and Raynor in currant moth (Abraxasgrassulariata) but female was heterogametic.

1908:

Gates was the first to observe a ring of chromosomes in Oenotherarubrinervis.

1909:

Frans Alfons Janssens (1863-1924) advanced the Partial chiasma-type theory of crossing over. According to this theory, chiasma is the result of crossing over (exchange) during meiosis. At the point of crossing over, during meiosis, a cross-shaped configuration (chiasma) is formed when the paired homologues become separated.

Number and position of chiasmata in early diplotene indicate the number and the place of crossing over occurred. Thus the partial chiasm type theory explains the relationship between the genetic crossing over and the cytologically visible chiasmata.

1910:

Thomas Hunt Morgan (1866-1945) discovered the ‘white eye’ mutant in Drosophila and sex linkage. In this fly, male is heterogametic, while female is homogametic. In 1911, he proposed that genes are arranged on chromosomes in a linear order.

The genes present on a chromosome are linked, and linkage breaks due to crossing over. In 1912, he presented linkage maps of Drosophila. In 1933, Morgan received Nobel prize in Medicine and Physiology and thus he is the first geneticist to be honoured by this prize.

1912:

Stevens observed the sex chromosome of Drosophila using acetocarmine smear technique and reported that the male fly has X and Y chromosomes.

1914:

J. Belling (1866-1933) working for the breeding programme of Florida velvet bean (StizolobiumdeeringianumBort.), Belling observed 50% pollen abortion and 50% seed abortion in this plant. He termed it semi sterility and explained it on the basis of segmental interchange between non-homologous chromosomes.

1916:

Winkler produced tetraploidSolanumnigrum from shoots arising from callus tissue. Calvin Blackman Bridges (1889-1938) published his work on the X chromosome non-disjunction in Drosophila, which presented the first direct proof that genes are present in chromosomes.

1917:

Allen reported the first case of chromosomal difference between the sexes in a plant, the liverwort (Sphaerocarposdonnellii) where the female gametophyte is 7 + X, and the male gametophyte is 7 + Y.

1917, 1919:

C.B. Bridges discovered “deficiency” in 1917 and “duplication” in 1919 in Drosophila melanogaster.

1921:

A.H. Sturtevant, using genetic tests in Drosophila melanogaster and D. simulans, recognized “inversion”. Small inversions were also designated as crossover suppressor (C). Later the “CIB” stock was used by Muller in detection of mutations.

Such a CIB stock has a crossover reducer in X chromosome associated with lethal (l) and has a Bar eye (B) as a dominant marker. The cross-over suppressor behaves like a “dominant gene”. (In 1926, Sturtevant discovered that the dominant C genes were inversions).

Albert Francis Blakeslee (1874-1954) discovered trisomies in plant Daturastramonium (Jimson weed) in 1921. This plant has 12 pairs of chromosomes, and all the 12 trisomies were identified and characterized by Blakeslee and his associates. With Belling, he reported that off type plants had extra chromosome in Datura (trisomies).

He observed various chromosome configurations in trisomies during meiosis. (In 1924, he coined the terms “primary trisomies” and “secondary trisomies”).

1922:

LV. Morgan described the attached X-chromosomes in Drosophila females which regularly showed the unusual transmission pattern of the sex-linked recessive y (yellow body) gene. This confirmed the observations of Bridges, and showed a direct correlation between gene and chromosome behaviour.

1923:

Cases of chromosomal differences between sexes in different plants were reported by Santos in dioecious water weed (Elodea gigan-tea); by Kihara and Ono in Rumexacetosa; by Blackburn in Melandrium and by Winge in Humulus and Vallisneria.

In 1923, C.B. Bridges developed the gene balance theory of sex determination. He found a triploid (3x) Drosophila female and mated it to a normal male. In the progeny, eight sexually distinct kinds of individuals were obtained. He analysed them cytologically and compared the sex.

It was found that the Y chromosome in Drosophila is not necessary for maleness. He concluded that every individual has in its genotype both male and female potentialities. The development in a particular sex is determined by the balance of the genes, i.e., the preponderance of male tendency or of female tendency genes.

