Genes and How They Work

1. Genes and How They Work Genes and How They Work Chapter 15

2. Outline • Cells Use RNA to Make Protein • Gene Expression • Genetic Code • Transcription • Translation • Spliced Genes • Introns and Exons • Prokaryotic and Eukaryotic Gene Expression

3. Cells Use RNA to Make Protein • Three types of RNA molecules • During polypeptide synthesis, ribosomal RNA (rRNA) is the site of polypeptide assembly. • Transfer RNA (tRNA) transports and positions amino acids. • Messenger RNA (mRNA) directs which amino acids are assembled into polypeptides. • Central Dogma • DNA  RNA Protein

4. Central Dogma of Gene Expression

5. Gene Expression • Transcription • DNA sequence is transcribed into RNA sequence • initiated when RNA polymerase binds to promoter binding site • moves along DNA strand and adds corresponding complementary RNA nucleotide • disengages at stop signal

6. Gene Expression • Translation • nucleotide sequence of mRNA transcript is translated into amino acid sequence in the polypeptide • rRNA recognizes and binds to start sequence • moves three nucleotides at a time • disengages at stop signal • Gene expression - collective of transcription and translation

7. Genetic Code • Genetic code consists of a series of information blocks called codons. • reading frame (triplet) • each codes for one amino acid • genetic code is nearly universal • mitochondria • chloroplasts

9. Transcription • RNA polymerase • only one of two DNA strands (template or antisense strand) is transcribed • non-transcribed strand is termed coding strand or sense strand • In both bacteria and eukaryotes, the polymerase adds ribonucleotides to the growing 3’ end of an RNA chain. • synthesis proceeds in 5’3’ direction

10. Transcription Bubble

11. Transcription • Promoter • Transcription starts at RNA polymerase binding sites called promoters on DNA template strand. • Initiation • Other eukaryotic factors bind, assembling a transcription complex. • RNA polymerase begins to unwind DNA helix.

12. Transcription • Elongation • Transcription bubble moves down DNA at constant rate leaving growing RNA strands protruding from the bubble. • Termination • Stop sequences at the end of the gene cause phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA.

13. Transcription • Eukaryotic transcription differs from prokaryotic transcription: • three RNA polymerase enzymes • initiation complex forms at promoter • RNAs are modified after transcription

14. Translation • Begins when initial portion of mRNA molecule binds to rRNA in a ribosome • tRNA molecule with complimentary anticodon binds to exposed codon on mRNA • some tRNA molecules recognize more than one codon

15. Translation • Activating enzymes • tRNA molecules attach to specific amino acids through the action of activating enzymes (aminoacyl-tRNA syntheases). • must correspond to specific anticodon sequences on a tRNA molecule as well as particular amino acids

16. Translation • Start and stop signals • start signal coded by AUG codon • stop signal coded by one of three nonsense codons: UAA - UAG - UGA • Initiation • Polypeptide synthesis begins with the formation of an initiation complex. • initiation factors

17. Formation of Initiation Factor

18. Translation • Elongation • After initiation complex forms, large ribosome subunit binds, exposing mRNA codon adjacent to the initiating codon, positioning it for interaction with another amino acid-bearing tRNA molecule.

19. Translation • Translocation • ribosome moves nucleotides along mRNA molecule

20. Translation • Termination • Nonsense codons are recognized by release factors that release the newly made polypeptide from the ribosome.

21. Spliced Gene Transcripts • DNA sequence specifying a protein is broken into segments (exons) scattered among longer noncoding segments (introns). • Initially, primary RNA transcript is produced for the entire gene. • Small nuclear ribonuclearproteins (snRNPs) associate with proteins to form spliceosomes. • Lariat forms, excising introns and splicing exons to form mature mRNA. • alternative splicing

22. RNA Splicing • During RNA processing, intron sequences are cut out of primary transcript before it is used in polypeptide synthesis. • remaining sequences are not translated • remaining exon sequences are spliced together to form final processed mRNA

23. Eukaryotic Genes are Fragmented

24. Differences Between Prokaryotic and Eukaryotic Gene Expression • Most eukaryotic genes possess introns. • Individual bacterial mRNA molecules often contain transcripts of several genes. • Eukaryotic mRNA molecules must be completely formed and must pass across the nuclear membrane before translation. • In prokaryotes, translation begins at the AUG codon preceded by a special nucleotide sequence.

25. Differences Between Prokaryotic and Eukaryotic Gene Expression • Eukaryotic mRNA molecules have introns cut out and exons joined together before translation. • Eukaryotic ribosomes are larger than prokaryotic ribosomes.

26. Summary • Cells Use RNA to Make Protein • Gene Expression • Genetic Code • Transcription • Translation • Spliced Genes • Introns and Exons • Prokaryotic and Eukaryotic Gene Expression