Transcription and Translation Explained.
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What is transcription? Transcription is the process of making RNA from a DNA template. Several key factors are involved in this process. Including, DNA, transcription factors, RNA polymerase, and ATP. Transcription begins with a strand of DNA. It is divided into several important regions. The largest of these is the transcription unit. This portion of the DNA will be used to produce RNA. Upstream of the transcription unit is the TATA box. An enhancer region may also be involved. Several complexes, known as transcription factors, are required for successful transcription. The first is TFIID, the largest of the general factors. A component of this factor, TBP, binds to the DNA using the TATA box to position TFIID near the transcription initiation site. Other transcription factors, including TFIIA and TFIIB, then attach. These complexes prepare the DNA for the successful binding of RNA polymerase. One RNA polymerase is bound, other transcription factors complete the mature transcription complex. Now, energy must be added to the system for transcription to begin. This energy is provided by the reduction of ATP into ADP and Pi. RNA polymerase then synthesizes an RNA template from the strand of DNA. Most factors are released after transcription begins. When the end of the transcription unit is reached, the RNA polymerase dissociates, and the newly formed strand of RNA is released.
What is Translation? Translation is the synthesis of a protein from an mRNA template. This process involves several key molecules including mRNA, the small and large subunits of the ribosome, tRNA, and finally, the release factor. The process is broken into three stages: initiation, elongation, and termination. Eukaryotic mRNA, the substrate for translation, has a unique 3′-end called the poly-A tail. mRNA also contains codons that will encode specific amino acids. A methylated cap is found at the 5′-end. Translation initiation begins when the small subunit of the ribosome attaches to the cap and moves to the translation initiation site. tRNA is another key molecule. It contains an anticodon that is complementary to the mRNA codon to which it binds. The first codon is typically AUG. Attached to the end of tRNA is the corresponding amino acid. Methionine corresponds to the AUG codon. The large subunit now binds to create the peptidyl (or P) site and the aminoacyl (or A) site. The first tRNA occupies the P-site. The second tRNA enters the A-site and is complementary to the second codon. The methionine is transferred to the A-site amino acid, the first tRNA exits, the ribosome moves along the mRNA, and the next tRNA enters. These are the basic steps of elongation. As elongation continues, the growing peptide is continually transferred to the A-site tRNA, the ribosome moves along the mRNA, and new tRNAs enter. When a stop codon is encountered in the A-site, a release factor enters the A-site and translation is terminated. When termination is reached, the ribosome dissociates, and the newly formed protein is released.
