Messenger RNA Definition
Messenger ribonucleic acids (mRNAs) transfer the information from DNA to the cell machinery that makes proteins. Tightly packed into every cell nucleus, which measures just 10 microns in diameter, is a three-meter long double-stranded DNA “instruction manual” on how to build and maintain a human body.
In order for each cell to maintain its structure and perform all of its functions, it must continuously manufacture cell-type-specific parts (proteins). Inside each nucleus, a multi-subunit protein called RNA polymerase II (RNAP II) reads DNA and simultaneously fabricates a “message” or transcript, which is called messenger RNA (mRNA), in a process called transcription.
Molecules of mRNA are composed of relatively short, single strands of molecules made up of adenine, cytosine, guanine, and uracil bases held together by a sugar-phosphate backbone. When RNA polymerase finishes reading a section of the DNA, the pre-mRNA copy is processed to form mature mRNA and then transferred out of the cell nucleus.
Ribosomes read the mRNA and translate the message into functional proteins in a process called translation. Depending on the newly synthesized protein’s structure and function, it will be further modified by the cell, exported to the extracellular space, or will remain inside the cell.
Precursor mRNA contains introns and exons. Introns are removed before translation, while exons code for the amino acid sequence of proteins. To make mature mRNA, the cell machinery removes “non-translatable” introns from the pre-mRNA, leaving only translatable exon sequences in the mRNA.
Types of mRNA
Pre-mRNA and hnRNA
Precursor mRNA (pre-mRNA) is the primary transcript of eukaryotic mRNA as it comes off the DNA template. Pre-mRNA is part of a group of RNAs called heterogeneous nuclear RNA (hnRNA). hnRNA refers to all single strand RNA located inside the nucleus of the cell where transcription takes place (DNA->RNA) and pre-mRNA form a large part of these ribonucleic acids. Pre-mRNA contains sequences that need to be removed or “spliced out” before being translated into a protein. These sequences can either be removed through the catalytic activity of the RNA itself, or through the action of a multi-protein structure called spliceosome. After this processing step, the pre-mRNA is considered as a mature mRNA transcript.
A monocistronic mRNA molecule contains the exon sequences coding for a single protein. Most eukaryotic mRNAs are monocistronic.
A bicistronic mRNA molecule contains the exon coding sequences for two proteins.
A polycistronic mRNA molecule contains the exon coding sequences for multiple proteins. Most mRNA of bacteria and bacteriophages (viruses that live in bacterial hosts) are polycistronic.
Prokaryotic vs. Eukaryotic mRNA
Polycistronic prokaryotic mRNAs contain multiple sites for initiating and terminating protein synthesis. Eukaryotes have just one site for translation initiation and eukaryotic mRNAs are primarily monocistronic.Prokaryotes lack organelles and a well defined nuclear envelope, and therefore mRNA translation can be coupled with mRNA transcription in the cytoplasm.
In eukaryotes, mRNA is transcribed on chromosomes in the nucleus, and after processing, is shuttled through nuclear pores and into the cytoplasm. Unlike prokaryotes, translation in eukaryotes takes place only after transcription has been completed. Prokaryotic mRNA is constantly degraded by ribonucleases, enzymes that cut RNA.
For example, the half-life of mRNA in E. Coli is approximately two minutes. Bacterial mRNAs are short-lived to allow for flexibility in adjusting to rapidly changing environmental conditions. Eukaryotic mRNAs are more metabolically stable. For example, precursors of mammalian red blood cells (reticulocytes), which have lost their nuclei, synthesize hemoglobin for several days by translating mRNAs that were transcribed when the nucleus was still present.
Lastly, the mRNAs of prokaryotes undergo minimal processing. In eukaryotes, the pre-mRNA must undergo processing before being translated, involving the removal of introns, the addition of the 5’ –cap as well as the 3’ poly-adenylated tail before mature mRNA is formed and ready to be translated.
Functions of mRNA
The primary function of mRNA is to act as an intermediary between the genetic information in DNA and the amino acid sequence of proteins. mRNA contains codons that are complementary to the sequence of nucleotides on the template DNA and direct the formation of amino acids through the action of ribosomes and tRNA. mRNA also contains multiple regulatory regions that can determine the timing and rate of translation. In addition, it ensures that translation proceeds in an orderly fashion because it contains sites for the docking of ribosomes, tRNA as well as various helper proteins.
Proteins produced by the cells play a variety of roles, either as enzymes, structural molecules or as transport machinery for various cellular components. Some cells are also specialized for secreting proteins, such as the glands that produce digestive enzymes or hormones which influence the metabolism of the entire organism.
mRNA can be translated on free ribosomes in the cytoplasm with the help of transfer RNA (tRNA) molecules and multiple proteins called initiation, elongation and termination factors. Proteins that are synthesized on free ribosomes in the cytoplasm are often used by the cell in the cytoplasm itself or targeted for use inside intracellular organelles.
Alternatively, proteins that have to be secreted begin to be translated in the cytoplasm but as soon as the first few residues are translated, specific proteins transport the entire translation machinery to the membrane of the endoplasmic reticulum (ER). The initial few amino acids get embedded in the ER membrane and the rest of the protein is released into the interior space of the ER. The short sequence is removed from proteins that need to be secreted from the cell, while those intended for internal membranes retain that short stretch providing a membrane anchor.
Examples of Disorders Related to mRNA Processing
Over 200 diseases are associated with defects in the processing of pre-mRNA to mRNA. Mutations in DNA or splicing machinery majorly affect pre-mRNA splicing accuracy. For example, an abnormal DNA sequence can eliminate, weaken, or activate hidden splice sites in pre-mRNA. Likewise, if the splicing machinery is not working properly, the spliceosome may cut the pre-mRNA incorrectly regardless of the sequence.
These mutations result in the processing of pre-mRNA to mRNAs that will go on to encode malfunctioning proteins. The abnormal mRNAs themselves are also sometimes the targets for nonsense-mediated mRNA decay as well as co-transcriptional degradation of nascent pre-mRNAs. Cells derived from patients with a variety of diseases including progeria, breast cancer, and cystic fibrosis display RNA splicing defects, with cancer and neuropathological diseases being the most common.