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- 2. Central Dogma ofMolecular Biology The flow of information in the cell starts at DNA, which replicates to form more DNA. Information is then ‘transcribed” into RNA, and then it is “translated” into protein.
- 3. Transcription Transcription isthe process of making an RNA copy of a single gene. The enzyme used in transcription is “RNA polymerase”. There are several forms of RNA polymerase. In eukaryotes, most genes are transcribed by RNA polymerase
- 4. Process of Transcription 1)Transcription starts with RNA polymerase binding to the promoter. 2) This binding only occurs under some conditions: when the gene is “on”. Various other proteins (transcription factors) help RNA polymerase bind to the promoter. 1) Once it is bound to the promoter, RNA polymerase unwinds a small section of the DNA and uses it as a template to synthesize an exact RNA copy of the DNA strand.
- 5. 4) The DNAstrand used as a template is the “coding strand” the other strand is the “non-coding strand”. RNA is made from 5’ end to 3’ end so the coding strand is actually read from 3’ to 5’.
- 6. RNA Polymerase Theenzyme responsible for the RNA synthesis is DNA-dependent RNA polymerase. The prokaryotic RNA polymerase is a multiple- subunit protein of ~480kD Eukaryotic systems have three kinds of RNA polymerases, each of which is a multiple-subunit protein and responsible for transcription of different RNAs(mRNA, t-RNA & rRNA).
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- 8. After Transcription • Inprokaryotes, the RNA copy of a gene is messenger RNA, ready to be translated into protein.
- 9. • In eukaryotes,the primary RNA transcript of a gene needs further processing before it can be translated. • This step is called “RNA processing”. • Also, it needs to be transported out of the nucleus into the cytoplasm. • Steps in RNA processing: 1. Add a cap to the 5’ end 2. Add a poly-A tail to the 3’ end 3. Splice out Introns.
- 10. Capping • RNA isinherently unstable, especially at the ends. The ends are modified to protect it • At the 5’ end, a slightly modified guanine (7- methyl G) is attached • At the 3’ end, the primary transcript RNA is cut at a specific site and 100-200 adenine nucleotides are attached: the poly-A tail.
- 11. Introns • Introns areregions within a gene that don’t code for protein • Protein-coding sections of a gene (called exons) are interrupted by introns.
- 12. Intron Splicing • Intronsare removed from the primary RNA transcript while it is still in the nucleus. • Introns are “spliced out” by RNA/protein hybrids called “spliceosomes”. • The intron sequences are removed, and the remaining ends are re- attached so the final RNA consists of exons only. www.biology.arizona.edu
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- 14. Summary of RNAprocessing • In eukaryotes, RNA polymerase produces a “primary transcript”, an exact RNA copy of the gene. • A cap is put on the 5’ end • The RNA is terminated and poly-A is added to the 3’ end • All introns are spliced out • At this point, the RNA can be called messenger RNA. It is then transported out of the nucleus into the cytoplasm, where it is translated.
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