Concept of gene and Complementation test.pdf
- 1. CONCEPT OF GENE Structural & genetic definitions – complementation test Submitted to, Smt. I K Nishitha Asst.Professor Dept.of Botany St.Teresa’s college,EKM. Submitted by, AncyVarghese 1 M Sc. Botany Roll No.6 St.Teresa’s college,EKM.
- 2. Our concept of the gene has undergone a remarkable evolution as biologists have learned more about the nature of inheritance. genes were considered discrete factors that were retained throughout the life of an organism and then passed on to each of its progeny. Over the following century, these hereditary factors were shown to reside on chromosomes and consist of DNA. A genome contains all of the genes required to “build” a particular organism. INTRODUCTION
- 3. MILESTONES
- 4. CONTRIBUTIONS OF MENDEL – CONCEPT OF GENE The gene was formerly called a factor.The existence of a gene as an independentheritable factor was first identified by Gregor Mendel who is now credited as the father of genetics. Mendel formulated the laws of heredity based on his careful breeding experiments on garden pea plants. An individual plant possessed two copies of a gene that controlled the development of each trait,one derived from each parent. The two copies could be either identical to one another or nonidentical. Alternate forms of a gene are called alleles.
- 5. WILHELM JOHANSEN The factor was called gene as introduced by Wilhelm Johannsen, a Danish botanist and plant physiologist, in 1905. Also coined the terms Genotype to refer to genetic make up Phenotype to refer outward appearance of an organism
- 6. T H MORGAN Gene theory of T.H Morgan in 1911 Chromosomes contain hereditary units & each chromosomes carries thousands of genes. Genes are arranged on the chromosome in linear order and on the special regions of locus and looks like beads on string. After discovering of DNA as carriers of genetic information Morgan’s theory was discarded.
- 7. CHANGING CONCEPT Prior to Mendel’s work, the popular belief is that male and female parents provide fluids that blend or mix during fertilization. Charles Darwin referred to the particles that mix at fertilization as gemmule. Mendel’s work was published in 1886. However, it was in the late 1900s that it was accepted as it was supported by the same results from the works of Hugo deVries, Carl Correns, and Erich von Tschermak. Further study in the next century led to an understanding of the molecular structure and function of the gene. It was found out that the gene is comprised of expressed DNA. The works of Rosalind Franklin and MauriceWilkins and later on of James D. Watson and Francis Crick identified DNA structure.
- 8. Genes are discrete particles inherited in Mendelian pattern. Responsible for expression of specific phenotypic characters. eg.skin colour, texture of hair…etc If position of gene changes then character also changes. May exist in alternate forms. Eg.Human blood group A,B,AB and O. Genes may undergo sudden changes in position and composition- mutation. Capable of self duplication producing their own copies.
- 9. G W BEADLE & E L TATUM Demonstrated one gene – one enzyme hypothesis in Neurospora crassa. Genes express themselves through the synthesis of enzymes. Genes control a biochemical reaction. Later one gene – one polypeptide hypothesis was put forward. Stating that an enzyme contain many polypeptides.
- 10. A change even in one nucleotide of the polypeptide specifying gene may mutate and produce a varient of wild type chain.- differs in one amino acid residue. Therefore functional gene is not the same as mutational gene – it consists of many mutable sites. There are also many sites where recombination occurs in a gene.- studies of x- linked lozenge locus of drosophila melanogaster by c . P. Oliver in 1940. He demonstrated crossing over occurred between 2 mutants such as alleles lzs & lzy of sex linked lozenge locus of drosophila melanogaster at a low frequency of 0.2%. First evidence for intra genic recombination. Till that time it was thought gene is not subdivisible and crossing over does not occur within a gene..
- 11. CHROMOSOMES – PHYSICAL CARRIERS OF GENES Although Mendel provided convincing evidence that inherited traits were governed by discrete factors,or genes, His studies were totally unconcerned with the physical nature of these elements or their location within the organism. During the time between Mendel’s work and its rediscovery,a number of biologists became concerned with this other aspect of heredity—its physical basis within the cell. 1903,Walter Sutton,a graduate student at Columbia University,published a paper that pointed directly to the chromosomes as the physical carriers of Mendel’s genetic factors (genes).
- 12. GENE : STRUCTURE & FUNCTION A gene is the fundamental, physical, and functional unit of heredity.Tranfer information from one generation to next generation. In modern view, gene is a long nucleotide sequence of nucleotide pairs that is capable of mutating and recombining at many different sites along its length. The genes make up the sequences of DNA (genotypes). The genotypes, together with the other factors (e.g. environmental factors), determine the phenotypic trait of an organism. Eg.skin colour, hair colour….etc These changes in the genes drive evolution and natural selection.
- 13. Chemically :A gene is a segment or region of DNA containing a sequence of nucleotides that encodes any function – a nucleotide sequence that can make a protein. If it produce pigment protein for eye – it is a gene for eye colour . Here, that particular gene is expressed. Complete DNA has millions of genes.
