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Biology_A-level_Cie

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  1. 1-1-the-microscope-in-cell-studies
    5 主题
  2. 1-2-cells-as-the-basic-units-of-living-organisms
    5 主题
  3. 2-1-testing-for-biological-molecules
    3 主题
  4. 2-2-carbohydrates-and-lipids
    8 主题
  5. 2-3-proteins
    6 主题
  6. 2-4-water
    2 主题
  7. 3-1-mode-of-action-of-enzymes
    5 主题
  8. 3-2-factors-that-affect-enzyme-action
    8 主题
  9. 4-1-fluid-mosaic-membranes
    4 主题
  10. 4-2-movement-into-and-out-of-cells
    12 主题
  11. 5-1-replication-and-division-of-nuclei-and-cells
    6 主题
  12. 5-2-chromosome-behaviour-in-mitosis
    2 主题
  13. 6-1-structure-of-nucleic-acids-and-replication-of-dna
    4 主题
  14. 6-2-protein-synthesis
    5 主题
  15. 7-1-structure-of-transport-tissues
    4 主题
  16. 7-2-transport-mechanisms
    7 主题
  17. 8-1-the-circulatory-system
    7 主题
  18. 8-2-transport-of-oxygen-and-carbon-dioxide
    5 主题
  19. 8-3-the-heart
    4 主题
  20. 9-1-the-gas-exchange-system
    6 主题
  21. 10-1-infectious-diseases
    3 主题
  22. 10-2-antibiotics
    3 主题
  23. 11-1-the-immune-system
    4 主题
  24. 11-2-antibodies-and-vaccination
    6 主题
  25. 12-1-energy
    5 主题
  26. 12-2-respiration
    11 主题
  27. 13-1-photosynthesis-as-an-energy-transfer-process
    8 主题
  28. 13-2-investigation-of-limiting-factors
    2 主题
  29. 14-1-homeostasis-in-mammals
    8 主题
  30. 14-2-homeostasis-in-plants
    3 主题
  31. 15-1-control-and-coordination-in-mammals
    12 主题
  32. 15-2-control-and-coordination-in-plants
    3 主题
  33. 16-1-passage-of-information-from-parents-to-offspring
    5 主题
  34. 16-2-the-roles-of-genes-in-determining-the-phenotype
    7 主题
  35. 16-3-gene-control
    3 主题
  36. 17-1-variation
    4 主题
  37. 17-2-natural-and-artificial-selection
    7 主题
  38. 17-3-evolution
    2 主题
  39. 18-1-classification
    5 主题
  40. 18-2-biodiversity
    7 主题
  41. 18-3-conservation
    6 主题
  42. 19-1-principles-of-genetic-technology
    11 主题
  43. 19-2-genetic-technology-applied-to-medicine
    4 主题
  44. 19-3-genetically-modified-organisms-in-agriculture
    2 主题
  45. 1-1-the-microscope-in-cell-studies
  46. 1-2-cells-as-the-basic-units-of-living-organisms
  47. 2-1-testing-for-biological-molecules
  48. 2-2-carbohydrates-and-lipids
  49. 2-3-proteins
  50. 2-4-water
  51. 3-1-mode-of-action-of-enzymes
  52. 3-2-factors-that-affect-enzyme-action
  53. 4-1-fluid-mosaic-membranes
  54. 4-2-movement-into-and-out-of-cells
  55. 5-1-replication-and-division-of-nuclei-and-cells
  56. 5-2-chromosome-behaviour-in-mitosis
  57. 6-1-structure-of-nucleic-acids-and-replication-of-dna
  58. 6-2-protein-synthesis
  59. 7-1-structure-of-transport-tissues
  60. 7-2-transport-mechanisms
  61. 8-1-the-circulatory-system
  62. 8-2-transport-of-oxygen-and-carbon-dioxide
  63. 8-3-the-heart
  64. 9-1-the-gas-exchange-system
  65. 10-1-infectious-diseases
  66. 10-2-antibiotics
  67. 11-1-the-immune-system
  68. 11-2-antibodies-and-vaccination
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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:

    • Transcription:

      • DNA is transcribed and an mRNA molecule is produced

    • Translation:

      • mRNA (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 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

    • The double-stranded DNA molecule then re-forms

  • The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

Diagram illustrating DNA transcription in a cell nucleus, showing DNA unwinding, RNA polymerase activity, and mRNA formation and exit to cytoplasm.
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

    • 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

Diagram illustrating mRNA leaving the nucleus via a nuclear pore to reach the ribosome. tRNA binds specific amino acids, Met and Tyr, based on anticodons.
The translation stage of protein synthesis—tRNA molecules bind with their specific amino acids
Diagram illustrating protein synthesis: mRNA attaches to ribosome; tRNA with anticodon bonds to codon; peptide bond forms; ribosome reads mRNA sequence.
Diagram showing tRNA adding amino acids to a growing polypeptide chain in a ribosome on mRNA, illustrating protein synthesis stages with labels.
The translation stage of protein synthesis—an amino acid chain is formed

Examiner Tips and Tricks

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.