Orthogenesis- “Straight-line evolution,” held that the variation that arises is directed toward fixed goals.

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#Id: 3040

#Unit 3. Fundamental Processes

DNA ligase makes the bond that connects the 3 ' end of one Okazaki fragment to the 5 ' beginning of the next fragment.

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#Id: 3041

#Unit 3. Fundamental Processes

Synthesis of an Okazaki fragment terminates just before the start of the RNA primer of the preceding fragment.

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#Id: 3042

#Unit 3. Fundamental Processes

FENl is an exo-jendonuclease that recognizes the structure created when one strand of DNA is displaced from a duplex as a "flap." In mammalian replication it cleaves at the base of the flap to remove the RNA primer.

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#Id: 3043

#Unit 3. Fundamental Processes

The gap is filled by DNA polymerase I; pol A mutants fail to join their Okazaki fragments properly. 

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#Id: 3044

#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. 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 (flap endonuclease 1 ) .

TLS Online TPP Program

#Id: 3045

#Unit 3. Fundamental Processes

FEN 1 functions as an endonuclease, but it also has a 5 '-3' exonuclease activity. In DNA repair reactions, FEN 1 may cleave next to a displaced nucleotide and then use its exonuclease activity to remove adjacent material.