Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    3' 3' DNA polymerase 5' 3' 3' 5' exonuclease III 5' 3' Figure 4.5: removal and restoration of cohesive ends. DNA polymerase I extends the 3' ends to fill in the duplex up to each 5' end (bases added are shown in grey). This causes loss of infectivity. The added bases can be removed by exonuclease III to restore the protruding cohesive single stranded ends. restores them. In these experiments, about 20 nucleotides appeared to be added and removed at each end. Treatment of native A. DNA with exonu clease III also abolishes infectivity, although it may be restored by treat ment with DNA polymerase; this suggests that the activity of the cohesive ends is lost when the single strand length is too great, again being restored when the appropriate length is regenerated. Wu and Kaiser (1968) showed that the same inactivation and reactivation by polymerase and exonu clease is displayed by the isolated halves of X'DNA; this suggests that both cohesive ends possess the same structure. Are the two cohesive ends the same length? The open circles of X DNA can be covalently sealed by the action of polynucleotide ligase alone. Since ligase is able only to form covalent bonds between adjacent nu cleotides, this observation implies that there is only a single strand break and no nucleotide gap in the open circles (see Geliert, 1967; Gefter et al., 1967; Kaiser and Wu, 1968). To achieve this result, the two cohesive ends must be exactly the same length.