Transcription Translation

transcription and translation Gene expression -process by which DNA directs protein synthesis two stages: transcription and translation

Figure 14.UN01 Central Dogma DNA RNA Protein Gene expression

Differences Between DNA and RNA

Prokaryotes Figure 14.4a-2 translation of mRNA can begin before transcription has finished DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell

Figure 14.4b-3 Eukaryotes Nuclear envelope DNA TRANSCRIPTION -the nuclear envelope separates transcription from translation; mRNA must be transported out of the nucleus to be translated Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA Pre-mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide Overview: http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/ (b) Eukaryotic cell

DNA template strand Protein mRNA 3 Trp TRANSCRIPTION TRANSLATION Figure 14.5 DNA template strand Protein mRNA 3 Trp TRANSCRIPTION TRANSLATION Amino acid Codon 5 Phe Gly Ser G U C A T

Genetic Code 64 codons; 20 amino acids Figure 14.6 UUU Second mRNA base UUC UUA UUG UCU UCC UCA UCG UAU UAC UAA UAG UGU UGC UGA UGG CUU CUC CUA CUG CCU CCC CCA CCG CAU CAC CAA CAG CGU CGC CGA CGG AUU AUC AUA AUG ACU ACC ACA ACG AAU AAC AAA AAG AGU AGC AGA AGG GUU GUC GUA GUG GCU GCC GCA GCG GAU GAC GAA GAG GGU GGC GGA GGG First mRNA base (5 end of codon) U C A G Phe Leu Ser Tyr Cys Trp Met or start Stop Arg Gln His Pro Val Ala Asp Glu Gly IIe Thr Lys Asn Third mRNA base (3 end of codon) Genetic Code 64 codons; 20 amino acids The genetic code is redundant: more than one codon may specify a particular amino acid

Universal Genetic Code Figure 14.7 (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene

Transcription Nontemplate strand of DNA RNA nucleotides RNA polymerase Figure 14.10 Nontemplate strand of DNA Direction of transcription RNA polymerase 3 5 RNA nucleotides Template strand of DNA Newly made 3 end U C G A T RNA polymerase assembles 5’ to 3’ -can start a chain without a primer

Prokaryotes Transcription unit RNA polymerase Promoter Figure 14.8-3 Transcription unit RNA polymerase Promoter Template strand of DNA Start point Termination Completed RNA transcript RNA transcript Unwound DNA Rewound Direction of transcription (“downstream”) Initiation Elongation 3 5 3 2 1 Prokaryotes http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

Eukaryotes Transcription factors mediate the binding of RNA polymerase and the initiation of transcription Figure 14.9 Transcription factors TATA box Promoter Nontemplate strand Start point initiation complex forms. Transcription initiation complex DNA RNA transcript A eukaryotic promoter Several transcription factors bind to DNA. 3 5 3 2 1 Template strand RNA polymerase II T A

Eukaryotic cells modify RNA after transcription Figure 14.UN03 DNA Pre-mRNA mRNA Ribosome Polypeptide TRANSLATION TRANSCRIPTION RNA PROCESSING

RNA Processing Modifications: Figure 14.11 Protein-coding segment Polyadenylation signal G P A modified guanine nucleotide added to the 5 end 50–250 adenine nucleotides added to the 3 end 3 5 5 Cap 5 UTR 3 UTR Poly-A tail Start codon Stop AAUAAA … AAA RNA Processing Modifications: The 5 end receives a modified G nucleotide 5 cap The 3 end gets a poly-A tail Functions: Facilitating the export of mRNA to the cytoplasm Protecting mRNA from hydrolytic enzymes Helping ribosomes attach to the 5 end

Alternative RNA Splicing Figure 14.12 Introns cut out and exons spliced together 31–104 5 Cap 5 UTR 3 UTR Poly-A tail Coding segment 1–146 AAUAAA 105– 146 1–30 mRNA Pre-mRNA Intron Alternative RNA Splicing RNA splicing removes introns (noncoding) and joins exons (translates to amino acids), creating an mRNA molecule with a continuous coding sequence

Figure 14.14 5 tRNA Polypeptide Ribosome Anticodon mRNA Codons 3 tRNA with amino acid attached Amino acids Gly Trp Phe A C G U Translation A cell translates 
an mRNA message 
into protein with the help of 
transfer RNA (tRNA)

tRNA 5 Anticodon 3 Amino acid attachment site Hydrogen bonds Figure 14.15 5 Anticodon 3 Amino acid attachment site A G Hydrogen bonds attachment site C U * (b) Three-dimensional structure (c) Symbol used in this book (a) Two-dimensional structure

Ribosome Structure P E A tRNA molecules Large subunit Small Figure 14.17 P E A tRNA molecules Large subunit Small Growing polypeptide Exit tunnel mRNA 5 3 (a) Computer model of functioning ribosome (c) Schematic model with mRNA and tRNA Codons Amino end Next amino acid to be added to polypeptide chain A site (Aminoacyl- tRNA binding site) P site (Peptidyl-tRNA binding site) E site (Exit site) binding site (b) Schematic model showing binding sites Ribosome Structure

initiation 3 stages: Initiation Elongation Termination Figure 14.18 A Small ribosomal subunit binds to mRNA. GTP P site U mRNA 5 3 Met P i mRNA binding site Start codon Small ribosomal subunit Initiator tRNA Large ribosomal subunit completes the initiation complex. G C Translation initiation complex Large E GDP  1 2 3 stages: Initiation Elongation Termination http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/

Elongation Amino end of polypeptide Codon recognition Figure 14.19-3 Amino end of polypeptide mRNA Ribosome ready for next aminoacyl tRNA P site i 5 3 E GTP  A GDP Peptide bond formation Codon recognition Translocation 3 2 1

Termination Free polypeptide Ribosome reaches a stop codon on mRNA. Figure 14.20-3 Free polypeptide Ribosome reaches a stop codon on mRNA. 5 3 2 GTP  2 GDP P i Release factor Stop codon (UAG, UAA, or UGA) Ribosomal subunits and other components dissociate. Release factor promotes hydrolysis. 3 1

Figure 14.22 Incoming ribosomal subunits (b) A large polyribosome in a bacterial cell (TEM) Ribosomes mRNA (a) Several ribosomes simultaneously translating one mRNA molecule 0.1 m Start of (5 end) End of mRNA (3 end) Growing polypeptides Completed polypeptide Polyribosome polyribosomes A number of 
ribosomes can 
translate a 
single mRNA 
molecule 
simultaneously

Review http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/ Figure 14.24 A U tRNA 5 Cap 3 mRNA Ribosomal subunits Aminoacyl (charged) P E G 5 Ribosome Codon Anticodon TRANSLATION Amino acid Aminoacyl-tRNA synthetase CYTOPLASM NUCLEUS AMINO ACID ACTIVATION Intron Poly-A TRANSCRIPTION RNA PROCESSING transcript RNA transcript (pre-mRNA) polymerase Exon DNA C Review http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/