临床症状消失

临床症状消失后3天

1次大便培养阴性

2次大便培养阴性

3次大便培养阴性

临床症状消失

临床症状消失后3天

1次大便培养阴性

2次大便培养阴性

3次大便培养阴性

成熟细胞被重新编程的过程中细胞的分化程度逐渐降低

诱导多功能干细胞能分化成神经细胞，体现了诱导多功能干细胞的全能性

诱导多功能干细胞与成熟细胞相比，细胞核内DNA的含量不变

诱导多功能干细胞分化的过程不仅与基因有关，也与细胞的生活环境有关

microRNAs (miRNAs)

small interfering RNAs (siRNAs)

l ong noncoding RNAs (lncRNAs)

tRNAs

正确

错误

物理学

化学

生物学或医学

经济学

文学

正确

错误

Some of the same components that process and package miRNAs also play another crucial part in the life of a cell: they serve as a powerful cell defense mechanism. In this case, the system is used to eliminate “foreign” RNA molecules—in particular, long, double-stranded RNA molecules. Such RNAs are rarely produced by normal genes, but they often serve as intermediates in the life cycles of viruses and in the movement of some transposable genetic elements. This form of RNA targeting, called RNA interference (RNAi), keeps these potentially destructive elements in check.

In the first step of RNAi, double-stranded, foreign RNAs are cut into short fragments ( ~ 22 nucleotide pairs in length) in the cytosol by a protein called Dicer—the same protein used to generate the double-stranded RNA intermediate in miRNA production. The resulting double-stranded RNA fragments, called small interfering RNAs (siRNAs), are then taken up by the same RISC proteins that carry miRNAs. The RISC discards one strand of the siRNA duplex and uses the remaining single-stranded RNA to seek and destroy complementary RNA molecules. In this way, the infected cell effectively turns the foreign RNA against itself.

At the same time, RNAi can also selectively shut off the synthesis of foreign RNAs by the host’s RNA polymerase. In this case, the siRNAs produced by Dicer are packaged into a protein complex called RITS (for RNA-induced transcriptional silencing). Using its single-stranded siRNA as a guide, the RITS complex attaches itself to complementary RNA sequences as they emerge from an actively transcribing RNA polymerase.

RNAi operates in a wide variety of organisms, including single-celled fungi, plants, and worms, indicating that it is an evolutionarily ancient defense mechanism, particularly against viral infection. In some organisms, including many plants, the RNAi defense response can spread from tissue to tissue, allowing an entire organism to become resistant to a virus after only a few of its cells have been infected. In this sense, RNAi resembles certain aspects of the adaptive immune responses of vertebrates; in both cases, an invading pathogen elicits the production of molecules—either siRNAs or antibodies—that are custom-made to inactivate the specific invader and thereby protect the host.