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【多选题】
减免税的基本形式有( )。A.
税种式减免
B.
税基式减免
C.
税额式减免
D.
税率式减免
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【简答题】国外交易所规定，套利的佣金支出与一个回合单盘交易的佣金相比（ ）。
【单选题】Ribozymes ______.
A.
are RNA molecules
B.
act as catalysts
C.
can build RNA molecules
D.
can carry out exchanges of RNA fragments
E.
all of the above
【多选题】What are the functions of ribozymes?
A.
The best-characterized ribozymes are the self-splicing group I introns, RNase P, and the hammerhead ribozyme. Most of the activities of these ribozymes are based on two fundamental reactions: transesterification and phosphodiester bond hydrolysis (cleavage).
B.
The substrate for a ribozyme is often an RNA molecule, and it may even be part of the ribozyme itself. When its substrate is RNA, the RNA catalyst can make use of base-pairing interactions to align the substrate for the reaction.
C.
Ribozymes vary greatly in size. A self-splicing group I intron may have more than 400 nucleotides. The hammerhead ribozyme consists of two RNA strands with only 41 nucleotides in all. As with protein enzymes, the 3D structure of ribozymes is important for function.
D.
Ribozymes are inactivated by heating above their melting temperature or by addition of denaturing agents or complementary oligonucleotides, which disrupt normal base-pairing patterns. Ribozymes can also be inactivated if essential nucleotides are changed.
【简答题】核酶（ribozymes）
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