Keystone hypothesis

TLS Online TPP Program

#Id: 6029

#Unit 3. Fundamental Processes

The crystal structure of PCNA closely resembles the E. coli b subunit: A trimer forms a ring that surrounds the DNA.  The sequence and subunit organization are different from the dimeric b2 clamp; however, the function is likely to be similar.

TLS Online TPP Program

#Id: 6030

#Unit 3. Fundamental Processes

The exonuclease FEN1 removes the RNA primers of Okazaki fragments. The complex of DNA polymerase d and FEN1 carries out the same type of nick translation that E. coli DNA polymerase I carries out during Okazaki fragment maturation

TLS Online TPP Program

#Id: 6031

#Unit 3. Fundamental Processes

In mammalian systems where the DNA polymerase does not have a 5’–3’ exonuclease activity, Okazaki fragments are connected by a two-step process. 

TLS Online TPP Program

#Id: 6032

#Unit 3. Fundamental Processes

Synthesis of an Okazaki fragment displaces the RNA primer of the preceding fragment in the form of a “flap.”

the base of the flap is cleaved by the enzyme FEN1 (f lap endonuclease 1).  In this reaction, FEN1 functions as an endonuclease, but it also has a 5’–3’; exonuclease activity. In DNA repair reactions, FEN1 may cleave next to a displaced nucleotide and then use its exonuclease activity to remove adjacent material.

TLS Online TPP Program

#Id: 6033

#Unit 3. Fundamental Processes

Failure to remove a flap rapidly can have important consequences in regions of repeated sequences such as deletion and duplication

TLS Online TPP Program

#Id: 6034

#Unit 3. Fundamental Processes

Direct repeats can be displaced and misaligned with the template; palindromic sequences can form hairpins. These structures may change the number of repeats. The general importance of FEN1 is that it prevents flaps of DNA from generating structures that may cause deletions or duplications in the genome.