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First teaching 2023

First exams 2025

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Constructing Polypeptides (CIE AS Biology)

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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 complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
    • The DNA triplets of bases are transcribed as mRNA codons
    • The single-stranded mRNA molecule can fit through the pores in the nuclear envelope, unlike double-stranded DNA
  • 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

The Process of Transcription Diagram

transcription-in-the-nucleus-diagram

The transcription stage of protein synthesis – DNA is transcribed and an mRNA molecule is produced

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 anticodon on each tRNA molecule pairs with a complementary 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 
    • The 'stop' codon 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

tRNA-molecules-bind-with-amino-acids-diagram

The translation stage of protein synthesis – tRNA molecules bind with their specific amino acids

The Process of Translation Diagram

translation-(1)-diagramtranslation-(2)-diagramThe translation stage of protein synthesis – an amino acid chain is formed

Examiner 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.

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.