Heterogeneous nuclear RNA hnRNA

Figure 11-3. The prokaryotic RNA transcription bubble. RNA pol II, RNA polymerase II hnRNA, heterogeneous nuclear RNA.

DAG 1,2-diacy lgly cerol hnRNA heterogeneous nuclear RNA... 

HLA human leucocyte antigens. hnRNA heterogenous nuclear RNA. 

Each mRNA molecule contains a nucleotide sequence that is converted into the amino acid sequence of a polypeptide chain in the process of translation. In eukaryotes, messenger RNA (mRNA) is transcribed from protein-coding genes as a long primary transcript that is processed in the nucleus to form mRNA. The various processing intermediates, which are mRNA precursors, are called pre-mRNA or hnRNA (heterogenous nuclear RNA). mRNA travels through nuclear pores to the cytoplasm, where it binds to ribosomes and tRNAs and directs the sequential insertion of the appropriate amino acids into a polypeptide chain. 

The terms hnRNA (heterogeneous nuclear RNA) and pre-mRNA are both used to denote mRNA precursors. The term hnRNA was originally applied to a pool of RNA molecules in the nucleus that were rapidly synthesized and varied greatly in size. These RNA molecules are now known to be the mRNA precursors that vary greatly in size because they contain exons that encode different sizes of polypeptide chains and introns that vary in amount and size. 

Heterogeneous kinetic rate law, 25 286 Heterogeneous nuclear RNA (hnRNA), 22 454... 

A stretch of DNA that is transcribed as a single continuous RNA strand is called a transcription unit. A unit of transcription may contain one or more sequences encoding different polypeptide chains (translational open reading frames, ORF) or cistrons. The transcription unit is sometimes termed the primary transcript, pre-messenger RNA or heterogeneous nuclear RNA (hnRNA). The primary transcript is further processed to produce mRNA in a form that is relatively stable and readily participates in translation. In order to understand the primary need for processing of this RNA, the biochemical definition of a gene must be discussed. 

Translation arrest or nuclease digestion by exogenously applied antisense oligonucleotides or by antisense mRNA produced from DNA delivered by a plasmid. Heterogeneous nuclear RNA is hnRNA. 

The RNA molecule synthesized by RNA polymerase II (the primary transcript) contains the sequences that are found in cytosolic mRNA. The collection of all the precursor molecules for mRNA is known heterogeneous nuclear RNA (hnRNA). The primary transcripts are extensively modified in the nucleus after transcription. 

RNA polymerase I is located in the nucleolus and synthesizes a large precursor that is later processed to form rRNA. It is completely resistant to inhibition by a-amanitin. RNA polymerase II is located in the nucleoplasm and synthesizes large precursor RNAs (sometimes called heterogeneous nuclear RNA, or hnRNA) that are processed to form cytoplasmic mRNAs. It is also responsible for the synthesis of most viral RNA in virus-infected cells. PolII is very sensitive to a-amanitin, being inhibited by 50% at 0.05 /u,g/ml. RNA polymerase III is also located in the nucleoplasm and synthesizes small RNAs, such as 5S RNA and the precursors to tRNAs. This enzyme is somewhat resistant to a-amanitin, requiring about 5 /u,g/ml to reach 50% inhibition. 

A primary transcript corresponding to the full length of the gene is first made. This is then chemically modified, and introns (two in the case of /3-globin) are removed by splicing. The mixture of primary transcripts present in the nucleus is known as heterogeneous nuclear RNA (hnRNA). 

Mature mRNA is formed by extensively modifying the primary transcript also called heterogeneous nuclear RNA (hnRNA). The hnRNA must undergo three major modifications before maturing into mRNA capping, polyadenylation and splicing. 

Before RNA splicing was discovered, the nucleus was observed to contain a significant amount of seemingly untranslated RNA. The collection of RNA molecules of widely varied sizes was given the name heterogeneous nuclear RNA (hnRNA), a term that is still sometimes used for nuclear RNA. 

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