m-RNA ( messager RNA )
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Answer:
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 extra-cellular space, or will remain inside the cell. The diagram below shows transcription (DNA->RNA) taking place in the cell nucleus where RNAP is RNA polymerase II enzyme synthesizing RNA.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.
The diagram below describes the structure of pre-mRNA. Pre-mRNA includes introns and may or may not include the 5’ cap and poly-adenylated 3’ tail:
Pre-mRNA to mRNA
Monocistronic mRNA
A monocistronic mRNA molecule contains the exon sequences coding for a single protein. Most eukaryotic mRNAs are monocistronic.
Bicistronic mRNA
A bicistronic mRNA molecule contains the exon coding sequences for two proteins.
Polycistronic mRNA
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.