Ribozymes. Ribozymes. cataly&c RNA molecules. func&on as enzymes and can splice RNA. Their discovery rendered obsolete

Transcription

1 Fig ʹ RNA transcript (pre-mrna) Exon 1 Intron Exon 2 Protein snrna snrnps Other proteins Spliceosome 5ʹ Fig ʹ RNA transcript (pre-mrna) Exon 1 Intron Exon 2 Protein snrna snrnps Other proteins Spliceosome 5ʹ Spliceosome components 5ʹ mrna Exon 1 Exon 2 Cut-out intron Ribozymes Ribozymes cataly&c RNA molecules func&on as enzymes and can splice RNA Their discovery rendered obsolete the belief that all biological catalysts were proteins 1

2 Ribozymes Three proper&es of RNA enable it to func&on as an enzyme It can form a three- dimensional structure because of its ability to base pair with itself Some bases in RNA contain func&onal groups Amine, carbonyl RNA may hydrogen- bond with other nucleic acid molecules Alterna9ve RNA splicing Alterna9ve Splicing Some genes can encode more than one kind of polypep&de depending on which segments are treated as exons during RNA splicing Therefore the number of polypep&des an organism can produce is greater than its number of genes Humans: ~25,000 genes ~150,000 proteins Alterna9ve Splicing Domains Regions of proteins with discrete modular architecture Different exons code for the different domains in a protein Exon shuffling may result in the evolu&on of new proteins 2

3 Fig DNA Gene Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide Transla9on Process of reading mrna to guide construc&on of specific polypep&de Occurs in cytoplasm At a ribosome Using transfer RNA (trna) Delivers amino acids to ribosome Transfer RNA Transfer RNA (trna) serves as a bridging molecule in protein synthesis ability to bind with both a specific amino acid and mrna Delivers specific amino acid to a ribosome in which an mrna transcript is being read. 3

4 trna Structure/Charging RNA molecule ~80nt long folded back on itself ~45 species per cell all have CCA at 3 end recogni&on for Addi&on of amino acid to trna catalyzed by aminoacyl- trna synthetase 20 different types one for each amino acid trna Structure An&codon Opposite end of the trna molecule from the amino acid binds with the complimentary codon in the mrna chain Therefore iden&cal to the original DNA template! Ribosome workbenches of protein synthesis composed of proteins and ribosomal RNA (rrna, ~60%) Enzyma&c RNA (performs catalysis) Protein only for structure Each ribosome exists as two separate subunits in the cytoplasm come together only with the ini&a&on of protein transla&on. 4

5 Small subunit Ribosome Structure mrna binding Large subunit Three trna binding sites Aminoacyl (A site) trna entry site Pep&dyl (P site) Polypep&de transfer site Exit (E site) Three Stages of Transla9on Chain Ini&a&on Brings together mrna, small subunit, first trna, and finally, large subunit Chain Elonga&on Addi&on of amino acids Chain Termina&on Signals end of polypep&de and stops transla&on Transla9on Ini9a9on mrna binds with small ribosomal subunit At recogni&on sequence on 5 end Upstream from start (ini&a&on) codon First trna binds to mrna start codon AUG trna an&codon UAC Codes for methionine 5

6 Transla9on Ini9a9on (cont.) Complex of transla&on ini&a&on factors (for deeper knowledge only) Transla9on Ini9a9on Large subunit binds GTP powered Met trna now in Pep&dyl site Aminoacyl site ready for next trna and elonga&on Transla9on Elonga9on Succession of trna molecules arriving at a ribosome Dictated by an&codon- codon complementa&on Fascilitates delivery of specific amino acids 6

7 Transla9on Elonga9on (cont.) Step 1 - Codon Recogni&on Elonga&on Factor guides trna into A site Requires hydrolysis of more GTP Hydrogen bonds form base pairs between codon- an&codon Transla9on Elonga9on (cont.) Step 2 - Pep&de bond forma&on Between first Met amino acid in P site and next amino acid in A site Catalyzed by pep&dyl transferase Part of large subunit Met detachment From Met trna Growing polypep&de now aeached to trna in A site Transla9on Elonga9on (cont.) Step 3 - Transloca&on mrna ratcheted one codon from 5-3 Uses GTP Moves (empty) Met trna to E site Second trna now in P site Next codon now in A site ready for next trna Empty Met trna exits Back to step 1 codon recogni&on In A site 7

8 Transla9on Termina9on Step 1 - Termina&on (stop) mrna codon enters A site UAA, UAG, UGA Release Factor binds stop codon Step 2 - Polypep&de release Transla9on Termina9on Pep&dyl transferase adds water molecule instead of amino acid at stop codon Caused by release factor H 2 0 hydrolyzes polypep&de from trna in P site Transla9on Termina9on Step 3 - Ribosome Dissocia&on Small and large subunits separate Polypep&de now ready to fold May have begun already 8

9 Transla9on Translation Polyribosomes Mul&ple ribosomes translate single mrna Increases rate of protein produc&on for the cell Also called polysomes Postransla9onal Modifica9ons Protein folding Primary structure occurs as polypep&de formed Secondary, Ter&ary subsequent May be facilitated by chaperonin Further modifica&ons Sugars, Lipids, Phosphates, etc Targe&ng to loca&on Signal pep9de First ~20 amino acids Triggers delivery to ER Detected by signal recogni&on par&cle (SRP) 9

10 Polypep9de Targe9ng You should now be able to: Describe the contribu&ons made by Beadle, and Tatum to our understanding of the rela&onship between genes and enzymes Briefly explain how informa&on flows from gene to protein Compare transcrip&on and transla&on in bacteria and eukaryotes Explain what it means to say that the gene&c code is redundant and unambiguous Include the following terms in a descrip&on of transcrip&on: mrna, RNA polymerase, the promoter, the terminator, the transcrip&on unit, ini&a&on, elonga&on, termina&on, and introns Include the following terms in a descrip&on of transla&on: trna, wobble, ribosomes, ini&a&on, elonga&on, and termina&on 10