The DNA transcription is the process by which the information contained in deoxyribonucleic acid is copied in the form of a similar molecule, RNA, either as a previous step for protein synthesis or for the formation of RNA molecules that participate in multiple cellular processes of great importance (regulation of gene expression, signaling, etc.).
Although it is not true that all the genes of an organism code for proteins, it is true that all the proteins of a cell, whether eukaryotic or prokaryotic, are encoded by one or more genes, where each amino acid is represented by a set of three DNA bases (codon).
The synthesis of the polypeptide chain belonging to any cellular protein occurs thanks to two fundamental processes: transcription and translation; both highly regulated, since they are two processes of great importance for the functioning of any living organism.
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Transcription involves the formation of a "template" for an RNA molecule known as "messenger RNA" (mRNA) from the "standard" sequence encoded in the region of DNA corresponding to the gene to be transcribed..
This process is carried out by an enzyme called RNA polymerase, which recognizes special places in the DNA sequence, binds to them, opens the DNA strand and synthesizes an RNA molecule using one of these complementary DNA strands as a template or pattern, even when it encounters another special stop sequence.
Translation, on the other hand, is the process through which protein synthesis occurs. It consists of the “reading” of the information contained in the mRNA that was transcribed from a gene, the “translation” of the DNA codons into amino acids and the formation of a polypeptide chain.
The translation of the nucleotide sequences of the mRNA is carried out by enzymes known as aminoacyl-tRNA synthetases, thanks to the participation of other RNA molecules known as "transfer RNA" (tRNA), which are anticodons of the codons contained in the MRNAs, which are a true copy of the DNA sequence of a gene.
In eukaryotic cells, the transcription process occurs within the nucleus, which is the main intracellular organelle where DNA is contained in the form of chromosomes. It begins with the "copy" of the coding region of the gene that is transcribed into a single band molecule known as messenger RNA (mRNA)..
Since the DNA is confined in said organelle, the mRNA molecules function as intermediaries or transporters in the transmission of the genetic message from the nucleus to the cytosol, where the translation of the RNA occurs and the entire biosynthetic machinery for protein synthesis (the ribosomes).
A gene consists of a DNA sequence whose characteristics determine its function, since the order of the nucleotides in said sequence is what determines its transcription and subsequent translation (in the case of those that code for proteins)..
When a gene is transcribed, that is, when its information is copied in the form of RNA, the result can be a non-coding RNA (cRNA), which has direct functions in the regulation of gene expression, in cell signaling, etc. or it can be a messenger RNA (mRNA), which will then be translated into an amino acid sequence in a peptide.
Whether a gene has a functional product in the form of RNA or protein depends on certain elements or regions present in its sequence..
Genes, eukaryotic or prokaryotic, have two strands of DNA, one known as the "sense" strand and the other "antisense." The enzymes responsible for the transcription of these sequences "read" only one of the two strands, typically the "sense" or "coding" strand, which has a "direction" 5'-3 '.
Every gene has regulatory sequences at its ends:
- if the sequences are before the coding region (the one that will be transcribed) they are known as "promoters"
- if they are separated by many kilobases, they can be "silencing" or "enhancing"
- those sequences that are closer to the 3 'region of genes are usually terminator sequences, which tell the polymerase to stop and end transcription (or replication, as the case may be)
The promoter region is divided into distal and proximal, according to its proximity to the coding region. It is at the 5 'end of the gene and is the site that recognizes the enzyme RNA polymerase and other proteins to initiate transcription from DNA to RNA.
In the proximal part of the promoter region, transcription factors can bind, which have the ability to modify the affinity of the enzyme to the sequence to be transcribed, so they are responsible for regulating the transcription of genes positively or negatively.
The enhancer and silencing regions are also responsible for regulating gene transcription by modifying the "activity" of the promoter regions by their union with activator or repressor elements "upstream" of the coding sequence of the gene..
It is said that eukaryotic genes are always "off" or "repressed" by default, so they need their activation by promoter elements in order to be expressed (transcribed).
Regardless of the organism, transcription is carried out by a group of enzymes called RNA polymerases, which, similar to the enzymes responsible for DNA replication when a cell is about to divide, specialize in the synthesis of an RNA chain from one of the DNA strands of the gene being transcribed.
RNA polymerases are large enzyme complexes made up of many subunits. There are different types:
- RNA polymerase I (Pol I): which transcribe the genes that encode the “large” ribosomal subunit.
- RNA polymerase II (Pol II): which transcribe protein coding genes and produce micro RNAs.
- RNA polymerase III (Pol III): which produce the transfer RNAs used during translation and also the RNA corresponding to the small subunit of the ribosome.
- RNA polymerase IV and V (Pol IV and Pol V): are typical of plants and are responsible for the transcription of small interfering RNAs.
Genetic transcription is a process that can be studied as divided into three phases: initiation, elongation and termination..
RNA polymerase (say RNA polymerase II) binds to the promoter region sequence, which consists of a 6-10 base pair stretch at the 5 'end of the gene, usually about 35 base pairs away. transcription start site.
