The term change is used to indicate an irreversible modification of information Génétique and Héréditaire, in the Séquence of a Génome (DNA or ARN). The changes can be due to errors of copy of the genetic material during the cellular Division, or to the exposure to mutagen agents (Radiation S, chemical agents, Virus). Most of the errors made during the Réplication of the genome are corrected immediately by complex and effective mechanisms of repair of the DNA, and only a weak share of these errors become changes transmitted to the cell-girls.
At the pluricellular animal , the changes of the germinal line can be transmitted to the descent, contrary to the somatic changes .
The changes explain the existence of a variability between genes. Changes, which is the least favorable (noxious) to the survival of the individual who carries them, are eliminated by the play from the Natural selection, whereas the advantageous changes tend to accumulate. The majority of the changes are known as neutrals, they do not influence the selective Valeur and can be fixed or disappear by the play from the genetic Dérive. The spontaneous changes, generally rare and random, thus constitute the genetic independent source of Diversité, engine of the evolution. The causes of the spontaneous changes are unknown.
In Botanical, an organization transferring is called a Sport.
Agents Mutagen S
The change is a spontaneous phenomenon, due to errors in the process of Réplication of DNA. However, in certain circumstances, the rate of changes can be increased considerably by physical or chemical factors, called agents Mutagène S.
- the electromagnetic Ondes (x-rays, gamma rays, ultraviolet rays).
- Of chemical substances interacting with DNA (or possibly with ARN) such as Pesticide S, derived from Benzene, solvents, Colchicine, etc
- a modification of the system of repair of the DNA, which then ceases correcting the errors of réplications.
Transmission of the changes
-
If a change affects the germinal cells, it is transmitted to the descendants of the mutant individual. In certain cases, this change can get a selective advantage or on the contrary to be noxious, even lethal. It is the base of the process of the evolution. It is however allowed that the majority of the changes intervene between the Gène S, in the Intron S, or at places where them effect is tiny (synonymous Mutations) ; the majority of the changes are thus probably neutral, and are not preserved (or eliminated) only by chance (genetic Dérive).
- On the other hand, as it is the case for the majority of the accidental changes (caused by irradiation or chemical substances), if it affects only somatic cells, the change is not transmitted and will thus affect only the subject having undergone it directly. If the cells divide actively, there is possibility of creation of a tumor being able to evolve to a cancer. On the other hand, if there is no division the effect is negligible.
Various types of change
A change is known as sexual when it relates to a sexual chromosome, for example X/Y at the Mammifère S or W/Z in the birds. A change is known as autosomic losqu' it touches another gene that the genes of sexual determination.The changes can be classified according to their methods of modification of gene:
- SPECIFIC CHANGES (modify only one nucleotide of the sequence of DNA)
- Mutations by substitution:
- the changes misinterpretation . This specific change results in the change of a nucleotide by another. In certain cases, this nucleotide modification involves a modification of the Amino-acid coded. The change of an amino-acid can have or not a repercussion in term of function of protein produced by gene, in the case of a coding gene, or of a modification of affinity for a factor of transcription, in the case of a zone promotrice of the DNA. One speaks about change of transition (transition change) when there is substitution of a purine base by another purine base (or of a pyrimidic base by another pyrimidic base). On the contrary a change of transversion (transversion change) is a change caused by the substitution of a purine base by a pyrimidic base (or of a pyrimidic base by a purine base);
- the changes nonsense . The change of a nucleotide causes the replacement of a codon specifying an amino-acid by a codon stop. That involves the production of a truncated protein;
- the quiet changes . The change of a nucleotide but the new triplet codes the same amino-acid as the original triplet, therefore this change does not have any consequence.
- Insertions and délétions:
- DYNAMIC CHANGES
Consequence of a change
The changes can be classified according to their phenotypical consequences :-
the majority of the changes have more or less important phenotypical consequences (some of them can have serious consequences like the Cancer or of the genetic diseases, because the modification of only one amino-acid in the chain constituting a Protéine can completely modify its space structure, which conditions its operation);
- the neutral changes do not modify the operation of protein and do not have a macroscopic phenotypical consequence;
- the dumb changes quiet or do not involve any change in the sequence of amino-acids, which is due to the many redundancies in the genetic Code. Indeed, the third base of a codon is in general not coding (in fact, several let us code different code the same amino-acid). This property is called degeneration of coding.
See too
- molecular Clock
- recessive change
External bonds
-
Moroccan Human Database Change:
-
TPE of last year secondary school students on the change and the mutagenèse
Simple: Change
| Random links: | Katimavik | François Boucq | Cook bélizienne | Currency school | Building and loan association of France |