Transcription Initiation in Eukaryotes

Eukaryotic transcription is the elaborate process that eukaryotic cells use to copy genetic information stored in DNA into units of transportable complementary RNA replica.Gene transcription occurs in both eukaryotic and prokaryotic cells. Unlike prokaryotic RNA polymerase that initiates the transcription of all different types of RNA, RNA polymerase in eukaryotes (including humans) comes in three variations, each translating a different type of gene. A eukaryotic cell has a nucleus that separates the processes of transcription and translation. Eukaryotic transcription occurs within the nucleus where DNA is packaged into nucleosomes and higher order chromatin structures. The complexity of the eukaryotic genome necessitates a great variety and complexity of gene expression control.

Eukaryotic transcription proceeds in three sequential stages: initiation, elongation, and termination.

The RNAs transcribed serve diverse functions. For example, structual components of the ribosome are transcribed by RNA polymerase I. Protein coding genes are transcribed by RNA polymerase II into messenger RNAs (mRNAs) that carry the information from DNA to the site of protein synthesis.More abundantly made are the so-called non-coding RNAs account for the large majority of the transcriptional output of a cell. These non-coding RNAs perform a variety of important cellular functions.

RNA polymerase II (Pol II) is responsible for the transcription of all mRNAs, some snRNAs, siRNAs, and all miRNAs.[3][4] Many Pol II transcripts exist transiently as single strand precursor RNAs (pre-RNAs) that are further processed to generate mature RNAs.[1] For example, precursor mRNAs (pre-mRNAs) are extensively processed before exiting into the cytoplasm through the nuclear pore for protein translation.

The initiation of gene transcription in eukaryotes occurs in specific steps.[1] First, an RNA polymerase along with general transcription factors binds to the promoter region of the gene to form a closed complex called the preinitiation complex. The subsequent transition of the complex from the closed state to the open state results in the melting or separation of the two DNA strands and the positioning of the template strand to the active site of the RNA polymerase. Without the need of a primer, RNA polymerase can initiate the synthesis of a new RNA chain using the template DNA strand to guide ribonucleotide selection and polymerization chemistry.[1] However, many of the initiated syntheses are aborted before the transcripts reach a significant length (~10 nucleotides). During these abortive cycles, the polymerase keeps making and releasing short transcripts until it is able to produce a transcript that surpasses ten nucleotides in length. Once this threshold is attained, RNA polymerase passes the promoter and transcription proceeds to the elongation phase.[

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