The interphase nuclei of some tissues of the larvae of dipteran flies contain chromosomes that are greatly enlarged relative to their usual condition. They possess both increased diameter and greater length. shows an example of a chromosome set from the salivary gland of D. melanogaster. The members of this set are called polytene chromosomes.

TLS Online TPP Program

#Id: 3293

#Unit 3. Fundamental Processes

Base excision repair of a T∙G mismatch. 

TLS Online TPP Program

#Id: 3297

#Unit 3. Fundamental Processes

oxoG:A repair, Oxidation of guanine produces oxoG. 

The modified base can be repaired before replication by DNA glycosylase via the base excision pathway. 

If replication occurs before the oxoG is removed, resulting in the misincorporation of an A, then a fail-safe glycosylase can remove the A, allowing it to be replaced by a C. 

This provides a second opportunity for the DNA glycosylase to remove the modified base.

TLS Online TPP Program

#Id: 3299

#Unit 3. Fundamental Processes

Another example of a fail-safe system is a glycosylase that removes a T opposite a G. Such a T:G mismatch can arise, as we have seen, by spontaneous deamination of 5-methylcytosine,which occurs frequently in the DNA of vertebrates. 

TLS Online TPP Program

#Id: 3301

#Unit 3. Fundamental Processes

DNA lesions that cause large distortions in the helical structure of DNA generally are repaired by the nucleotide-excision system, a repair pathway critical to the survival of all free-living organisms. 

TLS Online TPP Program

#Id: 3303

#Unit 3. Fundamental Processes

Nucleotide excision repair in E. coli is largely accomplished by four proteins: UvrA, UvrB, UvrC, and UvrD

TLS Online TPP Program

#Id: 3306

#Unit 3. Fundamental Processes

Unlike base excision repair, the nucleotide excision repair enzymes do not recognize any particular lesion. Rather, this system works by recognizing distortions to the shape of the double helix, such as those

caused by a thymine dimer or by the presence of a bulky chemical adduct on a base.