mRNA is a [ribonucleic acid]
? that is used as an intermediate storage for genetic information during
protein biosynthesis. As a
RNA, it consists of the four bases adenine
?,
cytosine,
guanine and uracil
? (A, C, G, U). mRNA is usually single-stranded.
mRNA runs through several steps during its usually brief existence:
- During transcription, an enzyme called [RNA polymerase]? makes a copy of a gene from the DNA to mRNA on demand.
- In eukaryotes, this copy (called primary transcript or precursor mRNA) is spliced to remove introns (certain genetic sequences unwanted in protein biosynthesis). In prokaryotes, mRNA is generally not spliced. Also only in eukaryotes, the mRNA is then exported from the nucleus through special structures in the nuclear membrane known as nuclear pores.
- Ribosomes read the mRNA and produce the according polypeptide (or protein) in a process called translation.
- Finally, the mRNA is disassembled into its nucleotides.
Double-stranded mRNA
Double-stranded mRNA was discovered to inhibit translation in many eukaryotes for the matching single-stranded mRNA only. That means a gene will never be expressed as protein if a matching double-stranded mRNA is present in the cell. This is most probably the result of a cellular defense mechanism against transposons or viruses, since both of them can use double-stranded mRNA as an intermediate. In biochemical laboratories, this effect has been successfully used to study gene functions, with simply shutting down the studied gene by adding its double-stranded mRNA transcript.
- See also : genetics
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