22. 在大腸桿菌中，通過核苷酸切除修復(nucleotide excision repair)來修補胸腺嘧啶二聚體 (thymine dimer)，必要的酶依作用順序，下列何者正確？
(A)核酸酶(nuclease)，DNA 聚合酶 III(DNA polymerase III)，RNA 引發酶(primase)
(B)解旋酶(helicase)，DNA 聚合酶 I(DNA polymerase I)，DNA 連接酶(ligase)
(C)DNA 連接酶，核酸酶，解旋酶
(D)核酸酶，DNA 聚合酶 I，DNA 連接酶

In prokaryotes: Uvr proteins

The process of nucleotide excision repair is controlled in Escherichia coli by

the UvrABC endonuclease enzyme complex, which consists of four Uvr proteins:

UvrA, UvrB, UvrC, and DNA helicase II (sometimes also known as UvrD in this complex).

(複合體由皆內核酸酶組成)

辨識階段：

1. First, a UvrA-UvrB complex scans the DNA, with the UvrA subunit recognizing distortions in the helix, caused for example by pyrimidine dimers.
2. When the complex recognizes such a distortion, the UvrA subunit leaves and an UvrC protein comes in and binds to the UvrB monomer and, hence, forms a new UvrBC dimer.

切除階段：

1. UvrB cleaves a phosphodiester bond 4 nucleotides downstream of the DNA damage,
2. and the UvrC cleaves a phosphodiester bond 8 nucleotides upstream of the DNA damage and created 12 nucleotide excised segment.
3. DNA helicase II (sometimes called UvrD) then comes in and removes the excised segment by actively breaking the hydrogen bonds between the complementary bases.

填補、黏合階段：

1. The resultant gap is then filled in using DNA polymerase I and DNA ligase. The basic excision process is very similar in higher cells, but these cells usually involve many more proteins – E.coli is a simple example

##以上，可以知道Uvr B、C 切除單股中的酯鍵(分別往突變處的下游、上游下刀)，形成拋棄片段

後來解旋酶II(或稱為UvrD)才幽幽地出現，任務是撕開拋棄片段和互補股之間的藕斷絲連(移除氫鍵)

這題出的不是很好，B選項其實也符合過程的順序，

但是題目有問你""必要""，所以還是選核酸酶(內核酸酶)比較好~~

參考出處：維基百科