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Chapter 17

Chapter 17 - From Gene to Protein Overview The Flow of...

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Unformatted text preview: From Gene to Protein Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation • The ribosome is part of the cellular machinery for translation (polypeptide synthesis) Basic Principles of Transcription and Translation • Transcription is the synthesis of RNA under the direction of DNA • Transcription produces messenger RNA (mRNA) • Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA • Ribosomes are the sites of translation • In prokaryotes, mRNA produced by transcription is immediately translated without more processing • In a eukaryotic cell, the nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA • Cells are governed by a cellular chain of command: DNA → RNA → protein LE 17-3-1 TRANSCRIPTION DNA Prokaryotic cell LE 17-3-2 TRANSCRIPTION DNA mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell LE 17-3-3 TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Eukaryotic cell LE 17-3-4 TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Eukaryotic cell LE 17-3-5 TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Eukaryotic cell The Genetic Code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four nucleotide bases in DNA • So how many bases correspond to an amino acid? Codons: Triplets of Bases • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced • During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript • During translation, the mRNA base triplets, called codons, are read in the 5′ to 3′ direction • Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide LE 17-4 Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 3′ 5′ TRANSCRIPTION mRNA 5′ Codon 3′ TRANSLATION Protein Amino acid LE 17-5 First mRNA base (5′ end) Second mRNA base Third mRNA base (3′ end) Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine • The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator • The stretch of DNA that is transcribed is called a transcription unit LE 17-7 Promoter 5 3′ Start point RNA polymerase Transcription unit 3 5′ DNA Initiation 5′ 3′ RNA Template strand Unwound tran- of DNA DNA script Elongation Rewound DNA 5′ 3′ 5′ RNA transcript 3′ 3′ 5′ 3′ 5′ Termination 5′ 3′ 5′ Completed RNA transcript 3′ 3′ 5′ LE 17-7 Elongation Non-template strand of DNA RNA nucleotides RNA polymerase 3′ 3′ end 5′ 5′ Direction of transcription (“downstream”) Template strand of DNA Newly made RNA Eukaryotic cells modify pre-RNA after transcription • • Enzymes in the eukaryotic nucleus modify pre-mRNA before they are dispatched to the cytoplasm in the form of mRNA During RNA processing, both ends of the primary transcript are usually altered: - The 5′ end receives a modified nucleotide cap - The 3′ end gets a poly-A tail • These modifications at the two ends serve several functions: – They seem to facilitate the export of mRNA – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5’ end • Also, introns are cut out, and the exons are spliced together RNA Processing Molecular Components of Translation • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) • Molecules of tRNA are not identical: – Each carries a specific amino acid on one end – Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA LE 17-26 TRANSCRIPTION DNA 3′ 5′ RNA transcript RNA PROCESSING Exon RNA polymerase RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase NUCLEUS FORMATION OF INITIATION COMPLEX Amino acid tRNA CYTOPLASM AMINO ACID ACTIVATION mRNA Growing polypeptide Activated amino acid 3′ A P E Ribosomal subunits 5′ TRANSLATION E A Anticodon Codon Ribosome LE 17-13 Polypeptide Amino acids tRNA with amino acid attached Ribosome tRNA Anticodon 5′ mRNA Codons 3′ LE 17-12 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide ...
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