Streptomycin

Streptomycin is an antibiotic commonly known for being the first medical solution against tuberculosis. This was the first aminoglycoside molecule, consisting of amino-modified sugars, to be discovered (October 19, 1943 by Albert Schatz). This bactericide acts as a protein synthesis’ inhibitor; it binds to the ribosome of bacteria interfering with the binding of formyl-methionyl-tRNA. This phenomenon will directly affect the reading of codons, which will lead to inhibition of protein formation and, as consequence, the death of the bacteria.

Haploinsufficiency

Haploinsufficiency occurs when a cell is incapable of producing the total level of a gene product (typically a protein). It happens only in diploid organisms, when only one of the gene copies is functional, while the other copy carries a mutation (e.g. a deletion in the gene itself).

In other words, if the normal phenotype requires protein production of both alleles and one is not functioning, there will only exist about 50% of the quantity necessary for the proper functioning of the cell, resulting in an abnormal phenotype, leading to disease (some cancers).

DNA mismatch repair

DNA mismatch repair

Mismatch repair is a type pf DNA repair mechanism in which consists in the repair of a DNA strand that has suffered a alteration in a nucleotide. These changes occur only when replication occurs and in daughter DNA strands only. Furthermore, these changes are not mutagenic, meaning that the altered nucleotides possess the same base match properties of the “normal” nucleotides, making them harder to distinguish from “normal” base matches.  

Since this type of damage/error is not easily detected, it is necessary to distinguish daughter DNA strand, in which occurs the nucleotide alteration, from mother DNA strand . In E. coli, mother DNA strands are methylated, meaning that these strands have methyl groups, unlike daughter strands, which have not. Mother strands are methylated due to DNA adenine methylase, enzyme that convert adenines to 6-methyladenines in the sequence 5′-GATC-3′, and DNA cytosine methylase, enzyme that converts cytosine to 5-methylcytosines in the sequences 5′-CCAGG-3′ and 5′-CCTGG-3′. 

The methylation process does not occur immediately after replication. It takes some time for the daughter strand to be methylated after being created, making a time window long enough for the mismatch repair process to take place.

In E. coli, there are three types of mismatch repairs:

·         long patch -> the DNA that is repaliced can be over 1kb long and utilizes enzymes MutH, Muts and MutL, along with DNA helicase II. MutH recognizes the undermethylated strand (daughter strand) by binding to undermethylated 5′-GATC-3′ sequences and MutS binds to the mismatch zone. It is unclear the role of MutL in the process, but it may somehow coordinate the other two proteins, so that MutH binds only to 5′-GATC-3′ sequences in the proximity of the mismatch zone recognized by MutS. After binding, MutH cuts the phosphodiester bond immediately upstream of the G nucleotide of the 5′-GATC-3′ sequence and DNA helicase II separates the single strand. The region originated from this process is degenerated by an exonuclease. This leaves a blank in the DNA sequence which is filled by DNA created by DNA polymerase I and DNA ligase;

·         short patch -> the events that occur in this type of repair are similar, the difference lying in the specificities of proteins involved in the recognition of the mismatch and the size of the replaced strand, of about 10bp long;

·         very short patch -> similar to short strand, but different enzymes and shorter strand.  

               It seems necessary to warn that these processes were study in E. coli. In mammals, mismatch repair was proved to exist, but the methylated chains are short, which puts the strand identification method mentioned above in cause.

Conditional-lethal mutation

Conditional-lethal mutations are understood as mutations that can cause a lethal effect. However, this effect only occurs when dealing with certain environmental conditions, hence it is conditioning. At a restrictive environmental condition, the mutation assumes the lethal effect, however, at a permissive environmental condition, the mutation allows cell growth and reproduction of apparently normal manner.

The presence of these mutations has been observed in many organisms, including humans, serving as a genetic marker of much of essential genes. Moreover, the conditions which enable these mutations are subject, through his study, draw conclusions about the nature and behavior of genes that so-called normal mutations do not allow.

Conditional lethal mutation made by genetic recombination, In this case we can see how different conditions - absence or presence of Ara - affect the behavior of the mutation

Bibliografia:

Black A. K., Singh S.M.; Conditional Lethal Mutations as Experimental Tools; Encyclopedia of life sciences (2001)

Kima S.W., Kangb H.Y., Hurc J., Gald S.W., Banga W.Y., Choe K., Kime C.W., Bahkf J.D., Leec J.H.; Construction of a conditional lethal Salmonella mutant via genetic recombination using the ara system and asd gene; Journal of Microbiological Methods; Volume 87 (2011)

Nucleotide tautomerization

Tautomerization (or tautomerism) is a reaction that allows isomers of a chemical compound to interconvert (tautomers). This reaction normally results in the migration of a hydrogen atom or proton and switch of a single bond and adjacent double bound, and is a reversible reaction.  Unlike other classes of isomers, tautomeric compounds exist in mobile equilibrium with each other, so that attempts to prepare the separate substances usually result in the formation of a mixture that shows all the chemical and physical properties to be expected on the basis of the structures of the components.

An example is the Keto-enol tautomerism where the hydrogen of the ketone (an aldehyde) migrates near the oxygen molecule bound with it forming an enol (alcohol), this bounding make the double bound between the oxygen and carbon turn into a single bound, and so a double bond is created between two carbon molecules.

 

Keto-enol tautomerism.
Left the keto form; right the enol.

Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules. In these cases the lactam – lactim tautomerism is present, it is an amide - imidic acid tautomerism in heterocyclic rings that usually happen in the nucleobases guanine, thymine, and cytosine.
 



Thymine(a) and Guanine(b) tautomerism.

Clastogen

Clastogen is a mutagenic agent with the function of breaking chromosomes directly acting on DNA that produce the gain, loss, or rearrangements of chromosomal sections. If the cells don’t receive repair care or do not start apoptosis, the mutation can lead it to carcinogenesis. Caffeine and acridines are some examples of chemical clastogens and x-ray, y-radiation and ultra-violet rays are examples of the radiation type of clastogen.

Genetic reversion

Genetic reversion is a mutation that occurs when a mutant gene is reverted to wild type. This reversion is located in the same place of the first mutation, which reverses the modification made previously. Is important to refer that this process will alter the base that suffered mutation before, in other words, the base will be altered and will return to what was in the first place, in the exact same position. Reverse mutation is less common than the forward mutations.

Fig.1:a) Cells suffer mutations from natural variation and sexual recombination or by influence from external agents.

         b) Mutated cells will suffer another mutation, that is the reversion to wild type.