Gene expression


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Gene expression

  1. 1. GENE EXPRESSION by A.Arputha Selvaraj
  2. 2. Two steps are required 1. Transcription The synthesis of mRNA uses the gene on the DNA molecule as a template This happens in the nucleus of eukaryotes 2. Translation The synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide. © 2010 Paul Billiet ODWS
  3. 3. RIBONUCLEIC ACID (RNA) Found all over the cell (nucleus, mitochondria, chloroplasts, ribosomes and the soluble part of the cytoplasm). © 2010 Paul Billiet ODWS
  4. 4. Types  Messenger RNA (mRNA) <5%  Ribosomal RNA (rRNA) Up to 80%  Transfer RNA (tRNA) About 15%  In eukaryotes small nuclear ribonucleoproteins (snRNP). © 2010 Paul Billiet ODWS
  5. 5. Structural characteristics of RNA molecules  Single polynucleotide strand which may be looped or coiled (not a double helix)  Sugar Ribose (not deoxyribose)  Bases used: Adenine, Guanine, Cytosine and Uracil (not Thymine). © 2010 Paul Billiet ODWS
  6. 6. mRNA  A long molecule 1 million Daltons  Ephemeral  Difficult to isolate  mRNA provides the plan for the polypeptide chain © 2010 Paul Billiet ODWS
  7. 7. rRNA  Coiled  Two subunits: a long molecule 1 million Daltons a short molecule 42 000 Daltons  Fairly stable  Found in ribosomes  Made as subunits in the nucleolus  rRNA provides the platform for protein synthesis© 2010 Paul Billiet ODWS
  8. 8. tRNA  Short molecule about 25 000 Daltons  Soluble  At least 61 different forms each has a specific anticodon as part of its structure.  tRNA “translates” the message on the mRNA into a polypeptide chain © 2010 Paul Billiet ODWS
  9. 9. Transcription: The synthesis of a strand of mRNA (and other RNAs)  Uses an enzyme RNA polymerase  Proceeds in the same direction as replication (5’ to 3’)  Forms a complementary strand of mRNA  It begins at a promotor site which signals the beginning of gene is not much further down the molecule (about 20 to 30 nucleotides)  After the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing NB Terminator sequence ≠ terminator codon. © 2010 Paul Billiet ODWS
  10. 10. Editing the mRNA  In prokaryotes the transcribed mRNA goes straight to the ribosomes in the cytoplasm  In eukaryotes the freshly transcribed mRNA in the nucleus is about 5000 nucleotides long  When the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long  The mRNA has been edited  The parts which are kept for gene expression are called EXONS (exons = expressed)  The parts which are edited out (by snRNP molecules) are called INTRONS. © 2010 Paul Billiet ODWS
  11. 11. Transcription plan Transcription DNA messenger RNA Gene Nucleus © 2010 Paul Billiet ODWS
  12. 12. Translation plan TRANSLATION Complete protein Polypeptide chain Ribosomes Stop codon Start codon © 2010 Paul Billiet ODWS
  13. 13. Translation  Location: The ribosomes in the cytoplasm that provide the environment for translation  The genetic code is brought by the mRNA molecule. © 2010 Paul Billiet ODWS
  14. 14. What is the genetic code?  The genetic code consists of the sequence of bases found along the mRNA molecule  There are only four letters to this code (A, G, C and U)  The code needs to be complex enough to represent 20 different amino acids used to build proteins. © 2010 Paul Billiet ODWS
  15. 15. How many combinations?  If one base represented one amino acid this would only be able to produce 4 different combinations. (A, C, G and U)  If pairs of bases represented each amino acid this would only be able to produce 4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG, CU etc)  If triplets of bases represented each amino acid, this would be able to produce 4 x 4 x 4 = 64 combinations This is enough combinations to code for the 20 amino acids but is the code actually made of triplets? © 2010 Paul Billiet ODWS
  16. 16. Nature is logical!  Over 10 years biochemists synthesised bits of mRNA with different combinations  Then they used them to synthesise polypeptides  The results proved the logical answer was correct  The genetic code is made of triplets of bases called codons. © 2010 Paul Billiet ODWS
  17. 17. The Central Dogma  Proposed by Francis Crick 1958  DNA holds the coded hereditary information in the nucleus  This code is expressed at the ribosome during protein synthesis in the cytoplasm  The protein produced by the genetic information is what is influenced by natural selection  If a protein is modified it cannot influence the gene that codes for it  Therefore there is one way flow of information: DNARNAProtein © 2010 Paul Billiet ODWS
  18. 18. An important discovery  Retro viruses (e.g. HIV) carry RNA as their genetic information  When they invade their host cell they convert their RNA into a DNA copy using reverse transcriptase  Thus the central dogma is modified: DNA↔RNAProtein  This has helped to explain an important paradox in the evolution of life. Image Credit: Reverse transcriptase
  19. 19. The paradox of DNA  DNA is a very stable molecule  It is a good medium for storing genetic material but…  DNA can do nothing for itself  It requires enzymes for replication  It requires enzymes for gene expression  The information in DNA is required to synthesise enzymes (proteins) but enzymes are require to make DNA function  Which came first in the origin of life DNA or enzymes? © 2010 Paul Billiet ODWS
  20. 20. RIBOZYMES: Both genetic and catalytic  Certain forms of RNA have catalytic properties  RIBOZYMES  Ribosomes and snRNPs are ribozymes  RNA could have been the first genetic information synthesizing proteins…  …and at the same time a biocatalyst  Reverse transcriptase provides the possibility of producing DNA copies from RNA © 2010 Paul Billiet ODWS
  21. 21. The ribosome a ribozyme Image Credit: Ribosome
  22. 22. Thank You