Syllabus Edition

First teaching 2014

Last exams 2024

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Skills: Polysomes & Ribosomes (DP IB Biology: HL)

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Lára

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Lára

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Identifying Polysomes

  • Translation can occur simultaneously at multiple positions along mRNA
  • Polysomes (or polyribosomes) are groups of two or more ribosomes translating the same mRNA transcript
  • Multiple copies of the same polypeptide chain can be made simultaneously from a single mRNA transcript
    • Polysomes effectively increase the amount of polypeptide produced
  • In electron micrographs, polysomes look like ‘beads on a string’ with each bead representing a ribosome
    • There are visible differences between eukaryotes and prokaryotes
  • In prokaryotes, the lack of a nucleus means transcription and translation are coupled
    • Translation starts before the mRNA has finished being transcribed from the DNA
    • On an electron micrograph, multiple polysomes can appear on growing mRNA strands along the DNA molecule
  • In eukaryotes, mRNA is transported out of the nucleus prior to translation
    • On an electron micrograph, polysomes are seen on the mRNA with no involvement of DNA
  • As ribosomes move in the same 5’ to 3’ direction along the mRNA, ribosomes towards the 3’ end have longer polypeptide chains being synthesised

electron-micrograph-of-prokaryotic-polysomes

Electron micrograph of prokaryotic polysomes, the image shows simultaneous transcription and translation of a bacterial gene

electron-micrograph-of-eukaryotic-polysomes

Electron micrograph of eukaryotic polysomes, polypeptide chains can be seen emerging from the ribosomes

Visualising the Structure of Ribosomes

  • There are many ‘open source’ databases that contain data relating to the three-dimensional structure of proteins
    • The most commonly used ones are PDB (Protein Data Bank) and Swiss-Prot
  • Such databases allow researchers to analyse biological molecules and study interactions between them
    • This helps relate structure to biological function
  • Data relating to the three-dimensional structure of biological molecules can be visualised using molecular visualisation software such as Mol* or Jmol
  • Molecules can be represented in many different ways including ball and stick atom models or simplified ribbon representations that show the protein backbone
  • Most molecular visualisation software is freely available on the Internet or can be accessed through many bioinformatics repositories

Analysing the structure of the eukaryotic ribosome

  • Visit the PDB and search for: Yeast 80S ribosome 4V7R (do not put the search term in quotes)
  • Select the “3D view” to view the protein structure in mol*
  • Eukaryotic ribosome are complex molecules consisting predominantly of ribosomal RNAs and multiple proteins - these are represented in different colours
  • The molecule appears to be made of two distinct halves (subunits)
  • Rotate or zoom into the image to visualise the different components
  • Try and identify the ribosomal RNA in the two subunits (hover the cursor over)
  • An opening (groove) between the subunits should be visible - mRNA passes through here

Analysing the structure of a tRNA molecule

  • Search for: 1YFG in the PDB (1YFG is the database identifier for yeast initiator tRNA)
  • Select the “3D view” to view the protein structure in mol*
  • You should be able to recognise the loop structures
  • Consider how this structure is specific to the tRNA-activating enzyme
  • Try and identify the anticodon region and amino acid binding site (acceptor arm)
  • Try selecting a different viewer such as JSmol
  • Investigate changing settings in the viewer

structure-of-yeast-trna

Structure of yeast tRNA

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.