He also found that ratio between the number of X chromosomes and the sets of autosomes (X/A ratio) in the fertilized egg decides the sex. In Drosophila, the X/A ratio 1.00 develops into female, while the ratio 0.50 develops into male.

The ratio falling between 1.0 and 0.50 produces an individual intermediate between male and female, i.e., an intersex. The X/A ratio above 1.00 produces super-female while the ratio below 0.50 produces super male.

1924:

Robert Joachim Feulgen (1884-1955) developed a technique of staining of chromosomes. With Rossenbeck, he described the technique for the test of the presence of DNA in cells. It is now called the “Feulgen reaction”. On gentle hydrolysis, the pentose sugar (deoxyribose) of DNA liberates aldehyde which reacts with the basic fuchsin and gives purplish colour.

1925:

Anderson gave the proof that crossing over occurs at 4-stand stage, through the use of attached X chromosome stock in Drosophila. 1925, 1926 : J. Belling in 1925, coined the term “interchange” for translocation. With Blakeslee in 1926, he coined the term “tertiary trisomic” in which the extra chromosome is a trans-located chromosome.

1925:

Supernumerary chromosomes in maize were discovered by Y. Kuwada in 1925 and A.E. Longley in 1927. The term “B-chromosome” was given by Randolph in 1928 to such accessory chromosomes in plant and animal species, while the normal chromosomes were called “A-chromosomes.”

1926:

Curt Stern analysed cytologically the first translocation in Drosophila and showed that a piece of X chromosome was attached to one end of the Y chromosome. Hitoshi Kihara (1893-) with Ono coined the terms autopolyploid and allopolyploid.

1927:

Herman Joseph Muller (1890-1967) discovered that X-rays can induce mutations in Drosophila. He used the CIB- technique to detect the induced mutations in the X chromosome. For this work, Muller was awarded the Nobel prize in 1946.

1928:

L.J. Stadler discovered that X-rays can induce mutations in plants. Emil Heitz, using staining reactions, found two types of chromosomal regions, (i) light staining region, and (ii) dark staining region during interphase, early prophase and telophase. In 1928-29, he coined the term euchromatin for light staining region and the term heterochromatin for dark staining region.

1929:

Barbara McClintock (1902-1992) extensively studied maize cytogenetics and for the first time made the morphological identification of maize chromosomes in 1929. By using trisomies, she associated each chromosome with a particular linkage group.

1930:

McClintock showed that a cross (+) shaped configuration is formed at pachytene due to pairing of homologous regions in the heterozygous interchange. This work provided evidence of an exchange of terminal segments of non-homologous chromosomes.

H. Kihara in 1930, developed a method called genome analysis for detecting the diploid ancestors of allopolyploid species. He analysed the genome of bread wheat (Triticumaestivum) which is an allohexaploid species, consisting of full chromosome complement of three diploid species, viz., T. monococcum, Aegilopsspeltoides and Aesquarossa.

He named the three genomes as A, B and D genomes. The genome analysis involves crossing between polyploids and diploids and studying the chromosome pairing during meiosis in the hybrids.

1931:

Emil Heitz showed an association between number of nucleoli in interphase with the number of particular type of chromosomes (nucleolar chromosomes). Curt Stern, using translocations between the X and Y chromosomes presented the proof that genetic crossing over is accompanied by an exchange of parts between homologous chromosomes.

H.B. Creighton and B. McClintock demonstrated in maize that genetic recombination was accompanied by a reciprocal exchange of chromatin material between the two homologous chromosomes and thus presented a cytological proof of crossing over.

J. Belling in 1931 and 1933 put forth a hypothesis known as Belling’s Hypothesis to explain crossing over.

According to this hypothesis:

(i) Homologous unduplicated chromosomes are coiled together,

(ii) Replication of chromosomes occurs first,

(iii) Then the replication of inter-chromomeric (intergenic) region occurs,

(iv) During synthesis of inter-chromomeric regions, new connections occur between the chromomere of one chromosome and the chromomere of its homologue.

