Chemistry of Life

Molecular genetics, biochemistry

transcription


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Transcription (Animation) : Transcription : DNA : Journey of the Nucleotides : Lodish et al 2000 Gene Expression : Bio DNA RNA Aminoacid part 1 of 2 : Bio DNA RNA Aminoacid part 2 of 2 : Straddling on DNA: Human TFIIA/TBP/DNA Complex : DNA RNA : Transcription and Translation :

In transcription, an RNA polymerase enzyme (RNAp, or pol III in eukaryotes) directs generation of a complementary strand of mRNA from DNA. The mechanism of alternative splicing enables to production of different mature mRNA molecules, depending on what sequences are treated as introns and what remain as exons. The rate of gene transcription is controlled by complex interactions between cis-acting elements within regulatory DNA sequences and trans-acting factors, including transcription factors and the basal transcription complex, which comprises the RNA polymerase and proteins necessary for correct initiation and elongation.

Transcription, like replication, proceeds 3' to 5' on the template strand and 5' to 3' on the new strand.

Transcription involves 3 phases: initiation, elongation, and termination.
1. Having bound to the promoter region , RNA polymerase II initiates transcription at the first nucleotide of the first exon of a class II gene (PIC).
2. The 5' end of the nascent RNA is capped with 7-methylguanylate (capping).
3. Transcription by RNA polymerase II terminates at any one of multiple termination sites downstream from the poly(A) site, which is located at the 3' end of the final exon.
4. After the primary transcript is cleaved at the poly(A) site.
5. A string of adenine (A) residues is added (polyadenylation). The poly(A) tail contains ≈250 A residues in mammals, ≈150 in insects, and ≈100 in yeasts.

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In more detail:
RNA polymerase binds to the promoter region of one strand of DNA (5’ end), and the DNA double helix is un-zipped into single strands. First, RNA polymerase requires a number of general transcription factors (called TFIIA, TFIIB, etc.). Many promoters contain a DNA sequence called the TATA box, which is located 25 nucleotides away from the site of initiation of transcription. The TATA box is recognized and bound by transcription factor TFIID, which then enables the adjacent binding of TFIIB. The rest of the general transcription factors plus the RNA polymerase assemble at the promoter. diagram - initiation of transcription.

The RNAp enzyme moves toward the 3’ end, connecting complementary bases into an elongating chain of RNA nucleotides. At termination, the transcribed mRNA molecule is released from the DNA strand. In prokaryotic cells – without a nuclear membranetranslation may begin prior to termination. In eukaryotic cells – with a nuclear membrane – the processed mRNA moves through the nuclear pores into the cytoplasm, where ribosomes on the rough endoplasmic reticulum translate the mRNA code into a peptide or a protein. Epigenetic, alternative splicing mechanisms can edit the mRNA prior to its translation into protein.

Capping of the 5’ end on the pre-mRNA with 7-methylguanylate occurs soon after initiation of transcription, and the 5 cap is retained in mature mRNAs.

Cleavage, pre-mRNA splicing, and polyadenylation usually follow termination of transcription of short primary transcripts with few introns. However, introns often are spliced out of the nascent RNA before transcription of the gene is complete for large genes with multiple introns.

It was believed that most genes in higher eukaryotes are regulated by controlling their transcription. However, it is increasingly recognized that epigenetic mechanisms (such as alternative splicing) are important in generating many proteins from a single gene, accounting for the Human Genome Project’s discovery that a mere 30,000 genes code for about 100,000 proteins.

animation - start of transcription : animation - life cycle of an mRNA : animation ~ alternative splicing : ball-stick - dna double helix : animation - adenine tyrosine H bonds : animation - cytosine guanine H bonds :

More at NCBI Molecular Cell Biology - Transcription Initiation Complex : SUMMARY transcription initiation (NCBI MCB) : Processes of Transcription : Wikipedia : Central Dogma :



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. . . transcription begun 10/06/06