DNA transcription

The genetic code is often called the "plan" because it contains the instructions for the cell to maintain its existence. Now we know that there are more than these simple instructions in the sequence of letters in the code of nucleotides, however. For example, a large amount of evidence indicates that this code is the basis for the production of various molecules as RNA and proteins. Studies have also shown that the instructions are stored in the DNA "to read" in two stages: transcription and translation. In the transcript of the double-stranded DNA template results in the single-stranded RNA molecules. In some cases, the RNA molecule itself is a "finished product", which performs an important function in the cell. Often, however, transcription of the RNA molecule followed by a translation step, which ultimately leads to the production of the protein molecule.

Vizualizing Transcript

Transcription of the process can be captured with an electron microscope, in fact, first observed this method in 1970. In electron microscopy, these principles, the DNA molecules act as "logs" with multiple RNA "branches" that goes beyond them. If DNase and RNase (enzymes that cause degradation of DNA and RNA, respectively) were added to the molecule, the use of DNase eliminated the stem structures, whereas the use of RNase branches disappear.

DsDNA, but only one strand serves as a template for transcription in a given time, the other is called non-coding Strand chain. In most organisms, DNA strand, which serves as a model for coding genes can be no other genes coding for the same chromosome.

Transcription process

Transcription begins when the enzyme RNA polymerase (RNA POL) is associated with the template DNA strand and begins to stimulate the production of complementary RNA. Big polymerase enzymes consist of about a dozen units, and when he is active in DNA, as a whole set of other factors. In many cases, these factors signal, which is the transcription of genes.

Three different types of RNA polymerases exist in eukaryotes, while bacteria have only one.Eukaryotic RNA-Paul and restores the genes encoding most of the ribosomal RNA (rRNA) and RNA-POL III resumes rRNA genes of one small addition to the transfer RNA that play a key role in the process of translation and other small regulatory RNA molecules. Thus, an RNA-POL II that restores RNA, which serve as templates for the production of protein molecules.

Transcription initiation

Picture 2The first step in transcription initiation of RNA-POL binds to DNA up (5 ') genes in a sequence called the promoter specialist. In bacteria, promoters often consist of three elements of the sequence, while the eukaryotes, there are up to seven points.

In prokaryotes, most genes is a sequence of TATAAT Pribnow box consensus sequence located at a distance of about ten base pairs of the object, which serves as the initiation of transcription. Not all Pribnow boxes, it is an exact sequence of nucleotides, these nucleotides are simply the most commonly found at each site. Despite the substitutions made every frame of this consensus, however, seems fairly closely. Many genes have TTGCCA consensus sequence at position 35 bases upstream from the starting point, and some of them, called up the item that the AT-rich region from 40 to 60 nucleotides upstream, which improves the rate of transcription (Fig. 2 ). In any case, binding, RNA-POL "core enzyme" binds to another subunit called sigma subunit to form holoezyme possible to relax the DNA double helix, in order to facilitate access to genetics. Sigma-subunit conveys specificity of a promoter of RNA polymerase, ie, responsible for talking RNA polymerase, where they join. There are a number of different parts of Sigma, which connect the various promoters and thereby contribute to the transformation of genes and outside conditions change.

Eukaryotic promoters are more complex than their prokaryotic counterparts, partly because eukaryotes, these three classes of RNA polymerase to write different sets of genes. Many of eukaryotic genes as Enhancer, which is located at a considerable distance from the genes they affect. Enhancer sequences of the leaders of the activation of genes by binding to activator protein and change the 3-D structure of DNA, to help "draw" RNA-POL II, thus regulating transcription. Since eukaryotic DNA is packed tightly as chromatin, transcription also requires a number of specialized proteins that help to make available the chain coding.

In eukaryotes, the "core" promoter of the gene transcribed by POL II is found most often immediately before (5 '), the initiation site of the gene. Most POL II genes have a box Dad (consensus sequence TATTAA) from 25 to 35 bases upstream from the start site, which influences the transcription rate and determines the location of the starting point. Eukaryotic RNA polymerase using a number of key factors (called general transcription factors), and one of them, TFIID recognizes the TATA box and ensures that you are using the correct starting point.Other cofactor TFIIB, recognizes different sequences is a general consensus G / CG / CG / CGCCC, about 38 to 32 bases upstream.

Dates are "strong" and "weak" is often used to describe the promoters and accessories, in accordance with its effects on transcription rates and, thus, in gene expression. Changing promoter strength may have harmful effects on cells, which often leads to disease. For example, some tumors contribute to the virus to transform cells through the inclusion of healthy strong promoters in close proximity to stimulate growth genes, and translocations in cancer cells, and some genes are "Disabled" in the proximity of strong promoters or amplifiers.

