the lac operon - blinderl/documents/ch17_lacoperonpost.pdf1 the lac operon 1961, jacob and monod e....

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  • 1

    The Lac Operon 1961, Jacob and Monod E. coli and other bacteria

    Bacterial Genes

    Many genes constitutively expressed

    housekeeping genes

    Other genes are more regulated

    Can be turned on, or off depending on cell needs

  • Operon

    group of coordinately regulated genes

    1 promoter for a number of genes

    Polycistronic mRNA

    Inducer molecule turns operon on

    2

  • 3

    E. Coli Lac Operon

    E. coli cells normally grown in glucose

    BUT, if lactose is used instead:

    convert lactose to glucose and galactose

  • 4

    The Lac Operon allows for

    coordinate gene expression

    Note: 1 mRNA, promoter, 3 genes

  • 5

    THE OPERON HAS

    A. 3 STUCTURAL GENES = Z, Y, A

  • Lac Z gene encodes betagalactosidase

    b-gal

    lactose ------------- glucose + galactose

    substrate products

    No lactose present ~ 3 molecules of bgal

    Add lactose 3,000 molecules of bgal

    6

  • 7

    So..

    b- gal is an inducible enzyme

    What is the role of lactose?

    inducer

  • 8

    Lac Operon

    B. promoter = allows transcription of ZYA

    C. operator = must be unbound for P to be

    open

    What molecule are ALL of the components above?

  • 9

    Lac Y gene encodes permease that transports lactose into cell

    Lac A encodes a transacetylase

  • 10

    D. Lac I gene

    Encodes a repressor protein

    Repressor binds to operator

  • 11

    Is this operon ON or OFF?

    Is lactose PRESENT or ABSENT?

  • 12

    Which components act in cis?

    In trans?

  • 13

    Regulation of the Lac Operon

    Its normally repressed! = OFF

    Because lactose is absent

    Therefore, it is an inducible operon

    When lactose is present

  • INDUCER (LACTOSE SUGAR)

    1. Lactose enters cell

    14

  • 2. Binds repressor protein causing

    conformational change

    15

  • 3. repressor cannot bind operator

    16

  • 4. RNA polymerase transcribes genes

    5. Cell metabolizes lactose

    17

  • 18

    Lactose (the inducer) enters the cell

    Binds repressor protein causing a conformational change

  • 19

    repressor binds operator

    polymerase cannot bind promoter

    no transcription of ZYA genes

    No lactose:

  • Lac operon animation

    20

    http://www.youtube.com/watch?v=oBwtxdI1zvk

  • 21

    Operon mutants

    Mutant Mutant Phenotype

    lac I -

    constitutive expression because

  • 22

    This operon is not inducible

  • Mutant Mutant Phenotype

    Oc constitutive expression because

    23

  • Mutant Mutant Phenotype

    P- operon always off because

    24

  • Mutant phenotype

    Z- operon is?

    25

  • 26

    Operon on, or off in the absence of

    lactose?

    Lac Oc I+P+OcZ+Y+A+

    In presence of lactose?

    Is it Inducible?

  • Operon on, or off in the absence

    of lactose?

    Lac I- I-P+O+Z+Y+A+

    In presence of lactose?

    Is it Inducible?

    27

  • 28

    Partial diploid cells contain a plasmid

    F I+ Inducible?

    I-P+O+Z+Y+A+

  • F I- P- Z+Y-A-

    I+P+O+Z- Y+A+

    29

    Inducible?

  • 30

    Repressor and polymerase = proteins

    diffusible

    Proteins can bind DNA

    act in TRANS

    promoter, operator, and ZYA and I = genes

    cannot move

    act in CIS

    Tips for plasmid analysis