Gene Messenger RNA

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

Outline how the genetic message in a gene is translated to produce a polypeptide. Include the roles of the gene, messenger RNA, ribosomes, and transfer RNA. 

Section 28 11 Three RNAs are involved m gene expression In the transcription phase a strand of messenger RNA (mRNA) is synthesized from a DNA tern plate The four bases A G C and U taken three at a time generate 64 possible combinations called codons These 64 codons comprise the genetic code and code for the 20 ammo acids found m proteins plus start and stop signals The mRNA sequence is translated into a prescribed protein sequence at the ribosomes There small polynucleotides called... 

Messenger RNA (mRNA) serves to carry the information or message that is encoded in genes to the sites of protein synthesis in the cell, where this information is translated into a polypeptide sequence. Because mRNA molecules are transcribed copies of the protein-coding genetic units that comprise most of DNA, mRNA is said to be the DNA-like RNA. ... 

Messenger RNA (mRNA) is the intermediate template between DNA and proteins. The information from a particular gene is transferred from a strand of DNA by the construction of a complementary strand of RNA through a process known as transcription. The amount of any particular type of mRNA in a cell reflects the extent to which a gene has been expressed . 

Some RNA molecules have intrinsic catalytic activity. The activity of these ribozymes often involves the cleavage of a nucleic acid. An example is the role of RNA in catalyzing the processing of the primary transcript of a gene into mature messenger RNA. 

Fig. 24.4 Splicing of a messenger RNA molecule transcribed from a hypothetical insulin gene containing two introns.

Saito H., Mimmack M., Keveme E.B., Kishimoto J. and Emson P. (1998). Isolation of mouse vomeronasal receptor genes and their co-localization with specific G-protein messenger RNAs. Molec Brain Res 60, 215-227. 

The human TPI gene spans 3.5 kb of DNA located on the short arm of chromosome 12 (12pl3) and comprises seven exons encoding a 1.2-kb messenger RNA (mRNA) that is translated into a 248-amino-acid protein (B35, M10). 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

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