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From Gene to Protein A.P. Biology Typical Gene Structure Regulatory sites Promoter (RNA polymerase binding site) Start transcription DNA strand Stop transcription RNA •Contains ribose instead of deoxyribose •Single stranded •Nucleotide uracil replaces thymine 3 Types of RNA • Messenger RNA = mRNA -Carries DNA sequence to the ribosome • Ribosomal RNA = rRNA -Component of ribosome • Transfer RNA = tRNA -Carries amino acid (anticodon) to the codon on mRNA Basic Principles of Transcription and Translation • Transcription – Is the synthesis of RNA under the direction of DNA – Produces messenger RNA (mRNA) • Translation – Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA – Occurs on ribosomes •In prokaryotes –Transcription and translation occur together TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide (a) Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing. •In eukaryotes –RNA transcripts are modified before becoming true mRNA Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Figure 17.3b (b) Eukaryotic cell. The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. •During transcription –The gene determines the sequence of bases along the length of an mRNA molecule Gene 2 DNA molecule Gene 1 Gene 3 DNA strand 3 5 A C C A A A C C G A G T (template) TRANSCRIPTION mRNA 5 U G G U U U G G C U C A Codon TRANSLATION Protein Figure 17.4 Trp Amino acid Phe Gly Ser 3 •In laboratory experiments –Genes can be transcribed and translated after being transplanted from one species to another Molecular Components of Transcription • RNA synthesis – Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides – Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine Synthesis of an RNA Transcript • The stages of transcription are – Initiation – Elongation – Termination Promoter Transcription unit 5 3 3 5 Start point RNA polymerase DNA Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand. 1 5 3 Unwound DNA 3 5 Template strand of DNA transcript 2 Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix. Rewound RNA RNA 5 3 3 5 3 5 RNA transcript 3 Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA. 5 3 3 5 5 Figure 17.7 Completed RNA transcript 3 Non-template strand of DNA Elongation RNA nucleotides RNA polymerase A T C C A A 3 3 end U 5 A E G C A T A G G T T Direction of transcription (“downstream”) 5 Newly made RNA Template strand of DNA RNA Polymerase Binding and Initiation of Transcription •Promoters signal the initiation of RNA synthesis 1 Eukaryotic promoters TRANSCRIPTION DNA RNA PROCESSING Pre-mRNA mRNA Ribosome TRANSLATION Polypeptide Promoter 5 3 3 5 T A T A A AA AT AT T T T TATA box •Transcription factors Start point Several transcription factors 2 –Help eukaryotic RNA polymerase recognize promoter sequences Template DNA strand Transcription factors 5 3 3 5 3 Additional transcription factors RNA polymerase II Transcription factors –TATA box 5 3 3 5 5 RNA transcript Figure 17.8 Transcription initiation complex Transcription – Making a Copy of the DNA 1. RNA polymerase separates the DNA strands at a promoter region on the DNA (TATA box) 2. RNA polymerase adds nucleotides in sequence to mRNA 3. RNA polymerase falls off the DNA at a terminator sequence on the DNA Unwinding of the DNA Helix by RNA Polymerase Step #1 Transcription of mRNA Complementary to DNA Step #2 Alteration of mRNA Ends •Each end of a pre-mRNA molecule is modified –The 5 end receives a modified nucleotide cap –The 3 end gets a poly-A tail A modified guanine nucleotide added to the 5 end TRANSCRIPTION RNA PROCESSING 50 to 250 adenine nucleotides added to the 3 end DNA Pre-mRNA 5 mRNA G Protein-coding segment Polyadenylation signal 3 P P P AAUAAA AAA…AAA Ribosome TRANSLATION 5 Cap Polypeptide 5 UTR Start codon Stop codon 3 UTR Poly-A tail •RNA splicing –Removes introns and joins exons •Ribozymes –Are catalytic RNA molecules that function as enzymes and can splice RNA TRANSCRIPTION RNA PROCESSING DNA Pre-mRNA 5 Exon Intron Pre-mRNA 5 Cap 30 31 1 Coding segment mRNA Ribosome Intron Exon Exon 3 Poly-A tail 104 105 146 Introns cut out and exons spliced together TRANSLATION Polypeptide mRNA 5 Cap 1 3 UTR Poly-A tail 146 Introns - Intervening sequence = Junk DNA Exons – Expressed sequence 3 UTR mRNA Processing in Eukaryotic Cells Molecular Components of Translation •A cell translates an mRNA message into protein –With the help of transfer RNA (tRNA) TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide •Molecules of tRNA are not all identical Amino acids Polypeptide –Each carries a specific amino acid on one end Ribosome tRNA with amino acid attached Gly –Each has an anticodon on the other end tRNA Anticodon A A A U G G U U U G G C Codons 5 mRNA 3 •A tRNA molecule –Consists of a single RNA strand that is only about 80 nucleotides long –Is roughly L-shaped 3 A C C A 5 C G G C C G U G U A A U A U U C UA C A C AG * G * G U G U * C C * * U C * * G AG C (a) Two-dimensional structure. The four base-paired regions and three G C U A loops are characteristic of all tRNAs, as is the base sequence of the * G amino acid attachment site at the 3 end. The anticodon triplet is A A* unique to each tRNA type. (The asterisks mark bases that have been C U * chemically modified, a characteristic of tRNA.) A G A Amino acid attachment site Figure 17.14a Anticodon C U C G A G A G * * G A G G Hydrogen bonds 5 3 Amino acid attachment site Hydrogen bonds A AG 3 Anticodon (b) Three-dimensional structure 5 Anticodon (c) Symbol used in this book •A specific enzyme called an aminoacyl-tRNA synthetase –Joins each amino acid to the correct tRNA •The ribosome –Is part of the cellular machinery for translation, polypeptide synthesis –Facilitates the specific coupling of tRNA anticodons with mRNA codons during protein synthesis •The ribosome has three binding sites for tRNA –The P site P site (Peptidyl-tRNA binding site) –The A site –The E site E site (Exit site) mRNA binding site A site (Aminoacyl-tRNA binding site) Central Dogma in Biology Gene Mutations: Substitution, Insertion, & Deletion Substitution Insertion Deletion Point Mutation Frameshift Mutation Base Pair Substitution • Missense mutation – altered codon codes for a different amino acid • Nonsense mutation – altered codon changed to a stop codon (no amino acid delivered) Chromosomal Mutations Deletion Duplication Inversion Translocation