Chapter 14 Genetic Code and Transcription You Must Know • The differences between replication (from chapter 13), transcription and translation and the role of DNA and RNA in each process. (There will be more on this later.) • How eukaryotic cells modify RNA after transcription. Figure 14.5 DNA template strand 3 5 A C C A A A C C G A G T T G G T T T G G C T C A 5 3 TRANSCRIPTION U G mRNA G U U U G G C U C 3 5 Codon TRANSLATION Protein A Trp Amino acid Phe Gly Ser Second mRNA base A C U UUU U UUC First mRNA base (5 end of codon) UUA C Leu UAU UCC UCA UAC Ser UGU Tyr UGC U Cys C UAA Stop UGA Stop A UCG UAG Stop UGG Trp G CUU CCU CAU U CUC Leu CCC CCA Pro CAC CAA CUG CCG CAG AUU ACU AAU AUC IIe ACC AUA ACA AUG Met or start GUU G UCU UUG CUA A Phe G GUC GUA GUG AAC Thr ACG AAG GCU GAU GCC Val AAA GCA GCG GAC Ala GAA GAG CGU His Gln CGC CGA C Arg CGG AGU Asn Lys Asp Glu AGC AGA A G Ser Arg U C A AGG G GGU U C GGC GGA GGG Gly A G Third mRNA base (3 end of codon) Figure 14.6 The genetic code is nearly universal! Three stages of Transcription Transcription unit Promoter 5 3 1 Initiation 3 5 Start point RNA polymerase 5 3 3 5 Template strand of DNA RNA transcript Unwound DNA 2 Elongation Rewound DNA 5 3 3 5 3 5 RNA transcript 3 Termination Direction of transcription (“downstream”) 5 3 3 5 5 Completed RNA transcript 3 Figure 14.9 Promoter Nontemplate strand DNA 5 3 3 5 TAT A A A A ATAT T T T TATA box Start point Transcription factors 3 5 Transcription factors RNA polymerase II 5 3 promoter Template strand 5 3 5 3 2 Several transcription factors bind to DNA. 3 Transcription 3 5 RNA transcript Transcription initiation complex 1 A eukaryotic initiation complex forms. Figure 14.10 Nontemplate strand of DNA RNA nucleotides RNA polymerase A T 3 C C A A 5 3 end A U C C A 5 T A G G T T 5 Direction of transcription 3 Template strand of DNA Newly made RNA Termination of Transcription • The mechanisms of termination are different in bacteria and eukaryotes. • In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification. Termination of Transcription • In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence. 5 Polyadenylation signal 3 AAUAAA Figure 14.UN03 TRANSCRIPTION RNA PROCESSING DNA Pre-mRNA mRNA TRANSLATION Ribosome Polypeptide RNA Processing in Eukaryotes A modified guanine nucleotide added to the 5 end 5 G P P Protein-coding segment P 5 Cap 5 UTR 50–250 adenine nucleotides added to the 3 end Polyadenylation signal 3 AAUAAA Start codon Stop codon 3 UTR AAA …AAA Poly-A tail These modifications share several functions • Facilitating the export of mRNA to the cytoplasm • Protecting mRNA from hydrolytic enzymes • Helping ribosomes attach to the 5 end Figure 14.12 Exons Pre-mRNA Intron Intron Poly-A tail 5 Cap Introns cut out and exons spliced together mRNA 5 Cap 5 UTR Poly-A tail Coding segment 3 UTR AAUAAA Figure 14.13 Small RNAs Spliceosome 5 Pre-mRNA Exon 2 Exon 1 Intron Spliceosome components mRNA 5 Exon 1 Exon 2 Cut-out intron Concept check • How can human cells make 75,000-100,000 different proteins, given that there are about 20,000 human genes?