DNA reparation. The statements of gene theory

Some damage of DNA may occur due to action of different agents or during protein biosynthesis. Many of those damages are corrected by special reparative enzymes. Reparation (from Latin - reparatio - to recover) - is a process of restoring natural DNA structure, which has been damaged during protein biosynthesis or due to harmful influences of external agents, having presented in all organism cells. The reparation process is based on fact that DNA molecule contains two complementary chains. So if one of them has been damaged, it may be repaired corresponding to other chain.

The DNA reparation was discovered in bacteria subjected to ultraviolet radiation. The pyrimidine bases in DNA adsorb ultraviolet radiation. This changes the structure of these bases. Now they are able to make a covalent bond between two pyrimidine bases placed together on a one strand. The resulting cross-link between adjacent bases of the DNA strand is called a pyrimidine dimer. However, it was shown that cells subjected to ultraviolet radiation survive better while placing on light than in darkness. It was stated that here occur photoreactivation or light reparation. The pyrimidine dimers are replaced by special enzyme, which become activated by action of visible light (pic 6.5a).

Ріс. 6.5. The DNA reparation: A - plitopeactivation, В - “dark" reparation, C - postreplicative reparation (by F. Fogel, A. Motulsky, 1989)

Later it was founded that cells may replace damaged regions of DNA without light exposure (dark reparation). We may observe dark reparation when cell recovering from ionizing radiation exposure, chemical impact or from other factors. It has several stages involving several enzymes. The first enzyme (endonuclease) recognizes damaged region and cuts DNA strand around it. The second enzyme (endo- or exonuclease) makes a second cut on DNA strand. The third enzyme cuts off damaged nucleotides. The fourth enzyme (DNA polymerase) makes a new strand of DNA corresponded to undamaged one. The fifth enzyme (ligase) glues the ends of DNA strands (pic 6.5b).

The postreplicative reparation is performed by recombination (fragments exchange) between two newly made DNA molecules (pic 6.5c). It is useful when pyrimidine dimers have not been removed by light reparation.

If the reparation cannot successfully repair high number of DNA defects, cell block DNA replication to prevent defects transmission to next cell generation.

Working together replication and reparation enzymes provide a small level of DNA molecule mistakes.

The statements of gene theory. The findings listed above allow formulating the gene theory. Its statements are the following.

1. The gene has definite locus in chromosome.
2. The gene is a part of genomic nucleic acid. The number of nucleotides in genes is not same.
3. There is mutation and recombination inside of a gene.

4. There are structural and functional genes.
5. The structural genes encode the macromolecules structure (such as proteins, tRNA, rRNA).
6. The functional genes control the structural genes activity.

7. The triplet’s line in gene corresponds with amino acids line in polypeptide.
8. Genes are able to reparation.
9. The genotype on being discrete acts as integrated unit.