Transcription Initiation Complex To The Gene example essay topic

INTRODUCTION: Transcription, otherwise known as gene expression, is the transfer of genetic information from DNA to RNA. Transcription creates a partial copy of messenger RNA (mRNA), and transfer RNA (tRNA). Control and regulation of gene expression is a prerequisite for life; if all genes were switched on at the same time, it would consume far more energy than the eukaryote cell could provide. The processing of the RNA formed involves a number of steps cut out and adds certain parts and molecules to produce a final mRNA strand ready to begin the process of translation. TRANSCRIPTION: The production of RNA involves the usual base pairing and is catalyzed by RNA polymerase. Signals encoded in the DNA instruct the enzyme to begin transcribing which are called promoters.

The amount of RNA transcribed depends upon the promoter, as it is this, which binds to the enzymes. Several other factors (along with the promoter) play a crucial role in the regulation of transcription. There are three steps of RNA formation, which are: Initiation Elongation Termination RNA polymerises have a high affinity for promoters. The binding of the RNA polymerase causes melting of the DNA double helix (uncoiling) disrupting H+ bonds between bases. The substrates of the polymerase are ribonucleoside 5'-triphosphate's (ATP, CTP, GIP, UTP). The initiator is usually a purine (ATP or GTP), and once the second is aligned a 3'-5' phosphodiester linkage is formed because of the released pyrophosphate.

The RNA chain elongates in a 5'-3' direction along with the polymerase. Once the DNA-RNA hybrid reaches the 12 bp in length the 5' RNA chain dissociates and the DNA duplex reforms. The polymerase continues this until reaching a termination sequence, which stops elongation. At this point they both dissociate. REGULATION: Initiation of transcription is the most important control point of eukaryotic gene expression (Weaver, Hedrick, 1989).

There are many promoters with cis-acting DNA regions called enhancers which stimulate transcription. Some of these regions are linked to negative control elements called silencers. Transcription doesn't only depend on RNA polymerises but trans-acting proteins called transcription factors that interact with promoters and enhancers. ENHANCERS AND SILENCERS: Enhancers increase the rate of transcription and are effective up to 1000 bp up / downstream. One of the best cases is the upstream 72 bp repeat in the early region of a Simian monkey virus tumour 40 (SV 40), and can increase transcription by 200 fold in Beta-Globin. Enhancer proteins have no histones; therefore they act as a door for polymerises to bind.

Silencers also act at a distance to control transcription, but they prohibit, and cause chromatin to coil up and condense, thereby preventing transcription of neighbouring genes (Lewin, 1997). TRANSCRIPTION FACTORS: Each gene is classified and regulated by a different RNA polymerase: 1. RNA polymerase I - transcribes rRNA (Pol I) 2. RNA polymerase II - transcribes mRNA (Pol II) 3. RNA polymerase - transcribes tRNA and other small RNA's (Pol ). But these need transcription factors before they can initiate transcription.

They are responsible for recognising the specific sequence of the promoter of the gene. Pol I and Pol recognise a restricted number of promoters, so they rely on a small number of factors to regulate their expression. In Pol II however, the promoters show more variation in sequence and number, and as a result more factors. We can divide them into 3 groups: 1. General Factors - Required for the mechanics of RNA synthesis at all promoters. They perform a complex with Pol II and determine the sight of initiation.

2. Upstream Factors - Recognise short consensus elements located upstream of start point. They increase the efficiency of initiation, and are required for promoters to function adequately. 3. Regulated Factors - Function like upstream factors but they also regulate. They are activated at specific times or in specific tissues.

The DNA sequences to which they bind to are called response elements. The latter two factors combine upstream of the initiation point to their elements and become associated with the construction of a protein-protein complex that recruits the transcription initiation complex (general transcription factors and Pol II) to the gene to be transcribed. DNA BINDING MOTIFS: Transcription factors and enhanced binding proteins share a structure of two functional domains: a DNA binding domain and one a transcription activation domain. The former consists of: 1. Zinc Fingers - These have a 3 D structure and zinc fingers in a loop. It is the sequence of the fingers that determine the DNA sequence to which the protein will bind (Hartl, Jones, 1998).

2. Leucine Zippers - Several DNA binding proteins with an alpha-helix domain; with all leucines on one face of the domain, exactly opposite another domain, hence a leucine zipper. This structure permits transcription factors to form dimers necessary for DNA binding. 3. Helix-turn-helix - Have amphipathic helices that are responsible for dimerization adjacent to basic regions that bind to DNA. PROCESSING PRE-Mrna The immediate product of transcription (primary transcript) must be converted into mRNA.

It consists of modification of the ends, and excision of untranslated regions. The regions which express the code for a polypeptide are called exons, and the regions intervening in the mRNA which do not code for a polypeptide are introns. Introns are excised from the primary transcript, with which there is a rejoining of the coding sequence (exons) to form mRNA. This process of excision and rejoining is called RNA splicing and is done by SnRNA (small ribonucleoprotein). Every intron has specific characteristics sequences of nucleotides on both ends. SnRNA recognises the introns and cuts them out of the sequence.

The second step is the addition of a 5'Cap to the 5' end of mRNA. This is done by the addition of a modified guanosine (7-methyl) in an uncommon 5' to 5' linkage, and it is this terminal that is known as the cap. The 3' terminus is modified by the addition of a poly adenosine (polyA) tail which can consist of up to 200 nucleotides. The 5'Cap is necessary for the mRNA to bind to the ribosome and as a result start protein synthesis (translation). The polyA tail assists in determining the mRNA stability to protect from deration. SUMMARY Eukaryotic transcription begins with the synthesis of a RNA molecule which is the copy of one strand of the DNA.

This is the process known as transcription and is carried out by the enzyme RNA polymerase. It is initiated when it has bound to the promoter sequence, and carries on until it reaches a termination sequence. At this point the RNA molecule is then processed, introns are removed and exons spliced together. The mode of regulation of this process is positive, that is a transcription factor or a set of factors under tissue-specific control are needed to activate gene expression, and ultimately, regulation. Regulation by specific repression of a targeted promoter is rare.

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