1932:

Ernst August Friedrich Ruska and Knoll published the description of electron microscope which consisted of an electron source and two magnifying lenses. In 1934, Ruska described an improved version of the electron microscope with a condenser lens.

Cyril Dean Darlington advanced the precocity theory which says that meiotic prophase starts precociously before the chromosomes are in duplicate state. However, the theory became later invalid.

1934:

B. McClintock introduced the term nucleolar organizer for the chromosome region which is active in the formation of nucleolus.

1936:

Curt Stern discovered mitotic (somatic) crossing over.

1937:

Blakeslee and Avery discovered that polyploidy can be induced by the use of colchicine, an alkaloid obtained from the plant Colchicum autumnale.

1939:

C.D. Darlington coined the term “misdivision of centromere” for transverse divisions of centromere giving rise to iso-chromosomes.

1941:

Beadle and Tatum proposed the “one-gene-one-enzyme” hypothesis which was later named “one-gene-one-polypeptide chain” hypothesis. Artificial induction of mutations by ultraviolet light was reported by Stadler and Holaender and Emmons.

1944:

C. Auerbach and J.M. Robson, working with aliyl-isothiocyanate discovered that chemicals can induce mutations. In 1947, they used sulphur and nitrogen mustards to induce mutations.

1944:

Oswald T. Avery (1877-1955) and associates (MacLeod, and McCarty) followed the experiments of Griffith conducted in 1928, and found that the genetic material is DNA. They used the pathogenic (smooth) and non-pathogenic (rough) strains of Pneumococcus (Diplococcuspneumoniae) for transformation studies. Non- pathogenic strain was transformed into pathogenic and they found that DNA was involved in transformation.

1946:

Lederberg and Tatum discovered genetic exchange in bacteria.

1949:

Delbruck and Bailey showed genetic exchange in bacteriophage. Kelner discovered “photo reactivation” process of repairing the damage caused by ultraviolet light (290-310 nm wavelength). Murray L. Barr (1908-) together with Bertram discovered sex chromatin in interphase nucleus of human females.

The sex chromatin is absent in males. The small stainable body is called “Barr body”. Now it is known that the Barr body is one hetero-chromatinized X chromosome.

1950:

B. McClintock discovered controlling systems in maize. The most famous is the AC-DS system discovered by her in 1950. The AC (activator) acts as regulator and DS (dissociation) cannot function in absence of AC. Both the loci can change their position on a chromosome (transposition). However, DS when changes its position, it induces a breakage in the chromatid.

Thus presence of both the loci in the same nucleus causes spontaneous chromosome breakage. In presence of AC, DS affects the gene lying adjacent to it, which expresses as a recessive mutant. McClintock gave full data of AC-DS controlling elements (transposable elements) even before the structure of DNA was known. Her work was recognized and she was awarded Nobel prize in 1983.

John Albert Levan (1905-) together with Joe Hin Tjio in 1950 developed a technique of squashing for study of mitotic chromosome. He used oxyquinoline during fixation of the material and got that the chromosomes were contracted and spindle destroyed. They reported chromosome number in several plant species.

1951:

H. Kihara formulated a technique of production of seedless fruits in water melons. This method involves the induction of polyploidy and crossing of an autotetraploid with a diploid plant to produce triploids. The triploids produce seedless fruits because the gametes are sterile.

1952:

A.D. Hershey and M. Chase, by using radioactive ³⁵S and ³²P labelling of protein and DNA, respectively, in T₂bactriophage showed that DNA is the genetic material.

N.D. Zinder and J. Lederberg discovered the phenomenon of “transduction”. In transduction, bacteriophage transfers genes from the previously infected host bacterial cells to the newly infected ones.

J. Lederberg and associates discovered E. coli sex factor (F) as a transmissible factor.

E. Chargaff discovered the A-T and G-C ratios in DNA.

T.C. Hsu developed the technique of osmotic shock for the study of mammalian chromosomes.

1953:

J.D. Watson and Francis Harry Compton Crick (1916-) proposed the double helix model of DNA. This model was based on the results of X-ray diffraction of DNA studied by Franklin and Wilkins, and on chemical analysis data.

