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End Repair Enzymes

Two enzymes are typically used for repairing DNA with sticky ends (see Figure 1)

  • Polymerase: Polymerase fills in the missing bases for the strand, in the 5' to 3' direction. The resulting double-stranded DNA will be the same length as the initial longest DNA strand.
  • Exonuclease: Exonuclease removes the 3' overhangs. The resulting double-stranded DNA will be the same length as the original shortest DNA strand.

Two reactions showing how polymerase and exonuclease repair DNA sticky ends. The first reaction starts with the 5 prime to 3 prime containing the nucleotide, G, and the 3 prime to 5 prime containing nucleotides, C, T, T, A, A. This means there is a 5 prime overhang and a sticky end was formed after E c o R 1 cleavage. Above the reaction arrow is d N T P s, and below is 5 prime to 3 prime polymerase activity. At the end of the reaction, the sticky end is filled in to yield a blunt ended fragment. The 5 prime to 3 prime contains nucleotides, G, A, A, T, T, and the 3 prime to 5 prime contains nucleotides, C, T, T, A, A. The second reaction starts with the 5 prime to 3 prime containing the nucleotides, G, G, T, A, C, and a 3 prime to 5 prime containing the nucleotide, C. This means there is a 3 prime overhang and a sticky end was formed after K p n 1 cleavage. Above the reaction arrow is d N T P s, and below is 3 prime to 5 prime exonuclease activity of the polymerase. At the end of the reaction, the sticky end is removed to yield a blunt ended fragment. The 5 prime to 3 prime contains the nucleotide, G, and the 3 prime to 5 prime contains the nucleotide, C.

Figure 1. The Polymerase adding nucleotides in the 5' → 3' direction and the exonuclease removing nucleotides from the 3' overhangs.

Referred from:

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