Eukaryotic Transcription


Transcription is the process of copying genetic information from DNA into RNA, especially mRNA, by the enzyme RNA polymerase. It is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase. It results in a complementary, antiparallel RNA strand called a primary transcript.

Transcription in eukaryotes is the elaborate process that eukaryotic cells use to copy genetic information stored in DNA into units of transportable complementary RNA replica. Eukaryotic transcription is carried out in the nucleus of the cell by one of three RNA polymerases, depending on the RNA being transcribed, and proceeds in three sequential stages: initiation, elongation, and termination .

Initiation requires the binding of transcription factors to promoter sequences upstream of the gene . The most-extensively studied core promoter element in eukaryotes is a short DNA sequence known as a TATA box, found 25-30 base pairs upstream from the start site of transcription. The TATA box is recognized by a transcription factor called TATA-binding protein (TBP), which is part of a larger complex called TFIID. Other transcription factors and RNA polymerase then assemble on the promoter to form a pre-initiation complex (PIC). In addition, transcription is also regulated by upstream control elements that lie 5′ to the TATA box.

Elongation involves the synthesis of RNA by RNA polymerase along the DNA template. The RNA polymerase transcribes one strand, the antisense (-) strand, of the DNA template. RNA synthesis occurs in the 5’ → 3’ direction with the RNA polymerase catalyzing a nucleophilic attack by the 3-OH of the growing RNA chain on the alpha-phosphorus atom on an incoming ribonucleoside 5-triphosphate.

Termination occurs when RNA polymerase reaches a termination signal on the DNA template and releases the RNA transcript. Unlike prokaryotes, eukaryotes do not have specific termination sequences on their genes. Instead, termination is coupled with polyadenylation, a process that adds a poly-A tail to the 3` end of the mRNA transcript. A cleavage and polyadenylation specificity factor (CPSF) recognizes a polyadenylation signal (AAUAAA) on the pre-mRNA and cleaves it downstream of this signal. A poly-A polymerase then adds about 200 adenine nucleotides to form the poly-A tail.

The primary transcript undergoes various processing steps before becoming a mature mRNA that can be exported to the cytoplasm for translation. These steps include capping, splicing, and editing. Capping involves adding a modified guanine nucleotide (7-methylguanosine) to the 5` end of the pre-mRNA. This cap protects the mRNA from degradation and facilitates its transport and recognition by the ribosome. Splicing involves removing non-coding introns and joining coding exons to produce a continuous coding sequence. This is done by a complex of proteins and RNAs called spliceosome. Editing involves changing some nucleotides in the pre-mRNA to alter its sequence and function. This can be done by deamination, insertion, or deletion of nucleotides.

Transcription in eukaryotes is a complex and highly regulated process that ensures accurate and timely expression of genes. It is essential for cellular function and differentiation.