KDC shows Divergent evolution, in which the genes for an enzyme with one substrate specificity give rise, during evolution, to closely related enzymes with different substrate specificities but the same enzymatic mechanism.

TLS Online TPP Program

#Id: 3309

#Unit 3. Fundamental Processes

Nucleotide excision repair in human cells. 

A DNA lesion that causes distortion of the double helix, such as a thymine-thymine dimer, is initially recognized by a complex of the XP-C (xeroderma pigmentosum C protein) and 23B proteins.

TLS Online TPP Program

#Id: 3311

#Unit 3. Fundamental Processes

Not only is nucleotide excision repair capable of mending damage throughout the genome, but it is also capable of rescuing RNA polymerase, the progression of which has been arrested by the presence of a lesion in the transcribed (template) strand of a gene, this phenomenon, known as transcription-coupled repair.

TLS Online TPP Program

#Id: 3328

#Unit 3. Fundamental Processes

The significance of transcription-coupled repair is that it focuses repair enzymes on DNA (genes) being actively transcribed.

TLS Online TPP Program

#Id: 3330

#Unit 3. Fundamental Processes

Central to transcription-coupled repair in eukaryotes is the general transcription factor TFIIH.

TLS Online TPP Program

#Id: 3331

#Unit 3. Fundamental Processes

TFIIH is responsible for two separate functions: 

Its strand-separating helicases melt the DNA around a lesion during nucleotide excision repair (including transcription-coupled repair) and also help to open the DNA template during the process of gene transcription.

TLS Online TPP Program

#Id: 3333

#Unit 3. Fundamental Processes

Humans can exhibit a genetic disorder called xeroderma pigmentosum (XP), an autosomal-recessive disease that renders afflicted individuals highly sensitive to sunlight and results in skin lesions, including skin cancer.