- 14. A portion of gene may contain introns and exons as in a eukaryotic structure in a mixed way. Both are functional. Exons code for proteins. Introns can not make protein but it can make other RNA- functional non coding RNA like tRNA, rRNA, ribozymes...etc eg. X – inactivation by introns in females. Ribozyme was discovered by Thomas Cech in 1983. Ribozymes include peptidyl transferase in translation, Spliceosome in hnRNA processing……etc
- 15. MODERN CONCEPT OF GENE Along with introns and exons, mutable and recombinable sites, a particular gene also contain regulatory elements in it . Regulatory elements like enhancers, promoters…etc,are also mixed with non- coding sequences like introns- junk dna that is seen in between genes. Enhancers and promoters are not transcribed. So, Gene is a unit of heredity located on chromosomes , made up of DNA and regulatory elements that encodes a functional protein or family of proteins or a functional RNA molecule that influences phenotype.
- 16. SEYMOUR BENZER (1921-2007) The most extensive study on fine structure of gene was undertaken by Seymour benzer for a locus in T4 bacteriophage infecting e- coli. This locus was known as r11 locus He coined the term cistron in genetics, the basis for the genetic code (DNA) developed Cistron is the smallest unit of genetic material which codes for a single polypeptide and functions as a transmitter of genetic information.Thus saying a gene might consist more than one cistron.
- 17. TheT4 rII System Seymour Benzer developed a new genetic technique involving recombination in T4 bacteriophage rII mutants,the technique is called theT4 rII system. The data he collected provided the first evidence that the gene is not an indivisible entity, as previously believed, and that genes were linear. He further proved that mutations were distributed in many different parts of a single gene, and the resolving power of his system allowed him to discern mutants that differ at the level of a single nucleotide. Seymour Benzer also proposed distinct classes of mutations including deletions, point mutations,missense mutations,and nonsense mutations. Benzer’s work on the topic also influenced further scientist including Francis Crick and Sidney Brenner, and Richard Feynman.
- 18. Recon: unit of genetic recombination,the smallest section of a chromosome that is capable of recombination. Consist of not more than 2 pairs of nucleotide. There may be more than one recon in a gene. Muton Smallest segment of genetic material that can undergo mutation. When changes occur to this unit phenotypic changes may occur. Muton can be a single nucleotide. Humans have approximately 20,000 protein coding genes. This is 1.5% of the entire genome. 3,000,000,000 – the entire sequences of genes & based that make up our genome.
- 19. PRESENT STATUS OF A GENE The discoveries of DNA technology, beginning in the early 1970s, have led to the second revolution in the concept of the gene - in which none of the classical ( gene as the indivisible unit of genetic transmission, genetic recombination,gene mutation and gene function- during 1910- 1930) or neoclassical criteria for the definition of the gene ( which prevailed from 1940 until the 1970s. In this view the gene or cistron, as it was now called, was divided into its constituent parts, the mutons and recons, materially identified as nucleotides)hold strictly true - The discoveries concerning gene repetition and overlapping,movable genes, complex promoters,multiple polyadenylation sites, polyprotein genes, editing of the primary transcript,pseudogenes and gene nesting are improving the concept of gene.
- 20. A PEEP INTO GENE FOR EYE COLOUR It was thought that eye colour was a single gene trait – with the allelle for brown eyes being dominant over blue. Now, scientists realize it was wrong and atleast 8 genes influencethe final eye colour. The genes control the amount of melanin inside specialised cells called iris. The range – blue to hazel to brown depends on level of melanin pigment stored in the melanosome packets in the melanocytes of the iris. OCA2 gene located on chromosome 15 produces a protein – p protein involved in formation & processing of melanin. Other genes included areTYRP1,ASIALC42A5…etc.
- 21. COMPLEMENTATION TEST Complementation test determine whether different recessive mutations are in the same gene or in different genes Test uses genetic complementation –for restoration of wild type phenotype by mating 2 different mutants. If two recessive mutations,a and b , are in the same gene, then a diploid organism carrying one a allelle and one b allelle will exhibit the mutant phenotype because neither allelle provides a functional copy of the gene. In contrast , if mutations a and b are in separate genes, then the heterozygous carrying a single copy of each mutant allele will not exhibit the mutant phenotype because a wild type allelle of each gene is also present. In this case , the mutations are said to complement each other.
- 22. If more than one gene is required for the effect of a phenotype Gene a + gene b =eye colour Occurs if there is mutation in either of these gene and if it is in homozygous recessive condition. Particular phenotype will be expressed only if ,at least one copy of each gene is present;These two genes a & b complement each other to produce a phenotype. During segregation , one Chromosome is from father and one is from mother.So, if both genes are present in different chromosomes,there is high chance to get the complementation effect. Mutant alleles present in the same gene do not show complementation, while those located in different genes show complementation. If the result is complementing with each other,then the mutations are in different genes. Complementation analysis can not be performed on dominant mutations because the phenotype conferred by the mutant allelle is displayed even in the presence of a wild type allelle of the gene.
- 23. TWO INDEPENDENT MUTATIONS THAT RESULT IN NOWING DEVELOPMENT
- 24. REFERENCES Lordish,H.,Berk,A.,Zipursky,S.,Matsudaira,P., Baltimore,D.,& Darnell,J.(2018). Molecular cell biology (8th edition).NewYork:W.H.Freeman and company. Pubmed.ncbi.nlm.nih.gov.- Historical development of the concept of gene.- Petter portin.2002 June Pubmed.ncbi.nlm.nih.gov.-The evolving defenition of the term gene- .Portin P.et al . Genetics 2017 Medline.plus.gov.- What is a gene?