Part 7: Gene Structure and Function

1. Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

2. The E. coli Chromosome

3. Genes and Gene Expression • The genome is composed of one or more chromosomes • Chromosomes contain genes • Genes encode RNA, and most RNAs encode proteins • Gene expression refers to the formation of a product, usually a protein, encoded by a gene

4. The Central Dogma of Molecular Biology

5. The Structure of DNA

6. X Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

7. The Important Role of RNA in Gene Expression • Ribosomal RNA (rRNA) – plays a structural and catalytic role in the ribosome, where all proteins are synthesised • Transfer RNA (tRNA) – transfers amino acids to polypeptide chain in a ribosome • Messenger RNA (mRNA) – contains specific information transcribed from a gene which is then translated into protein

8. The Ribosome Consists of Two Subunits

9. The Complex Structure of a Ribosomal Subunit Grey = RNA Gold = protein

10. X Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

11. Transcription: a Summary • RNA polymerase binds to the promoter of a gene • RNA polymerase moves along one strand of the DNA within the gene (= the template strand) • RNA polymerase disengages from the DNA when it reaches a transcriptional stop signal • The RNA transcript is released

12. Transcription in Progress

13. The Genetic Code • The DNA is transcribed into mRNA • The mRNA is read, during translation, as a series of triplets or codons • Each codon (of three bases) codes for a specific amino acid • Some amino acids are specified by more than one codon

14. Some of the Genetic Code in More Detail

15. The Reading Frame Example of a reading frame: UUU UUG CCC ACA AAG AGG GUC GCU Phe Leu Pro Thr Lys Arg Val Ala C at position 11 is lost, reading frame is changed: UUU UUG CCC AAA AGA GGG UCG CU? Phe Leu Pro Lys Arg Gly Ser Leu

16. X Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

17. Transcription in Eukaryotes • Three RNA polymerases - RNA polymerase II transcribes protein-coding genes • The core promoter of the gene often contains a TATA box, required for building a transcription initiation complex • Other short DNA sequences, lying upstream of the promoter, bind regulatory proteins to control gene expression

18. Initiation of Transcription Transcription factor binds to TATA box Other transcription factors are recruited to build an initiation complex RNA polymerase binds to complex and transcription begins

19. Modifications to RNA • 5’ cap • chemical modification of the first nucleotide; protects 5’ end of final mRNA and also assists in initiation of translation • 3’ poly-A tail • the RNA is trimmed back to a specific sequence, then a poly-A tail is added; protects 3’ end of final mRNA from degradation

20. Modifications to RNA The primary transcript is modified post-transcriptionally in eukaryotic cells by capping and tailing

21. Many Eukaryotic Genes are Split • The protein-coding DNA of many genes is interrupted by non-coding sequences • The non-coding sequences appear in the primary transcript but not in mRNA • The non-coding sequences (introns) separate the coding DNA (exons) • In RNA splicing, the introns are removed and the exons are joined (spliced) together

22. Introns and Exons

23. Why do Introns Exist? • Exon shuffling • exons may encode functional regions (domains) of proteins, so introns provide scope for exon shuffling • Alternative splicing • introns and exons may be interpreted differently, giving rise to different exons appearing in the final mRNA • No reason at all

24. X Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

25. Translation: a Summary • Activating enzymes called aminoacyl-tRNA synthetases add amino acids to specific tRNAs • Activated tRNAs bind to codons of mRNA during translation at the ribosome • The anticodon of the tRNA binds to the codon of the mRNA • The amino acid is transferred from the tRNA to the peptide chain

26. The Structure of tRNA

27. Activating Enzyme Binds Amino Acid and tRNA

28. Amino Acid is Linked to tRNA

29. tRNA Binds to Codon in mRNA

30. X Part 7: Gene Structure and Function • Genes and gene expression • The important role of RNA in gene expression • Transcription and the genetic code • Transcription in eukaryotes • Translation • Eukaryotic gene expression: a review

31. Eukaryotic Gene Expression (1)

32. Eukaryotic Gene Expression (2)

33. Eukaryotic Gene Expression (3)

34. Eukaryotic Gene Expression (4)

35. Eukaryotic Gene Expression (5)

36. Eukaryotic Gene Expression (6)