HL 1 Biology

3.5 & 7.4 Translation


Takes place in cytoplasm (on ribosomes)


  • Initiation: mRNA binds to small subunit of ribosome. tRNA molecule (carrying specific amino acid) corresponds to its anti-condon
  • tRNA binds to ribosome at site where anti-condon and condon on mRNA match
  • Elongation: 2 tRNAs bind + first one transfers the growing polypeptide chain to the second one in
  • Termination: ribosome moves along mRNA until reaches a stop condon (polypeptide released)


  • translation always begins in cytoplasm
  • ribosomes complete translation while it binds to endoplasmic reticulum, if proteins are destined for lysosomes/export
  • proteins used within cytosol= synthesized by unbound ribosomes
  • Ribosomes:
      • composed of rRNA + individual proteins
      • consist 2 subunits
      • has three tRNA binding sites (E = exit site, P= peptidyl site ,A aminoacyl site)
  • Polyribosome/Polysome: resulting complex of ribosomes along a single mRNA
      • more than one ribosome can be actively translating same mRNA molecule at once
  • Amino acid attaches at 3′ end of tRNA (3′ end terminates nucleotide sequence CCA)



Interpreting Electron Micrographs (p.73)

The first figure shows the process of translation because the polysomes are apparent as well as the growing peptide chains from the tRNA, which is surrounded by the sub unit of a ribosome. The multiple ribosomes and peptide chains on the single strand of mRNA suggests the process of translation occurring in the figure.

The figure in the middle shows the process of DNA replication as a distinct replication bubble and replication forks are visible.

The very right figure shows the process of transcription because of the long RNA transcript apparent from the DNA strand.

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