Powerpoint Presentation: Gene Expression

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  • 1. GENE EXPRESSION © 2007 Paul Billiet ODWS
  • 2. Two steps are required
    • Transcription The synthesis of mRNA use the gene on the DNA molecule as a template This happens in the nucleus of eukaryotes
    • Translation The synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide
    © 2007 Paul Billiet ODWS
    • Found all over the cell (nucleus, mitochondria, chloroplasts, ribosomes and the soluble part of the cytoplasm)
    © 2007 Paul Billiet ODWS
  • 4. Types
    • Messenger RNA (mRNA) <5%
    • Ribosomal RNA (rRNA) Up to 80%
    • Transfer RNA (tRNA) About 15%
    • In eukaryotes small nuclear ribonucleoproteins (snRNP)
    © 2007 Paul Billiet ODWS
  • 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).
    © 2007 Paul Billiet ODWS
  • 6. mRNA
    • A long molecule 1 million Daltons
    • Ephemeral
    • Difficult to isolate
    • mRNA provides the plan for the polypeptide chain
    © 2007 Paul Billiet ODWS
  • 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 from protein synthesis
    © 2007 Paul Billiet ODWS
  • 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
    © 2007 Paul Billiet ODWS
  • 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
    © 2007 Paul Billiet ODWS
  • 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 ( ex ons = ex pressed)
    • The parts which are edited out (by snRNP molecules) are called INTRONS
    © 2007 Paul Billiet ODWS
  • 11. Transcription plan Transcription DNA messenger RNA Gene Nucleus © 2007 Paul Billiet ODWS
  • 12. Translation plan TRANSLATION Ribosomes Stop codon Start codon © 2007 Paul Billiet ODWS Complete protein Polypeptide chain
  • 13. Translation
    • Location: The ribosomes in the cytoplasm that provide the environment for translation
    • The genetic code is brought by the mRNA molecule
    © 2007 Paul Billiet ODWS
  • 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
    © 2007 Paul Billiet ODWS
  • 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 ?
    © 2007 Paul Billiet ODWS
  • 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
    © 2007 Paul Billiet ODWS