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The Prokaryotic And Eukaryotic Rna And Polymerase

BY: Zandro Cabaral | Category: Bioinformatics | Submitted: 2011-01-29 20:13:16
 

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There is only one RNA polymerase in prokaryotes. This is a Deoxyribonucleic Acid-directed RNA polymerase. A holoenzyme is composed of 5 subunits. Two of the alpha unit, one each of beta and beta prime units, plus a cofactor called sigma, which is required for the recognition of the promoter during initiation. The Sigma factor, a subunit of RNA polymerase, gives the specificity of initiating transcription of Ribonucleic Acid. In bacteria, both the sigma factor and the core enzyme of RNA polymerase are required for RNA synthesis.

The Messenger RNA contains the codons, the 3 nucleotide sequences, which specify each of the amino acids in a polypeptide chain. Messenger RNA may be translated into a protein without further processing. Translation can begin at the free 5' end before the mRNA is completely transcribed. Prokaryotic messenger RNA does not contain a "cap" on its 5' end-eukaryotic messenger RNA does. Messenger RNA contains many "cistrons" or it is polycistronic. One after the other, each cistron will be coding for one protein. Many proteins may be translated from the same mRNA at the same time, one after the other. It has a short half-life.

The Transfer RNA contains the anti-codons which are complementary to the codons of the messenger RNA. Codons and anti-codons base pair during translation or protein synthesis. Transports activated amino acids; is cleaved from a larger precursor. D-loop and Thymidine - Cytosine loop are areas that do not base pair. The cloverleaf structure of transfer RNA is short, has 70 to 90 bases, and many unusual bases.

The Ribosomal RNA is a 30S precursor molecule. It is cleaved to form one of each; a 5S ribosomal RNA, a 16S ribosomal RNA, and a 23S ribosomal RNA. The Svedberg unit is a measure of the sedimentation rate of a molecule.

In a Prokaryotic Cell, The small 30S ribosomal subunit is made from two 16S ribosomal RNA. The large 50S subunit is made from 5S and 23S ribosomal RNA. The entire 70S ribosome contains one 30S subunit and one 50S subunit.

The Transcription of Eukaryotic DNA to RNA takes place in the nucleus and then the finished RNA, except for small nuclear RNA, are transported into the cytoplasm, where protein synthesis takes place on the ribosome. Similar to transcription in prokaryotes with the following exceptions:

1. TATATAA known as Hogness box or TATA box that has 25 bases prior to transcripted DNA.
2. GGTCAATCT also known as CAAT box that has 70 bases before transcripted DNA.
3. GC rich areas or GC boxes occurring approximately 40 to 110 bases before the transcripted DNA.


The Transcription factor is similar to the prokaryotic sigma factor. Transcription factor binds to DNA in the major groove. There are 3 types of RNA polymerase. An RNA polymerase I which synthesizes a 45S precursor ribosomal RNA which is cleaved to form a 5.8S, a 18S, and a 28S ribosomal RNA. The RNA polymerase II which synthesizes messenger RNA and an RNA polymerase III: synthesizes transfer RNA and the 5S ribosomal RNA.

The Eukaryotic messenger RNA is always monocistronic. RNA is single stranded, it has a negative charge at neutral pH, and it has Ribose sugar. Modify eukaryotic messenger RNA after transcription with 3'-polyadenylate tails. The 5'-Caps only occur in eukaryotic messenger RNA.

The Eukaryotic ribosomal RNA is a 45S precursor synthesized by RNA polymerase I is cleaved to form the 5.8S, 18S, and 28S ribosomal RNA chains. RNA polymerase III synthesizes the 5S ribosomal RNA chain. Approximately 80% of RNA in cells is ribosomal RNA. It is found in ribosomes and the nucleolus.

The Eukaryotic messenger RNA contains codons, the 3 nucleotide sequences which specify the individual amino acids. The RNA chain synthesized by RNA polymerase II is called a heterogenous RNA. Some Heterogenous RNA contain intervening RNA sequences called "introns" which do not code for amino acids and which must be removed from the chain before translation can occur, and before mRNA is transported out of the nucleus known as post-transcriptional modification. "Exons" which contain the codons that are translated into proteins are spliced together in a process which uses Small Nuclear RNA. "Snurp," is Small Nuclear RNA and Protein. A Poly (A) polymerase synthesizes a tail connected to the 3' end made up of many adenosine nucleotides of some messenger RNA in the nucleus; after transport into the cytoplasm the tail is removed. A Guanosine triphosphate is attached to the 5' end and then gets a methyl-CH3 group on carbon number 7, forming a "Cap" on that end. Post-transcriptional modification occurs only with eukaryotic messenger RNA such as addition of 5'-7-met-Guanosine cap helps in translation, addition of 3' Poly-A tail for stabilization and the removal of introns or intervening sequences.

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