Synthesis of DNA in the RNA template

An outstanding achievement was the discovery of the enzyme reverse transcriptase or RT (RNA-dependent DNA polymerase) which catalyzes the biosynthesis of DNA in the RNA template in the oncogenic viruses, or retroviruses (Rauscher virus and Rous sarcoma). RT needs a primer, a role of which t-RNA can play.

On the 1^st stage RT enzyme synthesizes complementary DNA strand on the template of viral RNA. A so-called hybrid molecule is formed.

The 2^nd stage is the destruction of the original viral RNA.

On the 3^rd stage new DNA strand is complementary synthesized on the template of DNA.

RT was used in molecular biology for the synthesis of genes and gene fragments in genetic engineering, to decipher the primary structure of RNA and proteins.

3.1.2. RNA biosynthesis

Biosynthesis of RNA is a transcription process, i.e., the rewriting of information from DNA template. All types of RNA are synthesized on the nuclear DNA as a template. Biosynthesis of RNA is carried out on the DNA by means of RNA polymerase. The specific region on the DNA where the enzyme binds is known as promoter region.

In eukaryotes three different RNA polymerases were discovered. RNA polymerase I catalyzes the synthesis of ribosomal RNA, RNA polymerase IIcatalyzes messenger RNA synthesis, RNA polymerase IIIcatalyzes transfer RNA synthesis, as well as several low molecular weight RNA with a specific function. Set of regulatory proteins (transcription factors) are integrated with the enzyme in a common transcriptional complex.

The synthesis of mRNA. At the same time many RNA molecules can be synthesized on the DNA molecule. Eukaryotic gene along with the coding sequences (exons) also contains non-coding (introns). RNA polymerase catalyzes the transcription of both exons and introns with the formation of the primary transcript (RNA precursor). Along with the informative sites they contain uninformative sites. The process of transcription stops by termination signals.Further processing or the maturation of RNA occurs in the nucleus.

The main stages of processing.

1. The 5’-capping is chemical modification of the 5'-terminal end of the mRNA."Cap" is a 7-methylguanosine triphosphate. It is involved in mRNA translation initiation.

2.Polyadenylation is chemical modification of 3'-terminal end of the mRNA. It is formation of poly-A tail of 100-200 nucleotide residues at the 3-end, with the participation of the enzyme poly-A polymerase. It is possible that the poly-A tail protects mRNA from hydrolysis by cellular RNAses.

3. Splicing is removing of introns from the mRNA and linking of exons. This occurs with the participation of small nuclear RNA (snRNA), or ribozymes. This is the only known macromolecules, which perform both information and catalytic function. After splicing the mature mRNA enters the cytosol.

tRNA also undergo post-translational modification. Endonucleases cleave intron-containing tRNA precursors on both sides of the intron. The primary transcript of rRNA contains no introns, and by the action of specific RNasesis cleaved to form smaller molecules.

Date: 2016-04-22; view: 858