MicroRNA, or miRNA, is a type of molecule that plays an important role in post-transcriptional gene regulation. Most ribonucleic acid (RNA) serves as an intermediary between deoxyribonucleic acid (DNA), which stores genetic information, and proteins, which typically express the genetic information. Transcription is the process by which double stranded DNA is used to make a single-stranded RNA molecule containing coded information necessary to express genetic information. MicroRNA is used to regulate the expression of genes that have already been transcribed from DNA to RNA. It is important to note that, though microRNA is a form of ribonucleic acid, it does not contain genetic information that codes for proteins; it serves regulatory functions only.
DNA and RNA are both made up of nucleotides, and sequences of these nucleotides contain genetic information. RNA and DNA used for data storage commonly exist in strands of hundreds or thousands of nucleotides. MicroRNA is, as indicated by its name, appreciably shorter; on average, it is just over 20 nucleotides in length.
After it is transcribed from DNA that does not code for proteins, microRNA molecules are integrated into an RNA-induced silencing complex commonly referred to as RISC that contains miRNA and a variety of different proteins. The sequences of miRNAs in a RISC complex are used to target particular messenger RNA (mRNA) strands that would otherwise be translated into protein. Other proteins associated with the complex are able to prevent this from happening, thereby repressing translation and effectively silencing the gene. In some cases, proteins cleave the mRNA strand to which miRNA and the RISC complex have bound.
It is important to note that dsRNA, or double stranded RNA, may also form RISC complex and are also involved in post-transcriptional regulation. RISC complexes involving dsRNA, however, always directly cleave the targeted mRNA into useless pieces instead of repressing translation. MicroRNA complexes usually repress translation without cleaving the RNA, though sometimes miRNA complexes will cleave the mRNA strands.
MicroRNA is considered to be one of the earliest and most primitive mechanisms of gene regulation, and it has changed very little throughout evolutionary history. By allowing more controllable, specific, and adaptable expression of genes, it may have actually allowed for the formation of complex organisms. Almost all organisms have miRNA as part of their genomes.
Regulation of gene expression through microRNA and through other organisms is extremely important. There are many proteins that are absolutely necessary for a wide range of cellular processes, such as cellular metabolism, that allow organisms to function. Without them, the organism could not live for even a short period of time. Too many or too few of a specific protein at a given time can cause drastic changes in the subtle biochemical balance within each organism. There are actually many diseases, including cancer, that either cause or are caused by a deregulation of gene expression.
Initiator Transfer RNA
The starting amino acid in eukaryote protein synthesis is methionine, while in prokaryotes it is N-formyl methionine. The tRNA molecule3 specific for these two amino acids are methionyl tRNA (tRNAmet) and N-formyl- methionyl IRNA (tRNAfmet) respectively.
These tRNAs are called initiator tRNAs, because they initiate protein synthesis. Initiator tRNAs have certain features which distinguish them from other tRNAs, and the initiator tRNAs of prokaryotes' and eukaryotes also differ.
In most prokaryotes the 5' terminal nucleoside is C. It has opposite it (i.e. in the fifth position from the 3' end) an A nucleotide. There is no Watson-Crick base pairing between the two. In the blue green 'alga' Anacystis nidulans, however, the fifth nucleotide from the 3' end is C. In eukaryotes there is an A.U base pair at the acceptor stem.
As noted previously, prokaryotes use tRNAf-met for initiation of protein synthesis, while eukaryotes use tRNAmet. The prokaryote Halo bacterium cutirubrum is, however, reported to initiate protein synthesis with tRNA met and has an A.U base pair at the end of the accept or stem. In these respects it resembles eukaryotes
The D loop of prokaryote initiator tRNAs contains an A11, U24 base pair. All other tRNAs have a Y11, R24 base pair. Eukaryotic cytoplasmic initiator tRNAs have AU or AU* instead of Tψ in the TψC loop. Also, in eukaryotes instead of a pyrimidine nucleotide (Y) there is A at the 3' end of the TψC loop.
In some eukaryotic cytoplasmic initiator tRNAs the anticodon sequence CAU is preceeded by C instead of U as in all other tRNAs.In prokaryotes the purine nucleotide following C in the TψC loop is A, while in eukaryotes it is G. In tRNA f-met the nucleotide adjacent to the 3' side of the anticodon triplet is adenosine while in tRNA met it is alkylated adenosine.
These tRNAs are called initiator tRNAs, because they initiate protein synthesis. Initiator tRNAs have certain features which distinguish them from other tRNAs, and the initiator tRNAs of prokaryotes' and eukaryotes also differ.
In most prokaryotes the 5' terminal nucleoside is C. It has opposite it (i.e. in the fifth position from the 3' end) an A nucleotide. There is no Watson-Crick base pairing between the two. In the blue green 'alga' Anacystis nidulans, however, the fifth nucleotide from the 3' end is C. In eukaryotes there is an A.U base pair at the acceptor stem.
As noted previously, prokaryotes use tRNAf-met for initiation of protein synthesis, while eukaryotes use tRNAmet. The prokaryote Halo bacterium cutirubrum is, however, reported to initiate protein synthesis with tRNA met and has an A.U base pair at the end of the accept or stem. In these respects it resembles eukaryotes
The D loop of prokaryote initiator tRNAs contains an A11, U24 base pair. All other tRNAs have a Y11, R24 base pair. Eukaryotic cytoplasmic initiator tRNAs have AU or AU* instead of Tψ in the TψC loop. Also, in eukaryotes instead of a pyrimidine nucleotide (Y) there is A at the 3' end of the TψC loop.
In some eukaryotic cytoplasmic initiator tRNAs the anticodon sequence CAU is preceeded by C instead of U as in all other tRNAs.In prokaryotes the purine nucleotide following C in the TψC loop is A, while in eukaryotes it is G. In tRNA f-met the nucleotide adjacent to the 3' side of the anticodon triplet is adenosine while in tRNA met it is alkylated adenosine.
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