Errors occurred due to mistakes

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Cells have developed the system to prevent the errors in the DNA replication anh thus, helps in repairing the errors occurred due to mistakes. Such errors are caused called as mutation. Mutation is the change in the DNA sequence of cell genome which is heritable. It causes various changes in the gene number and the chromosomal number of the genome which causes due the mutagen. Mutagen causes manipulations in the genetic material duringcell division either by exposuring to ultraviolet,ionizing radiation,mutagenic chemicals orviruses.

Mutations are caused due to genetic disorders or diseases. Different types of mutation occur on a macroscopic level in the form of chromosomal mutations or by a change in the single base pair of DNA sequence. Mutations prevent the synthesis of the correct protein which occur in the gene promoter regions changing the expression levels of the protein causing disruption to the splicing process and causes the production of incorrect protein. Many mutations have detrimental effects on an organism and as it is the basis of many human genetic disorders and disease.

Mutations are the heritable changes in the genome. Some mutations may cause changes in the phenotypic characteristics of the gene expression. In microbial organisms they are designated as wild-type strains and agents that cause mutations are called as mutants. Therefore, mutants are characterized by the inherited differences between them and their ancestral wild-type strains. Variant forms of a specific genetic determinant are called alleles. Genotypic symbols are lower case, italicized specify individual genes with a (+) superscript indicating the wild type allele were as phenotypic symbols are capitalized and not italicized. For example: to produce ß-galactosidase, in order to ferment lactose, the genotypic symbol used is lacZ+ and mutants not producing ß-galactosidase are lacZ. The lactose-fermenting phenotype is designated as Lac+ and inability to ferment lactose is Lac-.


  • Tautomerism- there is change in the base by alteration of the hydrogen bonding of that base. It's the conversion of keto to enol form and vice -versa.
  • Depurination- it is the loss of a purine base (A or G).
  • Deamination-it involves the changes of the normal base pair to the other base pair due to hydrolysis.
  • Transition - the conversion of purine to another purine, or a pyrimidine to a pyrimidine.
  • Transversion - conversion of purine to pyrimidine and vice versa.


Selection does not eliminate the bad gene completely because mutation helps in repairing the defaulted gene. In this case the equilibrium frequency of the mutation will work, acc. to which, the equilibrium is between the rate at which the mutant gene arises by recurrent mutation and its elimination is done by the natural selection. This is known as mutation-selection balance.Therefore, mutation-selection balance helps in maintaing the genetic polymorphism.

There are certain traits orallelesof aspecieswhich are subjected toselection in the evolution. Under selection, individuals with adaptive traits are chosen means they contribute offspring to the succeeding generation than others. When traits have their genetic basis, selection has increased the prevalence of those traits i.e. offspring willinheritthose traits from their parents. When selection persists in the nature, adaptive traits become universal to the population and the species which haveevolved.


Mutations are classified as selectable and unselectable:-

  • A selectable mutation is that mutation in which there is a growth and survivality under specific environment conditions where the mutant can grow and wild type will die.
  • An unselectable mutant requires a cell by cell examination in order to find a mutant with desirable characteristics.
  • Selection can be direct or indirect:-the indirect selection is used for isolating mutant that is deficient in producing a necessary growth factor. Like E.coli can grow on glucose and mineral salts more than lactic acid.



Department of Molecular Biology, Princeton University, Princeton, New Jersey, 08546-5414

NM23/NDPk protein plays an important role in the cancer and the development. These proteins play a role in the DNA metabolism including, binding of the nucleotide and the transcription and the cleavage of the DNA strands using covalent DNA complexes. DNA cleavage reaction of NM23/NDPk is characteristics of DNA repair enzymes. It is seen that both DNA cleavage and the NDPk reactions are conserved between E.coli and te human beings. NM23/NDP kinases are regulators of gene expression during development and the cancer via unrecognized pathways and mechanism of genetic control and unrecognized role in DNA repair and recombination.

The Parkinson disease causing LRRK2 mutationI2020T is associated with increased kinase activity:-

Christian Johannes Gloeckner,Norbert Kinkl,Annette Schumacher,Ralf J. Braun,Eric O'Neill,Thomas Meitinger,Walter Kolch,Holger Prokisch andMarius Ueffing

Parkinson disease is the most common neurodegenerativedisorder. Recent studies have shown that mutations in theleucine-richrepeat kinase2 gene, located on the chromosomalregionare the cause for the disorder named as Parkinson disease. The LRRK2 protein consistsof multiple domainscontains a predicted kinase domain, with homologyto MAP kinases. Using cell fractionation and immuno fluorescencemicroscopy, visualized that LRRK2 is located in the cytoplasmwhich is associated with cellular membrane structures. LRRK2 protein shows the autokinase activity. The disease-associated I2020T mutant shows a significant increase in the autophosphorylationofwild-type proteinin vitro. This suggeststhat the PD is caused by the I2020T mutation which is associatedwith an increase rather than a loss in LRRK2 kinase activity.

Mediator protein mutations that selectively abolish activated transcription:-

The deletion of any one of three subunits of yeast mediator as (Med2, Pgd1 (Hrs1), and Sin4) of transcriptional regulation terminated the activation by Gal4-VP16. Other mediators functions like the stimulation of basal transcription and of TFIIH kinase activity were remain unaffected. The overlapping Mediator subunit was found for activation by Gcn4. So, the genetic requirements for activationin vitrowere closely related with thosein vivo. A whole genome expression of a?med2strain showed diminished transcription of a subset of inducible genes but that show only minor effects on basal transcription. These findings make an important connection between transcriptional activationin vitroandin vivo and identify Mediator as a "global" transcriptional co activator.

DNA gyrase and topoisomerase IV: biochemical activities, physiological roles during chromosome replication, and drug sensitivities:-

DNA gyrase and topoisomerase IV are two topoisomerases present in the bacteria. They play crucial role in the cell. Topoisomerase gyrase is involved in the chain elongation during the replication of the chromosome whereas topoisomerase IV separates the linked daughter chromosomes during the terminal stage of DNA replication. The different roles can be attributed to the various biochemical properties of the two enzymes. Therefore, biochemical activities, physiological roles, and drug sensitivities of the enzymes are reviewed and studied using gyrase and the topoisomerase.

Recurrent Mutation and Selection for Increased Penicillin Titre in Aspergillus nidulans

Recurrent mutation and selection are used to increase penicillin titre in two closely related strains of Aspergillus nidulans. The two strains (programmes A and B) were selected and continued through six cycles of mutation and selection. It was seen that near-ultraviolet light which was in conjuction with 8-methoxypsoralen was taken as the sole mutagen throughout programme A and ethyl methanesulphonate as the sole mutagen throughout programme B.100 survivors were assayed for penicillin titre, after each mutagenic treatment in each programme where as the best five strains were picked on the basis of a single yield test and carried forward to the next cycle. In both selection programmes A and B, 300% increase in penicillin titre was achieved. The yield obtained illustrates the effectiveness for the strain development of experimental designs having successive cycles of mutagenesis with a single-stage screen and the selection of the few percent survivors in of each cycle.


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