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Chapter 9 From DNA to Protein 9.1 The Aptly Acronymed RIPs • A tiny amount of ricin, a natural protein found in castor-oil seeds, can kill an adult human – there is no antidote • Ricin is a ribosome-inactivating protein (RIP) – it inactivates the organelles which assemble amino acids into proteins • Other RIPs include shiga toxin, made by Shigella dysenteriae bacteria, and enterotoxins made by E. coli bacteria, including the strain O157:H7 Some RIPs 9.2 DNA, RNA, and Gene Expression • Transcription converts information in a gene to RNA DNA → transcription → mRNA • Translation converts information in an mRNA to protein mRNA → translation → protein The Nature of Genetic Information • Each DNA strand consists of a chain of four kinds of nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C) • The sequence of the bases in the strand is the genetic code • All of a cell’s RNA and protein products are encoded by DNA sequences called genes Converting a Gene to an RNA • Transcription • Enzymes use the nucleotide sequence of a gene to synthesize a complementary strand of RNA • DNA is transcribed to RNA • Most RNA is single stranded • RNA uses uracil in place of thymine • RNA uses ribose in place of deoxyribose A DNA Nucleotide base (guanine) 3 phosphate groups A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate sugar (deoxyribose) An RNA Nucleotide base (guanine) 3 phosphate groups An RNA nucleotide: guanine (G), or guanosine triphosphate sugar (ribose) ANIMATED FIGURE: Gene transcription details To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE adenine A DNA deoxyribonucleic acid RNA ribonucleic acid sugar– phosphate backbone guanine G cytosine C adenine A guanine G cytosine C nucleotide base base pair thymine T Nucleotide bases of DNA uracil U DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring. RNAs have various functions. Some serve as disposable copies of DNA’s genetic message; others are catalytic. Still others have roles in gene control. Nucleotide bases of DNA Figure 9-3 p151 RNA in Protein Synthesis • Messenger RNA (mRNA) • Contains information transcribed from DNA • Ribosomal RNA (rRNA) • Main component of ribosomes, where polypeptide chains are built • Transfer RNA (tRNA) • Delivers amino acids to ribosomes Converting mRNA to Protein • Translation • The information carried by mRNA is decoded into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein • mRNA is translated to protein • rRNA and tRNA translate the sequence of base triplets in mRNA into a sequence of amino acids Gene Expression • A cell’s DNA sequence (genes) contains all the information needed to make the molecules of life • Gene expression • A multistep process including transcription and translation, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body Take-Home Message: What is the nature of genetic information carried by DNA? • Genetic information occurs in DNA sequences (genes) that encode instructions for building RNA or protein products • A cell transcribes the nucleotide sequence of a gene into RNA • Although RNA is structurally similar to a single strand of DNA, the two types of molecules differ functionally • A messenger RNA (mRNA) carries a protein-building code in its nucleotide sequence; rRNAs and tRNAs interact to translate the sequence into a protein 9.3 Transcription: DNA to RNA • RNA polymerase assembles RNA by linking RNA nucleotides into a chain, in the order dictated by the base sequence of a gene • A new RNA strand is complementary in sequence to the DNA strand from which it was transcribed DNA Replication and Transcription • DNA replication and transcription both synthesize new molecules by base-pairing • In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotides • Uracil (U) nucleotides pair with A nucleotides • RNA polymerase adds nucleotides to the transcript The Process of Transcription • RNA polymerase and regulatory proteins attach to a promoter (a specific binding site in DNA close to the start of a gene) • RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins free RNA nucleotides into a complementary strand of mRNA RNA polymerase gene region RNA promoter sequence in DNA 1 RNA polymerase binds to a promoter in the DNA. The binding positions the polymerase near a gene. In most cases, the base sequence of the gene occurs on only one of the two DNA strands. Only the DNA strand complementary to the gene sequence will be translated into RNA. DNA winding up DNA unwinding 2 The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA. The DNA winds up again after the polymerase passes. The structure of the “opened” DNA at the transcription site is called a transcription bubble, after its appearance. direction of transcription 3 Zooming in on the gene region, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template strand, so it is an RNA copy of the gene. Stepped Art Figure 9-4 p152 RNA transcripts DNA molecule Figure 9-5 p153 Post-Transcriptional Modifications • In eukaryotes, RNA is modified before it leaves the nucleus as a mature mRNA • Introns • Nucleotide sequences that are removed from a new RNA • Exons • Sequences that stay in the RNA Alternative Splicing • Alternative splicing • Allows one gene to encode different proteins • Some exons are removed from RNA and others are spliced together in various combinations • After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly-A tail at the 3' end gene promoter exon intron exon intron exon DNA transcription exon intron exon intron exon new transcript RNA processing exon finished RNA 5′ exon exon 3′ cap poly-A tail Figure 9-6 p153 ANIMATED FIGURE: Pre-mRNA transcript processing To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Take-Home Message: How is RNA assembled? • In transcription, RNA polymerase uses the nucleotide sequence of a gene region in a chromosome as a template to assemble a strand of RNA • The new strand of RNA is a copy of the gene from which it was transcribed 9.4 RNA and the Genetic Code • Base triplets in an mRNA encode a protein-building message • Ribosomal RNA (rRNA) and transfer RNA (tRNA) translate that message into a polypeptide chain mRNA – The Messenger • mRNA carries protein-building information to ribosomes and tRNA for translation • Codon • A sequence of three mRNA nucleotides that codes for a specific amino acid • The order of codons in mRNA determines the order of amino acids in a polypeptide chain Genetic Code • Genetic code • Consists of 64 mRNA codons (triplets) • Twenty kinds of amino acids are found in proteins • Some amino acids can be coded by more than one codon • Some codons signal the start or end of a gene • AUG (methionine) is a start codon • UAA, UAG, and UGA are stop codons Figure 9-7a p154 Figure 9-7b p154 From DNA to RNA to Amino Acids a gene region in DNA transcription codon codon codon mRNA translation methionine (met) tyrosine (tyr) serine (ser) amino acid sequence rRNA and tRNA – The Translators • tRNAs deliver amino acids to ribosomes • tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon • Ribosomes, which link amino acids into polypeptide chains, consist of two subunits of rRNA and proteins tRNA Structure anticodon anticodon amino acid attachment site Translation: Ribosome and tRNA Take-Home Message: What roles do mRNA, tRNA, and rRNA play during translation? • mRNA carries protein-building information; the bases in mRNA are “read” in sets of three during protein synthesis; most base triplets (codons) code for amino acids; the genetic code consists of all sixty-four codons • Ribosomes, which consist of two subunits of rRNA and proteins, assemble amino acids into polypeptide chains • A tRNA has an anticodon complementary to an mRNA codon, and it has a binding site for the amino acid specified by that codon; transfer RNAs deliver amino acids to ribosomes ANIMATED FIGURE: Structure of a ribosome To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE