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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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Gene control: lac operon

  • Regulatory genes control structural genes and their levels of protein production

  • Regulatory genes sometimes have control over several structural genes at once

  • Structural genes in prokaryotes can form an operon: a group or a cluster of genes that are controlled by the same promoter

  • The lac operon found in some bacteria is one of the most well-known of these

  • The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins

  • Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell

  • It is an inducible enzyme that is only synthesised when lactose is present

  • This helps prevent the bacteria from wasting energy and materials

Structure of the lac operon

  • The components of the lac operon are found in the following order:

    • Promoter for structural genes

    • Operator

    • Structural gene lacZ that codes for lactase

    • Structural gene lacY that codes for permease (allows lactose into the cell)

    • Structural gene lacA that codes for transacetylase

  • Located to the left (upstream) of the lac operon on the bacterium’s DNA there is also the:

    • Promoter for regulatory gene

    • Regulatory gene lacI that codes for the lac repressor protein

  • The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)

    • When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter

    • When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator

Diagram of the lac operon, showing regulatory gene, promotor, operator, and structural genes lacZ, lacY, and lacA. Includes labels and functions.
The components of the Lac operon along with the upstream regulatory gene and its associated promoter

When lactose is absent

  • The following processes take place when lactose is absent in the medium that the bacterium is growing in:

    • The regulatory gene is transcribed and translated to produce lac repressor protein

    • The lac repressor protein binds to the operator region upstream of lacZ

    • Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region

    • Transcription of the structural genes does not take place

    • No lactase enzyme is synthesized

Diagram explaining lac operon function, showing RNA polymerase blocked, repressor-protein binding, and no transcription of structural genes.
The repressor protein binding to the operator region of the Lac operon and preventing transcription of the structural gene

When lactose is present

  • The following processes take place when lactose is present in the medium that the bacterium is growing in:

    • There is an uptake of lactose by the bacterium

    • The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site

    • RNA polymerase is then able to bind to the promoter region and transcription takes place

    • The mRNA from all three structural genes is translated

    • Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

Diagram showing lactose binding to the lac repressor, RNA polymerase binding to the promoter, and transcription of structural genes in the lac operon.
Lactose binding to the repressor protein which frees up the operator region of the Lac operon so RNA polymerase can bind and begin transcription of the structural genes

Examiner Tips and Tricks

The example above explains how the genetic control of an inducible enzyme works. You could get some questions on the genetic control of repressible enzymes.

In this mechanism an effector molecule also binds to a repressor protein produced by a regulatory gene. However this binding actually helps the repressor bind to the operator region and prevent transcription of the structural genes. So it’s the opposite of the lac operon: when there is less of the effector molecule, the repressor protein cannot bind to the operator region and transcription of the structural genes goes ahead, meaning the enzyme is produced.