Messenger RNA
mRNAs are produced by transcription of structural genes. Because the size of proteins is quite diverse, ranging from a few thousand to over three hundred thousand Daltons, they are quite diverse in size.
Basic characteristics of mRNAs:
mRNAs are much more susceptible to hydrolysis than DNA due to the presence of the 2'-hydroxyl group. Additionally, cells contain relatively high RNase activities, making it difficult to quantitatively isolate full-length mRNAs. Laboratory work with mRNAs requires the use of RNase inhibitors and chemicals that will inactivate RNase activities. mRNAs tend to have a finite lifetime in the cell, especially in eukaryotic cells, where the turnover of mRNA species may be differentially regulated as part of the overall control of gene expression.
Because of their size differences, they can be fractionated by electrophoresis, usually in an agarose gel. Once fractionated, they can be transfered to a membrane and specific mRNAs identified using a labeled DNA probe. This process is commonly called northern blotting.
mRNAs are very different between prokaryotic and eukaryotic organisms. Some of these differences are outlined in the following table.
(>1 protein/mRNA)
(1 protein/mRNA)
ribosome binding
Prokaryotic mRNAs generally have the following elements [local] located within their primary structure in a 5'-->3' orientation.
5'-(transcription initiation sequence)--> (Shine-Dalgarno or ribosome-binding sequence)--> (initiation codon AUG)--> (structural gene coding for the protein)--> (termination codon)--> (primary and secondary structural elements to stop translation)-3'.
Eukaryotic production of mRNA is more complex [local]. Eukaryotic mRNAs are not readable as first transcribed and are classified as heterogeneous nuclear RNAs (hnRNA). They require processing [local] and transport from the nucleus to the cytoplasm. As soon as transcribed, the hnRNA is modified at its 5'-end by the addition of a methyl-guanosine. Next, processing of hnRNAs generally involves addition of a special 3'-tail. This tail is added by enzymes binding sequence elements at the 3' end of eukaryotic mRNA. The true 3' end of the transcript is shortened and then elongated with a tail consisting of dozens to hundreds of adenine nucleotides. This is generally called the 3'-poly(A) tail. This tail makes it possible to isolate eukaryotic mRNAs by chromatography on a column modified with oligo(dT) and is also important in the production of cDNA from mRNA [local]. Eukaryotic hnRNAs contain introns that to not appear in the final mRNA. A splicing [local] process cuts out the introns [local] and joins the exons together to from a continuous primary sequence that contains the exact codons that will code for the protein.
