Lecture on history of genetics

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  1. 1. GeneticsAbu sayeed
  2. 2. Genetics (from Ancient Greek genetikos, "genitive" and thatfrom genesis, "origin"), a discipline of biology, isthe science of genes, heredity, and variation in living organisms.Genetics deals with the molecular structure and function of genes, genebehavior in context of a cell or organism, patterns of inheritance fromparent to offspring, and gene distribution, variation and changein populations such as through Genome-Wide Association Studies. Giventhat genes are universal to living organisms, genetics can be applied to thestudy of all living systems, from viruses and bacteria,through plants and domestic animals, to humans.
  3. 3. The fact that living things inherit traits from their parents has been usedsince prehistoric times to improve crop plants and animalsthrough selective breeding. However, the modern science of genetics,which seeks to understand the process of inheritance, only began withthe work of Gregor Mendel in the mid-19th century. Although he did notknow the physical basis for heredity, Mendel observed that organismsinherit traits via discrete units of inheritance, which are now called genesModern genetics started with Gregor Johann Mendel, a German-CzechAugustinian monk and scientist who studied the nature of inheritance inplants. In his paper "Versuche ber Pflanzenhybriden" ("Experiments onPlant Hybridization"), presented in 1865 to the NaturforschenderVerein (Society for Research in Nature) in Brnn,
  4. 4. The importance of Mendel's work did not gain wide understanding until the1890s, after his death, when other scientists working on similar problems re-discoveredhis research. William Bateson, a proponent of Mendel's work, coinedthe word genetics in 1905. (The adjective genetic, derived from the Greekword genesis, "origin", predates the noun and was first used in abiological sense in 1860.) Bateson popularized the usage of the word genetics todescribe the study of inheritance in his inaugural address to the ThirdInternational Conference on Plant Hybridization in London, England, in 1906.After the rediscovery of Mendel's work, scientists tried to determine whichmolecules in the cell were responsible for inheritance. In 1911, Thomas HuntMorgan argued that genes are on chromosomes, based on observations of a sex-linkedwhite eye mutation in fruit flies. In 1913, his student AlfredSturtevant used the phenomenon of genetic linkage to show that genes arearranged linearly on the chromosome.
  5. 5. Although genes were known to exist on chromosomes, chromosomes arecomposed of both protein and DNAscientists did not know which of these isresponsible for inheritance. In 1928, Frederick Griffith discovered thephenomenon of transformation (see Griffith's experiment): dead bacteria couldtransfer genetic material to "transform" other still-living bacteria..
  6. 6. Griffith's experiment, reported in 1928 by Frederick Griffith, was one of the firstexperiments suggesting that bacteria are capable of transferring geneticinformation through a process known as transformation.Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteriawhich infect mice a type III-S (smooth) and type II-R (rough) strain. The III-Sstrain covers itself with a polysaccharide capsule that protects it from thehost's immune system, resulting in the death of the host, while the II-R straindoesn't have that protective capsule and is defeated by the host's immunesystem. A German bacteriologist, Fred Neufeld, had discovered the threepneumococcal types (Types I, II, and III) and discovered the Quellung reaction toidentify them in vitro. Until Griffith's experiment, bacteriologists believed that thetypes were fixed and unchangeable, from one generation to another.In this experiment, bacteria from the III-S strain were killed by heat, and theirremains were added to II-R strain bacteria. While neither alone harmed the mice,
  7. 7. the combination was able to kill its host. Griffith was also able to isolate both liveII-R and live III-S strains of pneumococcus from the blood of these dead mice.Griffith concluded that the type II-R had been "transformed" into the lethal III-Sstrain by a "transforming principle" that was somehow part of the dead III-S strainbacteria.Today, we know that the "transforming principle" Griffith observed was the DNA ofthe III-S strain bacteria. While the bacteria had been killed, the DNA had survivedthe heating process and was taken up by the II-R strain bacteria. The III-S strainDNA contains the genes that form the protective polysaccharide capsule.Equipped with this gene, the former II-R strain bacteria were now protected fromthe host's immune system and could kill the host. The exact nature of thetransforming principle (DNA) was verified in the experiments done by Avery,McLeod and McCarty and by Hershey and Chase
  8. 8. Sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and MaclynMcCarty identified the molecule responsible for transformation as DNA. The Hershey-Chase experiment in 1952 also showed that DNA (rather than protein) is the geneticmaterial of the viruses that infect bacteria, providing further evidence that DNA is themolecule responsible for inheritance.James D. Watson and Francis Crick determined the structure of DNA in 1953, usingthe X-ray crystallography work of Rosalind Franklin and Maurice Wilkins thatindicated DNA had a helical structure (i.e., shaped like a corkscrew). Their double-helixmodel had two strands of DNA with the nucleotides pointing inward, eachmatching a complementary nucleotide on the other strand to form what looks likerungs on a twisted ladder. This structure showed that genetic information exists inthe sequence of nucleotides on each strand of DNA. The structure also suggested asimple method for duplication: if the strands are separated, new partner strands canbe reconstructed for each based on the sequence of the old strand.
  9. 9. The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is composed of achain of nucleotides, of which there are fourtypes: adenine (A),cytosine (C), guanine (G), and thymine (T). Genetic informationexists in the sequence of these nucleotides, and genes exist as stretches ofsequence along the DNA chain. Viruses are the only exception to this rulesometimes viruses use the very similar molecule RNA instead of DNA as theirgenetic material.DNA normally exists as a double-stranded molecule, coiled into the shape ofa double-helix. Each nucleotide in DNA preferentially pairs with its partnernucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in itstwo-stranded form, each strand effectively contains all necessary information,redundant with its partner strand. This structure of DNA is the physical basis forinheritance: DNA replication duplicates the genetic information by splitting thestrands and using each strand as a template for synthesis of a new partner strand.Genes are arranged linearly along long chains of DNA base-pair sequences.In bacteria, each cell usually contains a single circular genophore whileeukaryotic organisms (including plants and animals) have their DNA arranged inmultiple linear chromosomes. These DNA strands are often extremely long; thelargest human chromosome, for example, is about 247 million base pairs in length.
  10. 10. The DNA of a chromosome is associated with structural proteins that organize,compact, and control access to the DNA, forming a material called chromatin; ineukaryotes, chromatin is usually composed of nucleosomes, segments of DNAwound around cores of histone proteins. The full set of hereditary material in anorganism (usually the combined DNA sequences of all chromosomes) is calledthe genome.While haploid organisms have only one copy of each chromosome, most animalsand many plants are diploid, containing two of each chromosome and thus twocopies of every gene. The two alleles for a gene are located on identical loci of thetwo homologous chromosomes, each allele inherited from a different parent.Walther Flemming's 1882 diagram of eukaryotic cell division. Chromosomes arecopied, condensed, and organized. Then, as the cell divides, chromosome copiesseparate into the daughter cells.Many species have so called sex chromosomes. They are special in that theydetermine the sex of the organism. In humans and many other animals, the Y-chromosomecontains the gene that triggers the development of the specificallymale characteristics. In evolution, this chromosome has lost most of its contentand also most of its genes, while the X chromosome is similar to the otherchromosomes and contains many genes. The X and Y chromosomes form a veryheterogeneous pair