This double stranded helical model of DNA structure is universally accepted and is popularly known as “Watson-Crick Model”. For this achievement, Nobel prize was awarded to Watson, Crick and Wilkins in 1962.

1955:

S.W. Brown and D. Zohary, studying crossing over in stocks carrying chromosomal deficiency in Liliumformosanum showed 1 : 1 ratio between cytologically visible chaismata and genetic crossing over. They demonstrated the direct relationship between crossing over and chiasma formation and presented the proof for the support of “partial chiasma type theory” of crossing over originally proposed by Janssens in 1909.

1956:

E.R. Sears, using cytogenetical techniques transferred gene for leaf rust resistance from the wild grass Aegilopsumbellulata (2x =14) to common wheat (Triticumaestivum, 6x = 42) variety Chinese Spring. He made a cross between tetraploid Emmer wheat (T. dicoccoides, 4x = 28) and the wild grass (Ae. umbellulata) carrying leaf rust resistance gene.

Chromosomes of this triploid hybrid (3x = 21) were doubled by using colchicine to produce a hexaploid plant (fix = 42). The grass-emmer hybrid was then crossed to the susceptible common wheat variety Chinese Spring. The resulting “grass-emmer-wheat” hybrid contained 35 chromosomes of common wheat and 7 chromosomes of the wild grass.

The hybrid plant was back crossed to Chinese Spring. In the second back cross generation, a plant was obtained that looked like Chinese Spring and was rust resistant. This plant had 43 chromosomes (42 chromosomes of Chinese Spring and one chromosome of wild grass).

It was then irradiated with X-rays before flowering, and the pollen were used to pollinate the plants of Chinese Spring. In the progeny, one resistant plant was obtained that contained 42 chromosomes and possessed all the characteristics of Chinese Spring. A segment of the grass chromosome carrying the rust resistance gene was trans located to the wheat chromosome.

1957:

S. Benzer gave the concept of “cistron”,”recon” and “muton” in the study of fine structure of gene. The “cistron” is the functional unit, “muton” is the basic unit of gene mutation, while the “recon” is the smallest unit of recombination.

J.H. Taylor, P.S. Woods and W.L. Hughes demonstrated semi conservative replication of eukaryotic chromosome, studying Vicia faba root tip chromosomes using radioactive precursor of DNA, ³H thymidine.

Ingram for the first time demonstrated that a mutation in a gene changes an amino acid in the corresponding protein.

1958:

M. Meselson and EW. Stahl using ¹⁴N and ¹⁵N, proved that DNA replication occurs in a semiconservative manner.

Crick gave the concept of adopter molecule (tRNA) with an anticodon.

1959:

J. Lejeune discovered that Down’s syndrome in human is a trisomic, while C.E.

Ford discovered that Turner’s syndrome is a monosomic. In 1959, chromosomal aberrations were discovered in human.

1961:

S. Benzer discovered the “hot spots” where mutations are concentrated.

Driskell and Adelberg discovered that E. coli ‘F’ factor is a DNA plasmid.

Francis Jacob (1920-) together with J. Monod gave the “Operon model” of gene regulation. An “operon” consists of a regulator, a promoter, an operator and genes. For this work, Jacob and Monod together with Lwoff were awarded the Nobel Prize in 1965.

M.W. Nirenberg and J.H. Matthei broke the genetic code.

F.H.C. Crick and associates furnished the evidence through the study of reading frame shift mutations that “codon” is of “triplet” nature.

J. Marmur and Doty discovered DNA renaturation and established the specificity and feasibility of nucleic acid hybridization reactions.

Mary Francis Lyon in 1961 made detailed study of sex chromatin and developed a hypothesis known as “Single active X-Hypothesis” of dosage compensation in human and mammals. This hypothesis is popularly known as “Lyon Hypothesis”.

According to this hypothesis, single X chromosome in males and one of the two X chromosomes of females are active like autosomes. But one X chromosome of the female is inactivated by the process of “heterochromatinization.”