Enhancer sequences to do what its name implies, measures to improve the speed with which the transcription of genes and their consequences can be very powerful. Accessories may be thousands of nucleotides of the promoters with whom they interact, they put them in the immediate vicinity of the DNA cycle. This relationship is the result of interaction between the protein linked to the Enhancer and promoter linked. Proteins that facilitate this cycle is called activators, and those that impede called repressors.

Eukaryotic gene transcription polymerase I and III begins the same way, but the promoter and transcription activator protein sequence change.

Dates are "strong" and "weak" is often used to describe the promoters and accessories, in accordance with its effects on transcription rates and, thus, in gene expression. Changing promoter strength may have harmful effects on cells, which often leads to disease. For example, some tumors contribute to the virus to transform cells through the inclusion of healthy strong promoters in close proximity to stimulate growth genes, and translocations in cancer cells, and some genes are "Disabled" in the proximity of strong promoters or amplifiers.

Enhancer sequences to do what its name implies, measures to improve the speed with which the transcription of genes and their consequences can be very powerful. Accessories may be thousands of nucleotides of the promoters with whom they interact, they put them in the immediate vicinity of the DNA cycle. This relationship is the result of interaction between the protein linked to an amplifier and a promoter linked. Proteins that facilitate this cycle is called activators, and those that impede called repressors.

Eukaryotic gene transcription polymerase I and III begins the same way, but the promoter and transcription activator protein sequence change.

Strand extension

As soon as he initiates transcription, unwinds the double helix of DNA and RNA polymerase reads the template Strand adding nucleotides to the 3 'end of the growing chain. At a temperature of 37 degrees Celsius, the new nucleotides are added at a rate of about 15-20 amino acids in the second bacteria (Dennis & Bremer, 1974), while eukaryotes move much slower rate of about five-eight amino acids per second (Izban & Luse, 1992 ).

As soon as he initiates transcription, unwinds the double helix of DNA and RNA polymerase reads the template Strand adding nucleotides to the 3 'end of the growing chain. At a temperature of 37 degrees Celsius, the new nucleotides are added at a rate of about 15-20 amino acids in the second bacteria (Dennis & Bremer, 1974), while eukaryotes move much slower rate of about five-eight amino acids per second (Izban & Luse, 1992 ).



Transcription termination

Terminator sequences near the ends of coding sequences. Bacteria have two types of these sequences. Rho-independent terminator, an inverted repeat sequences are transcribed, it can be concluded at myself in the brackets, resulting in RNA POL pause and as a result of the release of transcripts. In addition, Rho-dependent terminators, to use a factor called Rho, which is actively unwinds DNA-RNA hybrid formed during transcription, which will release the new synthesis of RNA.

Eukaryotes, termination of transcription occurs by several processes, depending on the exact polymerase used. For Paul and gene transcription ceases to use the factor of termination, through a mechanism similar to Rho-dependent termination of bacteria. Transcription POL III gene transcription ends after the termination sequence, which includes a section polyuracil, through a mechanism similar to prokaryotic Rho independent termination. Termination POL II transcripts, however, is more complex.

Transcription POL II genes may persist for hundreds or even thousands of nucleotides after the coding sequence. RNA cleaved complex, which appears to be linked with the polymerase. The split seems, along with the termination of transcription and occurs in the consensus sequence.MRNA polyadenylated POL II above, in the end of 3 ', as a result of political (A) tail, this process should be the Division and coordinates its activities with the termination.

Both polyadenylation and termination of the use of the same sequence, consensus, and the interdependence of the processes demonstrated in the 1980's through the work of several groups. A group of scientists, working with the mouse globin genes showed that mutations introduced in the sequence aataaa consensus, as is known, necessary for sex (except for), inhibits both polyadenylation and transcription termination. We measured the degree of completeness of hybridization of transcripts from various fields (A) consensus sequence of mutants with wild-type transcripts and were able to see a decrease of hybridization signal, suggesting that the termination was proper inhibited. Therefore polyadenylation to the conclusion that it was necessary for the termination (Logan ET. Al, 1987). The second group Similar results were obtained with the help of the monkey virus SV40 (Simian Virus 40). There have been mutations in the field () site, resulting in mRNAs accumulate to a level much higher than the wild type (Connelly & Manley, 1988).

The exact relationship between the Division and the termination must be defined. The gap model assumes that this has caused the termination, suggest that the polymerase activity is affected when it passes through the consensus sequence at the cleavage site, possibly through changes in factors associated with transcriptional activation. Thus, research in the field of transcription prokaryotes and eukaryotes remains focused on unraveling the molecular details of this complex process, these data will allow us to better understand how gene transcription and silent.