Molecular Biology

Dna Transcription Searching for the Source of Gene Expression

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DNA transcription is a method used by our cells to create templates from our naked DNA called mRNA, which is then translated into proteins to be used in our bodies. Different proteins are coded for by different genes, these genes are regulated through upstream cis-acting control elements, such as enhancer, and promoter sequences in the DNA (such as the TATA box consensus sequence). Enhancers are independent from the promoter sequence, and can be effective from quite a distance from the gene, or even from in the middle of the gene. Transcription is also controlled by trans-acting transcriptional factors (proteins), with two binding domains, the DNA binding domain, and the activation domain.

Eukaryotic transcription is controlled by three RNA polymerase enzymes, the RNA pol1 enzyme which transcribes rRNA, the RNA pol2 enzyme which transcribes mRNA, and the RNA pol3 enzyme which transcribes tRNA and other small RNAs. There are also general factors which form a basal transcriptional apparatus with RNA pol2, upstream accessory factors (TFIID, TFIIA, TFIIB, TFIIF, TFIIE, and TFIIH) which recognize short consensus sequences, and inducible factors that allow control of gene expression, though a stimulus.

The assembly of the pre-initiation complex (PIC) involves recognition of the promoter sequence by TFIID, TBP (TATA-binding protein) positions RNA pol2 in the correct position, along with TAFs (TBP associated factors, which bend the DNA). TFIIA stabilizes the TBP, allows binding of TFIIB, then TFIIF brings the RNA pol2 into the complex, which has helicase activity, and the first phosphodiester bond is formed. TFIIE is involved in promoter melting and clearance, whilst TFIIH has ATPase, helicase, and kinase activity at the c-terminal end of the RNA pol2, it allows dissociation from RNA pol2. Eventually only TFIIF remains associated with the RNA pol2 as it begins elongation.

After transcription of mRNA from DNA, a 5' cap is added to the strand, and a poly-A tail is added to the 3' end. The non-coding introns are then spliced out of the mRNA, snRNAs assemble on the mRNA to form a large ribonucleoprotein (splicosome), leaving only the coding DNA, and promoter sequences. The mRNA is then transported out of the nucleus, into the cytoplasm for translation, this is a process involving the nuclear exporter RNA binding protein, it associates with FG repeats of nucleoporin proteins that line the nuclear pores. The 5' cap leads the mRNA through the pore, and the exporter protein is recycled back into the nucleus.

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