Chapter 6 How Cells Read the Genome: Molecular Biology of The Cell PDF

Chapter 6 How Cells Read the Genome: From DNA to Protein Copyright © 2022 W. W. Norton & Company, Inc. Genes can be expressed with different efficiencies RNA Molecules Are Single-Stranded FROM DNA TO RNA Transcription Produces RNA Complementary to One Strand of DNA RNA Polymerases Carry Out DNA Tran...

Chapter 6 How Cells Read the Genome: From DNA to Protein Copyright © 2022 W. W. Norton & Company, Inc. Genes can be expressed with different efficiencies RNA Molecules Are Single-Stranded FROM DNA TO RNA Transcription Produces RNA Complementary to One Strand of DNA RNA Polymerases Carry Out DNA Transcription RNA polymerases catalyze the formation of the phosphodiester. The RNA polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead of its active site for polymerization to expose a new region of the template strand for complementary base-pairing. RNA polymerases make about one mistake for every 104 nucleotides copied into RNA (Compared to 107 nucleotides in DNA polymerase) RNA polymerase does not require a primer. The RNA strand is transcribed continuously by the same RNA polymerase. RNA polymerase has a proofreading mechanism. Cells Produce Different Categories of RNA Molecules Signals Encoded in DNA Tell RNA Polymerase Where to Start and Stop Bacteria 1- RNA polymerase holoenzyme (polymerase core enzyme plus  factor) assembles and then, by sliding, locates a promoter DNA sequence. 2- The polymerase opens (unwinds) the DNA at the position at which transcription is to begin and begins transcribing. 3- This initial RNA synthesis (abortive initiation) is relatively inefficient as short, unproductive transcripts are often released. 4- Once RNA polymerase has managed to synthesize about 10 nucleotides of RNA, it breaks its interactions with the promoter DNA and eventually releases  factor. 5- As the polymerase tightens around the DNA and shifts to the elongation mode of RNA synthesis, moving along the DNA. 6,7- During the elongation mode, transcription is highly processive, with the polymerase leaving the DNA template and releasing the newly transcribed RNA only when it encounters a termination signal. 8- Termination signals are typically encoded in DNA, and many function by forming an RNA hairpin–like structure that destabilizes the polymerase’s hold on the RNA. Transcription Initiation in Eukaryotes Requires Many Proteins Eukaryotic cells: To Initiate Transcription, RNA Polymerase II Requires a Set of General Transcription Factors General transcription factors: Help to position eukaryotic RNA polymerase correctly at the promoter,. Aid in pulling apart the two strands of DNA to allow transcription to begin. Release RNA polymerase from the promoter to start its elongation mode. In Eukaryotes: Transcription Initiation requires: Activator Mediator Chromatin-modifying Proteins Histone-modifying enzyme Transcription Elongation in Eukaryotes Requires Accessory Proteins Transcription Elongation in Eukaryotes Is Tightly Coupled to RNA Processing RNA Capping Is the First Modification of Eukaryotic Pre-mRNAs The pre-mRNA splicing reaction. RNA Splicing Is Performed by the Spliceosome The Spliceosome Uses ATP Hydrolysis to Produce a Complex Series of RNA– RNA Rearrangements Other Properties of Pre-mRNA and Its Synthesis Help to Explain the Choice of Proper Splice Sites The chemical modification and nucleolytic processing of ribosomal RNAs The Nucleolus Is: a Ribosome-producing Factory Translation An mRNA Sequence Is Decoded in Sets of Three Nucleotides tRNA Molecules Match Amino Acids to Codons in mRNA Wobble base-pairing between codons and anticodons. Specific Enzymes Couple Each Amino Acid to Its Appropriate tRNA Molecule Editing by tRNA Synthetases Ensures Accuracy Amino Acids Are Added to the C-terminal End of a Growing Polypeptide Chain The RNA Message Is Decoded in Ribosomes Elongation Factors Drive Translation Forward and Improve Its Accuracy Nucleotide Sequences in mRNA Signal Where to Start Protein Synthesis Stop Codons Mark the End of Translation Proteins Are Made on Polyribosomes There Are Minor Variations in the Standard Genetic Code The Ribosome Coordinates the Folding, Enzymatic Modification, and Assembly of Newly Synthesized Proteins Molecular Chaperones Help Guide the Folding of Most Proteins The RNA World and the Origins of Life Single-Strand RNA Molecules Can Fold into Highly Elaborate Structures