Synthesis and types of RNA

RNAs are molecules that play several fundamental roles for the cell and there are many types: the mRNAs carry DNA information outside the nucleus; tRNAs transport amino acids, the “building blocks” that make up proteins; rRNAs form ribosomes. There are also other RNAs that act as enzymes and RNAs can regulate the expression of genes.

Synthesis of RNA

It occurs in the same way that DNA is synthesized:

the helicase has the task of unrolling
the RNA polymerase has the task of synthesizing (no primer is needed, but flows the DNA strand until it finds a promoter: a sequence of nucleotides in the DNA that directly indicates where the synthesis should start) and is supported by general factors of transcription that aggregate to the promoter and place the polymerase where there is the TATA sequence (15 nucleotides upstream of the transcription start site) and start the whole
Nitrogen bases are added in the form of triphosphate nucleosides (ATP, CTP, GTP, UTP)
The synthesis continues until the RNA polymerase encounters a termination signal
The RNA detaches and the DNA spirals up again.
There are three types of RNA polymerase:

polymerase 1: tRNA, rRNA, miRNA
polymerase 2: mRNA
polymerase 3: tRNA, rRNA, miRNA
This also summarizes the other types of RNA (ribosomal, messenger and transfer).

An RNA is composed of introns and exons that must be eliminated and are located at the ends of the intron. The exons are eliminated with a process called splicing, operated by molecules of (nuclear RNA) and not by proteins. Splicing allows you to code proteins other than the same gene.

The mRNAs

MRNA is the complementary copy of the gene it transcribed. It allows the cell to amplify its synthesis activity. A DNA molecule contains information for numerous mRNA molecules. Each mRNA molecule can be translated into numerous polypeptide chains.

The mRNAs leave the nucleus to be translated into a protein thanks to the genetic code. The messenger RNA nucleotide sequences are read in triplets (codons) and transformed into amino acids. Since there are four nucleotides, 4x4x4 = 64 combinations of three nucleotides. However, there are only 20 amino acids, so an amino acid can correspond to several triplets.


All mature tRNA molecules have:

73-93 nucleotides
Traits in which the pairing of the bases makes the structure fold
Loops not paired because in those areas there are 10 unusual bases modified post-transcriptionally and therefore it is impossible to pair them
An anticodone
A binding site for amino acids (CCA) at the 3 ‘end
TRNA is used to bind amino acids together. There are 10 different aminoacyl-tRNA synthetase enzymes that bind amino acids and tRNA with an ester bond. They are fundamental for the translation of messenger RNA because they recognize and bind the codon of mRNA (thanks to their complementary anticodon) on one side and on the other the amino acid.

Some tRNAs can mate to more than one codon because they can tolerate an oscillating pair in third position. The aminoacyl-tRNA synthetase enzymes combine all the amino acids with their corresponding RNA tranfer.

The rRNAs

It represents the most stable form of RNA and is 70-80% of the total RNA. The genes that encode rRNA are highly repeated. Ribosomes are made up of rRNA and proteins. They provide support for protein synthesis because they catalyze the link between two amino acids, that is, they transform the language of mRNA to that of amino acids according to the genetic code.

Ribosomes are made up of:

Major subunit: RNA 28S, RNA 5.8S and RNA 5S and about 45 proteins
Minor subunit: RNA 18S and 33 proteins
The RNA 28S, 18S and 5.8S are derived from a single transcript cut from nucleases in rRNA 18S and 32S (28S and 5.8S). The 5S RNA comes from a different precursor RNA, synthesized outside the nucleolus and then transferred to the nucleolus.

Ribosomes have a binding site for the mRNA and tRNA molecule (site A, P and E). The amino acid-bound tRNAs are positioned at site A. The amino acid is bound to what is found at site P. The ribosome then slips and the now amino-free tRNA is found at site E waiting to be expelled.

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