IBBiologyHLRevision NotesContinuity & ChangeProtein SynthesisTranslation & the Proteome (HL)

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First teaching 2023

First exams 2025

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Translation & the Proteome (HL) (HL IB Biology)

Revision Note

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Biology

Initiation of Translation

  • During translation, the specific sequence of messenger RNA (mRNA) is translated to produce a polypeptide chain consisting of amino acids
    • mRNA is a single stranded, linear, RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm
  • Translation is categorised into three stages: initiation, elongation and termination
  • Translation occurs in the cytoplasm at complex molecules made of protein and RNA called ribosomes
    • Ribosomes have a two-subunit (large and small) structure that helps bind mRNA
    • Ribosomes have three tRNA binding sites termed “E” (exit), “P” (peptidyl) and “A” (aminoacyl)
      • At the A site the mRNA codon joins with the tRNA anticodon
      • At the P site the amino acids attached to the tRNA are joined by peptide bonds
      • At the E site the tRNA exits the ribosome
  • Another key molecule in translation is transfer RNA (tRNA) that decodes mRNA
    • tRNA molecules are single stranded RNA molecules that fold to form a clover-shaped structure
      • The folded structure is held together by hydrogen bonds between bases at different points on the strand
      • tRNA molecules are the shortest of the RNA molecules, being only around 80 nucleotides in length
      • There are 20 different types of tRNA molecule, one for each of the amino acids involved in protein synthesis
    • tRNA molecules have a region that binds to a specific amino acid as well as a three-nucleotide region called an anticodon that is complementary to the codon on mRNA
    • The role of tRNA molecule is to carry a specific amino acid to the ribosome
tRNA structure
Structure of tRNA
  • In eukaryotic cells, the mRNA molecule leaves the nucleus through the nuclear pores
  • Translation is initiated by the following process
    • A small ribosomal subunit attaches to the 5’ end of mRNA
    • An initiator tRNA molecule carrying the amino acid methionine binds to the small ribosomal subunit
      • The initiator tRNA occupies the “P” site on the ribosome
    • The ribosome moves along the mRNA until it locates a start codon (AUG)
    • The large ribosomal subunit binds to the small subunit
      • Elongation of the polypeptide can begin
  • The initiator tRNA currently occupies the “P” site, the next codon on the mRNA signals for the corresponding tRNA to bind at the “A” site
    • The two amino acids (attached to the tRNAs) are linked with a peptide bond, forming a dipeptide
  • Synthesis of the peptide chain now involves a repeated cycle of events
    • In the cytoplasm, free tRNA molecules bind to their corresponding amino acids and transport them to the ribosome
    • The ribosome shifts along the mRNA one codon (three bases) at a time
      • The initiator tRNA in the “P” site moves to the “E” site which releases it
      • The tRNA carrying the peptide chain moves from the “A” site to the “P” site
      • The next mRNA codon is exposed and a tRNA with the complementary anticodon binds to the unoccupied “A” site whilst its amino acid is linked to the polypeptide chain
  • The cyclical process is repeated as new amino acids are added to the growing chain

Modification of Polypeptides

  • Once the primary structure of the polypeptide has been synthesised during translation it is often not immediately usable by the cell
    • The polypeptide must be modified in order to be transformed into a functional protein
  • Some examples of modifications include:
    • Protein folding into the secondary, tertiary and quaternary structures, including the formation of disulfide bonds in the tertiary and quaternary stages
    • Folding can require molecular chaperones that help to prevent incorrect folding
  • The formation of insulin requires polypeptide modification
    • When insulin is first synthesised it is in the form of an 110 long polypeptide chain called pre-proinsulin, which is attached to the wall of the endoplasmic reticulum (ER)
    • It is then modified by an enzyme that removes a peptide called a signal peptide from the end, detaching it from the ER and transforming it to proinsulin
    • From there the proinsulin folds and disulfide bonds form between different sections of the polypeptide 
    • The proinsulin is packaged into vesicles at the Golgi apparatus
    • The proinsulin is then cleaved (during which a section called the C peptide is removed from the middle) resulting in two chains (A-chain and B-chain) attached together with two disulfide bonds
    • This is the final, mature form of insulin, ready to be secreted from the cell and used in the body

Recycling of Amino Acids

  • Unneeded, damaged, or misfolded proteins can be recycled in the body into usable proteins
  • This involves enzymes to break the peptide bonds in these proteins, and releasing the amino acids to be used in translation to synthesise new proteins
    • Proteases are enzymes that break down proteins in this way
    • This process is called proteolysis
  • The proteasome is an organelle found in eukaryotic cells and acts as the location for proteolysis in the cell
  • By containing the protease enzymes within an organelle it prevents other useful cellular proteins being broken down by mistake
  • Proteins identified as being unneeded, damaged, or misfolded are tagged with a chemical called ubiquitin, which begins the process of them being broken down in the proteasome 
  • This process is constantly taking place in the cell and is essential for sustaining a functional proteome
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Emma

Author: Emma

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!