(33)
3. Angive de tre hovedformer af RNA (mRNA, tRNA og rRNA) samt beskrive deres funktioner
 Stryer, s. 129
Devlin, s. 82-85

 

mRNA – the template for protein synthesis or translation. It can be:

 

In the cytoplasm, mRNAs have a relatively short lifespan. Some mRNAs are synthesised and stored in an inactive or dormant state in the cytoplasm, ready for a quick response when protein synthesis is required (fx. Fertilized egg synthesizes proteins without transcription).

mRNA is transcribed by RNA polymerase II. After the transcription, the mRNA is called the primary transcript or the pre-mRNA. It has to go through several modification: capping, slicing and polyadenylation, before it becomes mature mRNA.

mRNA comprises for only 5% of the entire amount of RNA-molecules.


tRNA – carries amino acids in an activated form to the ribosome for peptide bond formation, in a sequence dictated by the RNA template. There is at least one kind of tRNA for each amino acid.

tRNAs have a cloverleaf secondary structure, with three loops and a L-form tertiary structure. They contain an anticodon sequence on the anticodon loop, which is used for base pairing with the complementary sequence of the mRNA.

The acceptor stem - the CCA terminal is used for bringing the right amino acid, which is going to be incorporated in the growing nucleotide chain.

tRNA is synthesized by RNA polymerase III.

The average length of tRNA is ca. 75 nucleotides. It accounts for 15% of all RNAs.


rRNA – major component of ribosomes, constitutes nearly 2/3 of the entire ribosome masse. It plays both a catalytic and structural role in protein synthesis. It is the most abundant of the RNA molecules. (80% in E.Coli)

Different RNAs are formed from cleavage of one big primary transcript into smaller components. All rRNAs are synthesized by RNA polymerase I in the nucleolus, except 5S rRNA which is synthesized by RNA polymerase III in the nucleoplasma.

A short overview over which RNAs are synthesized by which polymerases:

mRNA - RNA pol II
tRNA - RNA pol III
rRNA - RNA pol I
5sRNA - RNA pol III

 

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