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【单选题】
Ry是表面粗糙度评定参数中（）的符号。A.
轮廓算术平均偏差
B.
微观不平度点高度
C.
轮廓最大高度
D.
轮廓不平程度
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【多选题】What are the functions of reverse transcriptases?
A.
Reverse transcriptases catalyze three different reactions: (1) RNAdependent DNA synthesis, (2) RNA degradation, and (3) DNA-dependent DNA synthesis. Like many DNA and RNA polymerases, reverse transcriptases contain Zn 2+ .
B.
Each transcriptase is most active with the RNA of its own virus, but each can be used experimentally to make DNA complementary to a variety of RNAs. The DNA and RNA synthesis and RNA degradation activities use separate active sites on the protein. For DNA synthesis to begin, the reverse transcriptase requires a primer, a cellular tRNA obtained during an earlier infection and carried in the viral particle.
C.
This tRNA is base-paired at its 3′ end with a complementary sequence in the viral RN The new DNA strand is synthesized in the 5′ → 3′ direction, as in all RNA and DNA polymerase reactions. Reverse transcriptases, like RNA polymerases, do not have 3′ → 5′ proofreading exonucleases.
D.
They generally have error rates of about 1 per 20,000 nucleotides added. An error rate this high is extremely unusual in DNA replication and seems to be a characteristic of most enzymes that replicate the genomes of RNA viruses. A consequence is a higher mutation rate and faster rate of viral evolution, which is a factor in the frequent appearance of new strains of disease-causing retroviruses.
【单选题】关于摇摆式颗粒机叙述正确的是( )
A.
可以直接加入粉末和黏合剂
B.
应先制好软材再加入机器中制粒
C.
使用时，先加入制好的软材，然后再开机运行
D.
加入软材时应很少量加入，防止机器负荷过大
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