The inactivation occurs early in embryonic development. The inactivated X chromosome may be paternal or maternal in origin in different cells of the same individual. Once an X chromosome is hetero-chromatinized, the same chromosome is always hetero-chromatinized in the cells produced from that cell.

1962:

Silver and Ozeki discovered, the “colicinogenic factor” (Col factor) as a DNA plasmid.

Arber provided the first evidence for the existence of DNA restriction enzymes.

1962-63:

Margit M.K. Nass (1931-) together with Sylvan Nass discovered the presence of DNA in mitochondria.

1964:

R.J. Britten and Waring discovered the repetitious DNA in eukaryotes.

R. Holliday gave the “Hybrid DNA model” of crossing over.

1965:

H.L.K. Whitehouse gave a “polaron Hybrid DNA model” of crossing over,

Holley and associates reported the first sequence of transfer RNA (tRNA).

Ernest Joseph DuPraw (1931-) developed a technique of “whole mount electron microscopy” to study the eukaryotic chromosomes. From the studies of human mitotic metaphase chromosomes, he gave the “Folded Fibre Model” of eukaryotic chromosome structure in 1965.

According to this model, each un-replicated chromosome (unit chromatid) is loosely packed in transverse and longitudinal folded spirals of a single 20 nm fibre. The chromosome (chromatid) contains a long single DNA double helix in a super-coiled condition held by protein molecules.

1966:

Genetic code was elucidated by H.G. Khorana, Nirenberg and Ochoa.

Crick proposed the “Wobble Hypothesis” to explain the degeneracy of genetic code (same amino acid specified by more than one codons). According to wobble hypothesis, there occurs unusual base paring between the 3^rd base of the codon (3′ end) and the first base of anticodon (5′ end). This position (3′ end of codon and 5′ end of anticodon) is called the “wobble position”.

1967:

Gellert discovered the enzyme polynucleotide ligase that joins DNA fragments together.

1968:

W. Gilbert and D. Dressier proposed the “rolling circle model” of DNA replication.

1969:

A. Travers and Burgess characterized the sigma factor associated with RNA polymerase.

Roberts discovered the rho (p) factor associated with transcription termination.

1970:

M.L. Pardue and J.G. Gall showed the DNA/DNA and DNA/RNA hybridization in situ.

1970:

T. Caspersson and associates developed the fluorescent and Giemsa banding technique of eukaryotic chromosomes.

D. Baltimore; and Temin and Mizutani discovered the enzyme “reverse transcriptase” in retroviruses.

1972:

Singer and Nicolson introduced the idea that biological membrane is a 2- dimensional fluid.

1972-73:

Stanely N. Cohen (1917-) together with Chang developed the technique of “DNA cloning.”

1973:

Daniel Nathans (1928-) and coworkers, for the first time used restriction enzymes for chromosome mapping. In 1978, Nobel prize was awarded to Nathans, Smith and Arber for this work.

1974:

Charles Allen Thomas Jr. (1927-) together with Wilson discovered wide spread occurrence of “palindromes”. The palindromes are the sequences that read the same both backward and forward.

1975:

Har Gobind Khorana f 1922-) for the first time made the artificial synthesis of a gene.

1977-78:

Sanger and various other investigators discovered overlapping, included and interrupted genes (split genes).

1981:

Anderson and associates reported the first mitochondrial DNA sequence.

1982:

Bloom and Carbon revealed the molecular structure of centromere.

1983:

Evans and associates discovered cyclins.

Murray and Szostak made a synthetic chromosome by putting together the functional elements needed by a chromosome. Bender and associates described chromosome walking.

1984:

Feldherr and associates demonstrated the ability of the nuclear pores to transport rigid structures.

1987:

Greider and Blackburn discovered the enzyme “telomerase” that is needed for synthesis of telomere.

Henderson and associates recognized the unusual structure of G-T rich tail (single-stranded extension) in the telomere.

Burke, Carle and Olson developed the “yeast artificial chromosomes” (YACs).

1989:

Draetta and associates discovered the role of cyclins in M-phase kinase.

Williamson, Raghuraman and Cech proposed a “G-quartet model” of telomere structure.