Nearly All of the Several Species of RNA Are Involved in Some Aspect of Protein Synthesis

Those cytoplasmic RNA molecules that serve as templates for protein synthesis (ie, that transfer genetic information from DNA to the protein-synthesizing machinery) are designated messenger RNAs, or mRNAs. Many other cytoplasmic RNA molecules (ribosomal RNAs; rRNAs) have structural roles wherein they con

Figure 35-6. A segment of a ribonucleic acid (RNA) molecule in which the purine and pyrimidine bases— guanine (G), cytosine (C), uracil (U), and adenine (A)—are held together by phosphodiester bonds between ribo-syl moieties attached to the nucleobases by W-glycosidic bonds. Note that the polymer has a polarity as indicated by the labeled 3'- and 5'-attached phosphates.

Figure 35-6. A segment of a ribonucleic acid (RNA) molecule in which the purine and pyrimidine bases— guanine (G), cytosine (C), uracil (U), and adenine (A)—are held together by phosphodiester bonds between ribo-syl moieties attached to the nucleobases by W-glycosidic bonds. Note that the polymer has a polarity as indicated by the labeled 3'- and 5'-attached phosphates.

tribute to the formation and function of ribosomes (the organellar machinery for protein synthesis) or serve as adapter molecules (transfer RNAs; tRNAs) for the translation of RNA information into specific sequences of polymerized amino acids.

Some RNA molecules have intrinsic catalytic activity. The activity of these ribozymes often involves the cleavage of a nucleic acid. An example is the role of RNA in catalyzing the processing of the primary transcript of a gene into mature messenger RNA.

Much of the RNA synthesized from DNA templates in eukaryotic cells, including mammalian cells, is degraded within the nucleus, and it never serves as either a structural or an informational entity within the cellular cytoplasm.

In all eukaryotic cells there are small nuclear RNA (snRNA) species that are not directly involved in protein synthesis but play pivotal roles in RNA processing. These relatively small molecules vary in size from 90 to about 300 nucleotides (Table 35-1).

The genetic material for some animal and plant viruses is RNA rather than DNA. Although some RNA viruses never have their information transcribed into a DNA molecule, many animal RNA viruses—specifically, the retroviruses (the HIV virus, for example)—are transcribed by an RNA-dependent DNA polymerase, the so-called reverse transcriptase, to produce a double-stranded DNA copy of their RNA genome. In many cases, the resulting double-stranded DNA transcript is integrated into the host genome and subsequently serves as a template for gene expression and from which new viral RNA genomes can be transcribed.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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