String mRNA

At their 3 end, most eukaryotic mRNAs have a string of 80 to 250 A residues, making up the poly(A) tail. This tail serves as a binding site for one or more specific proteins. The poly(A) tail and its associated proteins probably help protect mRNA from enzymatic destruction. Many prokaryotic mRNAs also acquire poly(A) tails, but these tails stimulate decay of mRNA rather than protecting it from degradation. 

RNA consists of long strings of ribonucleotides, polymerised in a similar way to DNA, but the chains are considerably shorter than those of DNA. RNA contains ribose rather than deoxyribose and also contains uracil instead of thymidine. This has important connotations in the secondary structure of RNA which does not form the long helices found in DNA. RNA is usually much more abundant than DNA in the cell and its concentration varies according to cell activity and growth. This is because RNA has several roles in protein synthesis. There are three major classes messenger RNA (mRNA) ribosomal RNA (rRNA) and transfer RNA (tRNA). 

During transcription of information from DNA into mRNA, the two complimentary strands of the DNA partly uncoil. The sense strand separates from the antisense strand. The antisense strand of DNA is used as a template for transcribing enzymes that assemble mRNA (transcription), which, in the process produces a copy of the sense strand. Then, mRNA migrates into the cell, where other cellular structures called ribosomes read the encoded information, its mRNA s base sequence, and in so doing, string together amino acids to form a specific protein. This process is called translation. ... 

Processing at the 3 end of a pre-mRNA involves cleavage by an endonuclease to yield a free 3 -hydroxyl group to which a string of adenylic acid residues is added one at a time by an enzyme called poIy(A) polymerase. The resulting poIy(A) tail contains 100-250 bases, being shorter in yeasts and Invertebrates than in vertebrates. Poly(A) polymerase is... 

The first step in preparing a cDNA library Is to Isolate the total mRNA from the cell type or tissue of Interest. Because of their poly(A) tails, mRNAs are easily separated from the much more prevalent rRNAs and tRNAs present In a cell extract by use of a column to which short strings of thymidyl-ate (oligo-dTs) are linked to the matrix. 

Three different experimental approaches were taken in the early 1960s to prove that the genetic code is based on a continuous string of triplet ribonucleotides in the mRNA. The general idea for each experiment is illustrated in Figure 26.4. 

A gene expresses itself by transcribing its DNA sequence into a complementary copy of messenger ribonucleic acid (mRNA). The RNA sequence is translated, by a ribosome enzyme, into the string of amino acids that make up a particular protein. A cell s response to a stimulus, such as a toxin, can be evaluated by determining the effect on a gene s expression, that is, whether the gene is turned on or off. 

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