Ribosomes

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  • Ribosomes
    • what are they
      • made of 2 subunits
        • larger 60S unit
          • has catalytic site
          • 49 proteins
        • smaller 40S subunit
          • 33 proteins
        • 2/3 RNA, 1/3 proteins
      • RNA core with proteins attached to it
    • How are they made
      • rRNA
        • synthesis and processing
      • ribosmoal proteins
        • transcription, translation
      • ribosmoal subunits
        • assembly
    • Nucleolus
      • discrete area within the nucleus, no additional membrane
      • Forms around parts of the chromosomes called nucleolar organisers
      • site of ribosomal subunit production
      • Nucleolar organiser
        • Involves 10 chromosomes in humans
        • 200 copies or rRNA genes
        • nucleolus associated with chromosomal regions that contain rRNA genes
      • Dense fibrillar component (RNA being transcribed)
      • granular component (maturing ribosomes)
      • fibrillar centre (DNA not being transcribed)
    • Translation
      • Initiation
        • ribosomes recognise and bind to the 5' terminus
        • Scan down mRNA until encounter a start codon
        • involves small subunit
      • elongation
        • large subunit joins complex
        • polypeptide chain elongates (addition of amino acids)
        • 3 binding sites
          • p- peptidyl
          • a- aminoacyl
          • e- exit
        • initiator tRNA binds to p site
          • next tRNA with anticodon complementary to the next codon binds to a site
        • relies on base pairing of codons and anticodons
      • termination
        • stop codon is recognized by release factors in the cell
        • stimulate hydrolysis of bond between tRNA and the polypeptide chain
    • Protein turnover
      • removal of misfolded proteins
        • newly synthesised but misfolded are quickly degraded
        • aging proteins
        • failure to remove these proteins can interfere with normal cell functions
      • How
        • ATP dependent protease
        • hollow tube of many subunits
        • multi unit cap structures at each end (gateways)
        • cap has ATPase activity, unfolds target protein
        • substrate retained until degraded
      • Control
        • ubiquitin can be attached NH2 side chains of lysine residues targeted for degradation
        • activated by binding to ubiquitin activating enzyme
        • transfered to ubiquitin ligase
        • E2E3 complex recognises targeted protein
        • Ub added to protein, further Ub added by E1
          • signal for degradation
  • rRNA Production
    • Ribosomes
      • what are they
        • made of 2 subunits
          • larger 60S unit
            • has catalytic site
            • 49 proteins
          • smaller 40S subunit
            • 33 proteins
          • 2/3 RNA, 1/3 proteins
        • RNA core with proteins attached to it
      • How are they made
        • rRNA
          • synthesis and processing
        • ribosmoal proteins
          • transcription, translation
        • ribosmoal subunits
          • assembly
      • Nucleolus
        • discrete area within the nucleus, no additional membrane
        • Forms around parts of the chromosomes called nucleolar organisers
        • site of ribosomal subunit production
        • Nucleolar organiser
          • Involves 10 chromosomes in humans
          • 200 copies or rRNA genes
          • nucleolus associated with chromosomal regions that contain rRNA genes
        • Dense fibrillar component (RNA being transcribed)
        • granular component (maturing ribosomes)
        • fibrillar centre (DNA not being transcribed)
      • Translation
        • Initiation
          • ribosomes recognise and bind to the 5' terminus
          • Scan down mRNA until encounter a start codon
          • involves small subunit
        • elongation
          • large subunit joins complex
          • polypeptide chain elongates (addition of amino acids)
          • 3 binding sites
            • p- peptidyl
            • a- aminoacyl
            • e- exit
          • initiator tRNA binds to p site
            • next tRNA with anticodon complementary to the next codon binds to a site
          • relies on base pairing of codons and anticodons
        • termination
          • stop codon is recognized by release factors in the cell
          • stimulate hydrolysis of bond between tRNA and the polypeptide chain
      • Protein turnover
        • removal of misfolded proteins
          • newly synthesised but misfolded are quickly degraded
          • aging proteins
          • failure to remove these proteins can interfere with normal cell functions
        • How
          • ATP dependent protease
          • hollow tube of many subunits
          • multi unit cap structures at each end (gateways)
          • cap has ATPase activity, unfolds target protein
          • substrate retained until degraded
        • Control
          • ubiquitin can be attached NH2 side chains of lysine residues targeted for degradation
          • activated by binding to ubiquitin activating enzyme
          • transfered to ubiquitin ligase
          • E2E3 complex recognises targeted protein
          • Ub added to protein, further Ub added by E1
            • signal for degradation
    • Transcription
      • Transcribed by RNA polymerase I
      • high rate of transcription
    • Formation
      • 45S cleaved into 3 segments
      • 5'-3' direction, amino to carboxy
      • Large Subunit
        • transcribed by RNA Pol III
        • Made from 2 of the 3 segments
      • Small subunit
        • 55 structural proteins
        • transcribed by RNA Pol II
      • subunits are transported into cytoplasm for their role
  • precursor RNA (45S) is processed
    • Transcription
      • Transcribed by RNA polymerase I
      • high rate of transcription
  • control of regulatory proteins
    • maintained at low levels by short half life
    • can be conditional
      • stabilised when needed, degraded when not

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