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Noncoding RNAs (ncRNAs) with different lengths have various roles, serving as regulatory, structural, and catalytic components of cells. Abundant and functionally important types of ncRNAs include transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), as well as small RNAs such as microRNAs, siRNAs, piRNAs, snoRNAs, snRNAs, exRNAs, scaRNAs and the long ncRNAs such as Xist and HOTA. It is also likely that many ncRNAs are non functional (sometimes referred to as junk RNA), and are the product of spurious transcription. Which of the following statements about ncRNAs are correct?A.
rRNAs form the structural and catalytic core of the ribosomes, which translate mRNAs into protein. tRNAs act as adaptors that select specific amino acids and hold them in place on a ribosome for their incorporation into protein.
B.
In higher eukaryotes, miRNAs regulate gene expression. A single miRNA can reduce the expression levels of hundreds of genes. The mechanism by which mature miRNA molecules act is through partial complementary to one or more mRNA molecules, generally in 3'-UTR. The main function of miRNAs is to down-regulate gene expression. Many other ncRNAs are involved in RNA splicing, gene regulation, telomere maintenance, and many other cellular processes.
C.
Piwi-interacting RNAs (piRNAs) expressed in mammalian testes and somatic cells form RNA-protein complexes with Piwi proteins. These piRNA complexes (piRCs) have been linked to transcriptional gene silencing of retrotransposons and other genetic elements in germ line cells, particularly those in spermatogenesis.
D.
The more conserved ncRNAs are thought to be molecular fossils or relics from the last universal common ancestor and the RNA world, and their current roles remain mostly in regulation of information flow from DNA to protein .
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B.
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