Edexcel IGCSE Biology

Revision Notes

3.2.2 Protein Synthesis

RNA

  • Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain
  • Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
  • Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
  • Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
  • Each RNA polynucleotide strand is made up of alternating ribose sugars and phosphate groups linked together, with the nitrogenous bases of each nucleotide projecting out sideways from the single-stranded RNA molecule
  • An example of an RNA molecule is messenger RNA (mRNA), which is the transcript copy of a gene that encodes a specific polypeptide. Two other examples are transfer RNA (tRNA) and ribosomal RNA (rRNA)

mRNA as an example of RNA structure, downloadable AS & A Level Biology revision notes

Messenger RNA (mRNA) provides a good example of the structure of RNA

Exam Tip

  • The main differences you need to know between RNA and DNA:
    • RNA is single-stranded
    • RNA contains uracil instead of thymine

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 (protein) 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)
  • The mRNA molecule 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 (protein)

Protein synthesis, IGCSE & GCSE Biology revision notes

An overview of protein synthesis

How ribosomes build proteins, IGCSE & GCSE Biology revision notes

The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein

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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