CIE A Level Biology (9700) 2019-2021

Revision Notes

6.2.2 Constructing Polypeptides

Constructing Polypeptides: Transcription & Translation

  • A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
  • This process of protein synthesis occurs in two stages:
    • TranscriptionDNA is transcribed and an mRNA molecule is produced
    • TranslationmRNA (messenger RNA) is translated and an amino acid sequence is produced

Transcription

  • This stage of protein synthesis occurs in the nucleus of the cell
  • Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)
  • This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
  • A complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
  • Free activated RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule
  • The sugar-phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule
  • When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule re-forms
  • The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

Translation

  • This stage of protein synthesis occurs in the cytoplasm of the cell
  • After leaving the nucleus, the mRNA molecule attaches to a ribosome
  • In the cytoplasm, there are free molecules of tRNA (transfer RNA)
  • These tRNA molecules have a triplet of unpaired bases at one end (known as the anticodon) and a region where a specific amino acid can attach at the other
  • There are at least 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
  • The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
  • The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule
  • Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
  • A peptide bond is then formed between the two amino acids
  • This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
  • This amino acid chain then forms the final polypeptide

Exam Tip

Make sure you learn both stages of protein synthesis fully. Don’t forget – transcription occurs in the nucleus but translation occurs in the cytoplasm! Be careful – DNA polymerase is the enzyme involved in DNA replication; RNA polymerase is the enzyme involved in transcription – don’t get these confused.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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