The union of RNA polymerase leads to the "opening" of the DNA double helix, separating the complementary strands. RNA synthesis begins at the site known as the "initiation site" and occurs in the 5'-3 'direction, that is, "downstream" or from left to right (by convention).
The initiation of transcription mediated by RNA polymerases depends on the concomitant presence of protein transcription factors known as general transcription factors, which contribute to the "location" of the enzyme in the promoter region..
After the enzyme has begun to polymerize, it is "shed" from both the promoter sequence and general transcription factors..
It occurs as RNA polymerase "moves" along the DNA sequence and adds ribonucleotides complementary to the DNA strand that serves as a "template" to the growing RNA. As RNA polymerase "passes" through the DNA strand, it rejoins its antisense strand.
The polymerization carried out by RNA polymerase consists of nucleophilic attacks of oxygen in position 3 'of the growing RNA chain to the phosphate "alpha" of the next nucleotide precursor to be added, with the consequent formation of phosphodiester bonds and the release of a pyrophosphate molecule (PPi).
The set comprised of the DNA strand, the RNA polymerase and the nascent RNA strand is known as a transcription bubble or complex..
Termination occurs when the polymerase reaches the termination sequence, which is logically "downstream" of the transcription initiation site. When this occurs, both the enzyme and the synthesized RNA become “detached” from the DNA sequence being transcribed..
The termination region normally consists of a DNA sequence that is capable of "folding" on itself, forming a "hairpin loop" type structure. hairpin loop).
After termination, the synthesized RNA strand is known as the primary transcript, which is released from the transcription complex, after which it may or may not be post-transcriptionally processed (prior to its translation into protein, if applicable) through a process called "cutting and splicing".
Since prokaryotic cells do not have a membrane-enclosed nucleus, transcription occurs in the cytosol, specifically in the "nuclear" region, where chromosomal DNA is concentrated (bacteria have a circular chromosome).
In this way, the increase in the cytosolic concentration of a given protein is substantially faster in prokaryotes than in eukaryotes, since the transcription and translation processes occur in the same compartment..
Prokaryotic organisms have genes that are very similar to eukaryotes: the former also use promoter and regulatory regions for their transcription, although an important difference has to do with the fact that the promoter region is often sufficient to achieve a “strong” expression of the genes.
In this sense, it is important to mention that, in general, prokaryotic genes are always "on" by default..
The promoter region is associated with another region, usually "upstream", which is regulated by repressor molecules and is known as the "operator region.".
A difference in transcription between prokaryotes and eukaryotes is that normally the messenger RNAs of eukaryotes are monocistronic, that is, each one contains the information to synthesize a single protein, while in prokaryotes these can be monocistronic or polycistronic, where only one MRNA can contain the information for two or more proteins.
Thus, it is well known that prokaryotic genes that code for proteins with similar metabolic functions, for example, are found in groups known as operons, which are simultaneously transcribed into a single-molecule form of messenger RNA..
Prokaryotic genes are densely packed, without many non-coding regions between them, so once transcribed into linear messenger RNA molecules, they can be translated into protein immediately (eukaryotic mRNAs often need further processing).
Prokaryotic organisms such as bacteria, for example, use the same RNA polymerase enzyme to transcribe all their genes, that is, those that code for ribosomal subunits and those that code for different cellular proteins.
In the bacteria E. coli RNA polymerase is composed of 5 polypeptide subunits, two of which are identical. The α, α, β, β 'subunits comprise the central portion of the enzyme and assemble and disassemble during each transcription event..
The α subunits are those that allow the union between the DNA and the enzyme; the β subunit binds to the triphosphate ribonucleotides that will be polymerized according to the DNA template in the nascent mRNA molecule and the β 'subunit binds to said template DNA strand.
The fifth subunit, known as σ participates in the initiation of transcription and is what confers specificity to the polymerase.
Transcription in prokaryotes is very similar to that of eukaryotes (it is also divided into initiation, elongation and termination), with some differences in terms of the identity of the promoter regions and that of the transcription factors necessary for RNA polymerase exercise your functions.
Although the promoter regions may vary between different prokaryotic species, there are two conserved "consensus" sequences that can be easily identified in the -10 region (TATAAT) and in the -35 region (TTGACA) upstream of the coding sequence..
It depends on the σ subunit of RNA polymerase, as it mediates the interaction between DNA and the enzyme, making it capable of recognizing promoter sequences. The initiation ends when some abortifacient transcripts of about 10 nucleotides are produced that are released.
When the σ subunit is detached from the enzyme, the elongation phase begins, which consists of the synthesis of an mRNA molecule in the 5'-3 'direction (approximately 40 nucleotides per second).
Termination in prokaryotes depends on two different types of signals, it can be Rho-dependent and Rho-independent.
The Rho-dependent protein is controlled by this protein that "follows" the polymerase as it advances in RNA synthesis until the latter, which achieves a sequence rich in guanines (G), stops and comes into contact with the Rho protein. dissociating from DNA and mRNA.
Rho-independent termination is controlled by specific sequences of the gene, usually rich in guanine-cytosine (GC) repeats..
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