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Chapter 14.1

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  1. Chapter 14.1 How is RNA Transcribed from DNA AP Biology Fall 2010

  2. Three Classes of RNA • It takes three classes of RNA to synthesize proteins • Messenger RNA (mRNA) • Ribosomal RNA (rRNA) • Transfer RNA (tRNA)

  3. Three Classes of RNA • Messenger RNA (mRNA) • Carries the “blueprint” to the ribosome • Ribosomal RNA (rRNA) • Combines with proteins to form ribosomes upon which polypeptides are assembled • Transfer RNA (tRNA) • Brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid

  4. Nature of Transcription • RNA differs from DNA • RNA uses ribose sugar, not deoxyribose • RNA bases are A, G, C, and U (uracil)

  5. Nature of Transcription • Transcription differs from DNA replication in three ways: • Only one region of one DNA strand is used as a template • RNA polymerase is used instead of DNA polymerase • The result of transcription is a single-stranded RNA

  6. Nature of Transcription

  7. Transcription • Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) • Then moves along to the end of a gene • Copies from 3 prime to 5 prime end of DNA molecule • Builds from 5 prime to 3 prime end of RNA molecule • After free ribonucleotides are complementary bonded to the template, an RNA transcript is created

  8. Direction of Transcription

  9. Transcription • Transcription ends when RNA polymerase reaches “the end” signal • RNA transcript is then released

  10. Finishing Touches on mRNA Transcripts • Newly formed mRNA is an unfinished molecule, not yet ready for use • mRNA transcripts are modified before leaving the nucleus

  11. Finishing Touches on mRNA Transcripts • The 5’ end is capped with a modified guanine that serves as a “start” signal for translation • The cap will also help bind the mRNA to a ribosome • A “poly-A tail” of about 100-200 molecules of adenine ribonucleotides is added to the 3’ end

  12. Finishing Touches on mRNA Transcripts • Noncoding portions (introns) are snipped out, and actual coding regions (exons) are spliced together to produce the mature transcript • Both are transcribed before transcript reaches cytoplasm • Alternative splicing of exons mixes up different parts of the same gene • Resulting in different proteins that increases the cell’s capacity to make diverse proteins • One gene can specify two or more proteins that differ slightly in form and function