1. 10-1 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 10: The genetic code

2. 10-2 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Genes A single gene provides the genetic instructions for one polypeptide There is a specific relationship between the DNA sequence of the gene and the amino acid sequence of the protein The process of converting DNA information into protein molecules is gene expression The information is deciphered using the genetic code

3. 10-3 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint RNA RNA has the same primary structure as DNA. It consists of a sugar-phosphate backbone, with nucleotides attached to the 1' carbon of the sugar The differences between DNA and RNA are that –RNA has a hydroxyl group on the 2' carbon of the sugar (the difference between deoxyribonucleic acid and ribonucleic acid) –RNA uses the pyrimidine base uracil (U) to pair with adenine (A) –RNA exists as a single-stranded molecule

4. 10-4 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Transcription DNA acts as a template for the synthesis of RNA in a process called transcription Only one strand of DNA is used as the template Like DNA replication, transcription proceeds 5’  3’ on the strand being produced Nucleotides are added according to the complementary base pairing rules

5. 10-5 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.2a: Transcription of DNA by RNA polymerase

6. 10-6 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.2b: The structure of RNA polymerase, shown in the act of transcribing DNA to produce RNA

7. 10-7 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Initiation RNA polymerases do not require priming The polymerase binds just downstream of the promoter Elongation of the RNA continues by addition of complementary nucleotides until a termination signal is reached The transcribed region is called a transcription unit with the RNA being a primary transcript

8. 10-8 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.4: Formation of the first phosphodiester bond to initiate transcription

9. 10-9 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Transcription Transcription generates three major RNAs –messenger RNA (mRNA) determines the amino acid sequence of the protein during translation –ribosomal RNA (rRNA) is one of the components of the ribosome involved in translation –transfer RNA (tRNA) is a small RNA that can bind an amino acid at one end, and mRNA at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA (cont.)

10. 10-10 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Transcription (cont.) In prokaryotes transcription occurs in the cytoplasm Since many bacterial genes are arranged in operons (Chapter 11) the RNA transcripts are polycistronic Transcription and translation are often coincident

11. 10-11 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.5: Coincident transcription and translation in prokaryotes

12. 10-12 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Eukaryotic transcription Eukaryotic transcription occurs in the nucleus Eukaryotic genes are usually monocistronic— coding for a single polypeptide The eukaryotic nucleus has an organelle called the nucleolus which is the site for ribosomal RNA synthesis The three major classes of RNA are transcribed by different RNA polymerases

13. 10-13 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint mRNA processing Both ends of the primary RNA transcript are modified before transport to the cytoplasm –A methyl guanidine cap is added to the 5’ end –A poly adenine or poly A tail is added to the 3’ end of the transcript One of the most important stages in RNA processing is RNA splicing. In many genes, the DNA sequence coding for proteins, or ‘exons’, may be interrupted by stretches of non-coding DNA, called ‘introns’ (cont.)

14. 10-14 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint mRNA processing (cont.) In the cell nucleus, the DNA that includes all the exons and introns of the gene is first transcribed into a complementary RNA copy called 'nuclear RNA’ or nRNA Introns are then removed from nRNA by a process called RNA splicing –splicing occurs at specific sequences on the intron–exon boundaries –exons are joined together –the edited sequence is the final mRNA

15. 10-15 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.6: Post-transcriptional processing of eukaryotic mRNA transcripts

16. 10-16 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint The genetic code mRNA transfers information to protein in the form of a code defined by a sequence of nucleotide bases Each amino acid is specified by three nucleotides called a codon Since RNA is constructed from four types of nucleotides, there are 64 possible codons (4x4x4). Three of these codons called stop codons specify the termination of the polypeptide chain (cont.)

17. 10-17 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint The genetic code (cont.) That leaves 61 codons to specify only 20 different amino acids Therefore, most of the amino acids are represented by more than one codon Thus, the genetic code is to be degenerate Particularly in the third nucleotide position, a base change often does not change the amino acid specified

18. 10-18 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.7: The genetic code

19. 10-19 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Reading frames In order for the protein to be synthesised, translation must start and stop correctly The region of the mRNA used to encode the amino acid sequence is called the open reading frame

20. 10-20 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.8: The concept of reading frames

21. 10-21 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Frame shift mutations Each mRNA potentially has three reading frames but only one gives the correct sequence for the protein The reading frame is set by the position of the start codon (AUG) Within a gene, small deletions or insertions of a number of bases not divisible by 3 will result in a frame shift in the mRNA during translation (cont.)

22. 10-22 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.9: Frameshift mutations

23. 10-23 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Reading frame mutations (cont.) A nonsense mutation creates a stop codon where none previously existed –this shortens the resulting protein, possibly removing essential regions A missense mutation changes the code of the mRNA –for example if an AGU is changed to an AGA, the protein will have an arginine instead of serine –the shape or function of the protein may be altered

24. 10-24 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Translation During protein synthesis, ribosomes move along the mRNA molecule and read its sequence one codon at a time from the 5' end to the 3' end Each amino acid is specified by the mRNA's codon Codons pair with a sequence of three complementary nucleotides (anticodon) carried by a particular tRNA

25. 10-25 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Transfer RNA (tRNA) tRNAs have an anticodon at one end and an amino acid at the other They act as adaptor molecules to bring the correct amino acid to the mRNA codon (cont.)

26. 10-26 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.10: Transfer RNA structure

27. 10-27 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Transfer RNA (tRNA) (cont.) Each tRNA only binds the appropriate amino acid for its anticodon and is recharged after depositing its amino acid into the growing chain

28. 10-28 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.11: Activation of RNA by aminoacylation

29. 10-29 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Ribosomes Consist of two protein subunits, large and small, and associated rRNAs Provide a precise method of aligning codons and tRNAs to ensure amino acids are synthesised in the correct order

30. 10-30 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.12: Ribosomes

31. 10-31 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Polypeptide synthesis Initiation –the ribosomes form from subunits –tRNA methionine binds to start codon –Large subunit of ribosome binds to form A and P sites (cont.)

32. 10-32 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.14: The three phases of protein synthesis

33. 10-33 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Polypeptide synthesis (cont.) Elongation –begins with the formation of a peptide bond between the methionine and the second amino acid –the process involves the addition of a sequence of amino acids, specified by the codons –as each new amino acid is brought into position, a peptide bond is formed with the preceding amino acid –translation proceeds 5’ to 3’ along the mRNA –the peptide is synthesised from the amino (NH 2 ) end to the carboxyl (COOH) end (cont.)

34. 10-34 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Polypeptide synthesis (cont.) Termination –when the ribosome arrives at a stop codon the elongation process stalls because there is no tRNA for stop codons –termination factors remove the last amino acid from its tRNA –the ribosome separates into its two subunits and leaves the mRNA

35. 10-35 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.15a: Initiation

36. 10-36 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.15b: Elongation

37. 10-37 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig 10.15c: Termination

38. 10-38 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Protein processing Polypeptide synthesis is only the first step in the production of a mature protein The protein may be further modified by the addition of chemical residues Proteins are targeted to particular organelles by the addition of signal sequences which bind to receptors at the correct location Proteins for secretion are sorted and packaged in the Golgi apparatus

39. 10-39 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 10.16: Pathways of targeting in eukaryotic cells