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.

Protein Trp TRANSLATION Amino acid Phe GlySer Figure 14.5 DNA template strand C AC A AAC C AGGT G TG TT T G G TCCA 3 5 mRNA TRANSCRIPTION G U G U UUG G UCC A 3 5 Codon

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 U C A G U C A G U C A G U C A G U C A G Phe Leu Ser Tyr Cys Trp Met or start Stop Arg Gln His Pro Leu Val Ala Asp Glu Gly I Ie Thr Lys Asn Arg Ser Third mRNA base (3 end of codon)

The genetic code is nearly universal!

Completed RNA transcript Direction of transcription (“downstream”) Transcription unit RNA polymerase Promoter Start point 3 Three stages of Transcription Rewound DNA RNA transcript Termination Initiation Elongation Unwound DNA RNA transcript Template strand of DNA

Figure 14.9 TATA box Promoter Nontemplate strand Start point DNA A eukaryotic promoter Template strand 3 5 TATAAAA ATATTTT Transcription factors Several transcription factors bind to DNA Transcription initiation complex forms. Transcription initiation complex RNA transcript Transcription factors RNA polymerase II

Figure Nontemplate strand of DNA Direction of transcription RNA polymerase RNA nucleotides Template strand of DNA Newly made RNA 3 end 5 U C U G A A A A A A A A A A T T T T T T T C C C CC C C G G G U

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. Polyadenylation signal 3 5 AAUAAA

Figure 14.UN03 DNA Pre-mRNA mRNA Ribosome Polypeptide TRANSLATION TRANSCRIPTION RNA PROCESSING

RNA Processing in Eukaryotes 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 Protein-coding segment Polyadenylation signal GP A modified guanine nucleotide added to the 5 end 50–250 adenine nucleotides added to the 3 end Cap 5 UTR 3 UTR Poly-A tail Start codon Stop codon PP AAUAAA … AAA

Figure AAUAAA 5 Cap Poly-A tail Pre-mRNA Intron Exons Introns cut out and exons spliced together 5 Cap 5 UTR 3 UTR Poly-A tail Coding segment mRNA

Figure Spliceosome components 5 Pre-mRNA 5 mRNA Intron Spliceosome Exon 1 Small RNAs Exon 2 Exon 1 Cut-out intron

Concept check How can human cells make 75, ,000 different proteins, given that there are about 20,000 human genes?