Transcript Translation
Transcription and Translation Transcription • the process by which chemical information encoded in DNA is copied into mRNA (messenger RNA) • transcription is classically described in three distinct steps: initiation, elongation and termination. • Initiation – an enzyme binds to specific sequence of bases on a DNA strand called the promoter region and begins to break H+ of the complementary bases, unwinding the DNA. • a segment of DNA of around 17 base pairs stays unwound throughout the transcription process. • Elongation – an enzyme helps free floating ribonucleotides add to the unwound strand OH OH OH – each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing • for each C encountered on the DNA strand, a G is inserted in the RNA • for each G, a C • for each T, an A • however, each A on the DNA guides the insertion of the pyrimidine uracil, U instead of A • Termination – when the enzyme reaches a specific sequence of nucleotides, it and its newly transcripted RNA are released from the DNA. • finally, two major processes are done to the new mRNA strand before it's ready to be used to make protein: – introns are regions of mRNA that do not code for proteins and must be removed by spliceosomes leaving only exons or coding regions that code for proteins • this is now considered mature mRNA – a cap and tail are added to the ends for stability/protection Splicing Out Introns Movie Translation • process of converting the information in mRNA into a protein. • 3 main parts- Initiation, Elongation, Termination University of Colorado Translation Movie 1. Initiation • an initiation complex is formed when: – small subunit of the ribosome binds to a site on the mRNA – it proceeds “downstream” until it encounters the start codon AUG. – here it is joined by the large ribosomal subunit and a special initiator tRNA. – in eukaryotes (cells with a nucleus), the initiator tRNA with the anticodon UAC carries the amino acid methionine (Met) that binds to the first site on the large ribosomal unit 2. Elongation • the process of adding more a.a. to the protein – another tRNA covalently bonded with its amino acid base pairs with the next codon on the mRNA at the next site on the ribosome – the preceding amino acid methionine (Met) is then bonded to the incoming amino acid – the first tRNA is released and the ribosome moves one codon downstream 3. Termination • the point where the translating ribosome reaches a stop codon – stop codons are UAA, UAG, and UGA because none of the tRNAs have anticodons that are complimentary to these three codons – instead, a protein binds to the mRNA and releases the polypeptide from the ribosome which splits back into its two subunits McGraw Hill Movies.