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Full Version: DNA to mRNA: Transcription in Eukaryotes
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According to the central dogma of molecular biology, gene expression is regulated through copying the DNA sequence into mRNA and production of encoded proteins. In this synthesizing mRNA complementary to DNA is called transcription. Transcription is generally different from prokaryotes to eukaryotes. Transcription in eukaryotes consists of initiation, elongation and termination.

Eukaryotic cells possess three different RNA Polymerases which transcribe the genes for three types of RNA’s: RNA polymerase I exclusively located in nucleolus catalyzes the synthesis of ribosomal RNA, RNA polymerase II found in nucleoplasm catalyzes the synthesis of messenger RNA and RNA polymerase III in nucleoplasm responsible for synthesis of transfer RNA. Nature of promoter recognition and initiation in transcription is different in eukaryotes than in prokaryotes.

Transcription requires the sequences on DNA that are accessible to RNA polymerase and other proteins. However, in eukaryotic cells, DNA is complexed with histone proteins in highly compressed chromatin. Therefore before transcription the chromatin structure is modified so that the DNA will come to a more open configuration and is more accessible to the transcription machinery. Enzymes involved in this modification are acetyl transferases; which adds acetyl groups to amino acids at the ends of histone proteins leading to destabilization of the nucleosome structure and makes the DNA more accessible. Chromatin remodeling proteins bind to the chromatin and displace nucleosome from promoters and other regions important for transcription.

Promoter sequences are adjacent to the genes that it regulates. Enhancer sequences are not always adjacent to the regulated genes. Promoters and enhancers are important sequences for the initiation of transcription. In eukaryotic cells, promoter recognition is carried out by accessory proteins that bind to the sequence and then recruit a specific RNA polymerase to the promoter. These comprises of general transcription factors and transcription activator proteins.

A promoter for a gene transcribed by RNA polymerase II which is the major enzyme involved typically includes one or more consensus sequences. The most common is the TATA box which has the consensus sequence TATAAA. Apart from these TFII B recognition element serves as a consensus sequence. These specific sequences in the core promoter are recognized by transcription factors that bind to them and serve as a platform for the assembly of the basal transcription apparatus. The basal transcription apparatus binds to the DNA at the start site and require for initiation. This consists of RNA polymerase, a series of general transcription factors and a complex of proteins called as mediator. General transcription factors include TFII A, TFII B, TFII D etc. These are involved in stabilizing interactions, selection of start site, active site for RNA polymerase and helicase activity to unwind DNA. Regulatory promoters are sequences located immediately upstream of the core promoter. Enhancers are involved in increasing the rate of transcription. Sometimes enhancers are act to repress transcription and these are known as silencers.

After several nucleotides have been linked together, RNA polymerase leaves the promoter and disassociates from transcription factors moving downstream. During elongation, the RNA polymerase maintains a transcription bubble in which about eight nucleotides of RNA remain base paired with DNA template.

In the course of elongation, the two strands of DNA are unwound and the Ribonucleotides that are complementary to the template strand are added to the growing 3’ end of the RNA molecule. As it funnels through the polymerase, the DNA-RNA hybrid hits the wall of amino acids, bend at almost right angle. At this bent position new nucleotides are added. The newly synthesized RNA is separated from DNA and runs through another groove before exiting from the polymerase.

Termination mechanism differs depending on the type of RNA polymerase. RNA polymerase I requires a termination factor which binds to the DNA sequence downstream of the termination site. RNA polymerase III terminates transcription after transcribing a terminator sequence that produces a string of Uracil nucleotides in the RNA molecule. Unlike the rho factor independent terminators in bacterial cells, RNA polymerase III does not require hairpin structure to proceed the string of Uracils. Research findings suggest that termination is coupled to a cleavage which is carried out by a cleavage complex that is probably associated with RNA polymerase.

At the end of transcription, a messenger RNA molecule complementary to the DNA is formed and this m RNA serve as the template for protein synthesis in translation.