This class is the most heterogeneous in abundance, size, and stability; for example, in brewer’s yeast, specific mRNAs are present in 100s/cell to, on average, ≤0.1/mRNA/cell in a genetically homogeneous population. Both specific transcriptional and posttranscriptional mechanisms contribute to this large dynamic range in mRNA content. In mammalian cells, specific mRNA abundance likely varies over a 10⁴-fold range. All members of this RNA class function as messengers conveying the information in a gene to the protein-synthesizing machinery, where each mRNA serves as a template on which a specific sequence of amino acids is polymerized to form a specific protein molecule, in this case the ultimate gene product (Figure 1).

Fig1. The expression of genetic information within DNA into the form of an mRNA transcript with 5′ to 3′ polarity and then into protein with N- to C-polarity is shown. DNA is transcribed into mRNA that is subsequently translated by ribosomes into a specific protein molecule that exhibits polarity, N-terminus (N) to C-terminus (C).

Eukaryotic mRNAs have unique chemical characteristics. The 5′ terminus of mRNA is “capped” by a 7-methylguanosine triphosphate that is linked to an adjacent 2′-O-methyl ribonucleoside at its 5′-hydroxyl through the three phosphates (Figure 2). mRNA molecules frequently contain internal N⁶-methyladenine and other 2′-O-ribose-methylated nucleotides. The cap is involved in the recognition of mRNA by the translation machinery, and also helps stabilize the mRNA by preventing the nucleolytic attack by 5′-exoribonucleases. The protein-synthesizing machinery begins translating the mRNA into proteins beginning downstream of the 5′ or capped terminus. At the other end of almost all eukaryotic mRNA molecules, the 3′-hydroxyl terminus has an attached, nongenetically encoded polymer of adenylate residues 20 to 250 nucleotides in length. The poly(A) “tail” at the 3′-end of mRNAs maintains the intracellular stability of the specific mRNA by preventing the attack of 3′-exoribonucleases and also facilitates translation. Both the mRNA “cap” and “poly(A) tail” are added posttranscriptionally by nontemplate-directed enzymes to mRNA precursor molecules (pre-mRNA). mRNA represents 2 to 5% of total eukaryotic cellular RNA.

Fig2. The cap structure attached to the 5′terminal of most eukaryotic messenger RNA molecules. A 7-methylguanosine triphosphate (black) is attached at the 5′ end of the mRNA (red), which usually also contains a 2′-O-methylpurine nucleotide. These modifications (the cap and methyl group) are added after the mRNA is transcribed from DNA. Note that the γ- and β-phosphates of the GTP added to form the cap (black in figure) are lost on cap addition while the γ-phosphate of the initiating nucleotide (here an A-residue; red in figure) is lost during cap addition.

In mammalian cells, including cells of humans, the mRNA molecules present in the cytoplasm are not the RNA products immediately synthesized from the DNA template but must be formed by processing from the precursor, or pre-mRNA before entering the cytoplasm. Thus, in mammalian cell nuclei, the immediate products of gene transcription (primary transcripts) are very heterogeneous and can be 10- to 50-fold longer than mature mRNA molecules. As discussed in Chapter 36, pre-mRNA molecules are processed to generate mRNA molecules, which then enter the cytoplasm to serve as templates for protein synthesis.

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الآخبار الصحية

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الآخبار التكنلوجية

تركيب مقعد الطفل بأمان في سيارتك: دليل كامل وسهل

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المزيد

مواضيع ذات صلة

Small RNA

Ribosomal RNA

Identification of the src and sis Gene

Cellular Counterparts of Retroviral Oncogenes

Retroviruses and Cancer

Identifying a Nucleotide Change Causing Cancer

Transformation and Oncogenesis by Damaging the Chromosome

Bacterially Induced Tumors in Plants

Phosphorylation and the Rapid Response

Chemotaxis: Methylation and Adaptation

The Energy for Chemotaxis

The Mechanism of Chemotaxis