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Cloning vectors

  1. 1. Gene Cloning & Cloning Vectors Dr Ravi Kant Agrawal, MVSc, PhD Senior Scientist (Veterinary Microbiology) Food Microbiology Laboratory Division of Livestock Products Technology ICAR-Indian Veterinary Research Institute Izatnagar 243122 (UP) India
  2. 2. Gene Cloning - definition • Clone: from the Greek - klon, a twig • The production of exact copies (clones) of a particular gene or DNA sequence using genetic engineering techniques. • The DNA containing the target gene(s) is split into fragments using restriction enzymes. These fragments are then inserted into cloning vectors which transfer the recombinant DNA to suitable host cells. • Inside the host cell the recombinant DNA undergoes replication; thus, a bacterial host will give rise to a colony of cells containing the cloned target gene.
  3. 3. Significance • A particular gene can be isolated and its nucleotide sequence determined • Control sequences of DNA can be identified & analyzed • Protein/enzyme/RNA function can be investigated • Mutations can be identified, e.g. gene defects related to specific diseases • Organisms can be ‘engineered’ for specific purposes
  4. 4. History • G. Mendel, in the middle of the 19th century coined the term gene for the factor controlling the inheritance of biological characteristics of an organism. • In 1903, W. Sutton proposed that genes reside on chromosomes. • In 1922 Gene mapping by T H Morgan and he analyzed the relative positions of 2000 genes on 4 chromosomes of the fruitfly, Drosophila melanogaster. • In 1944 Avery, Macleod and McCarty and in 1952 Hershey and Chase explained the molecular nature of the gene to be Deoxyribonucleic acid. • 1952 to 1966: The Structure of DNA, genetic code and the process of transcription and translation. • 1971-1973: New technology like recombinant DNA technology or genetic engineering or the process of Gene cloning developed. This technology helped in studying the regulation of genes, aberrations in the genes and then used genes for production of desired proteins. • In 1985, Kary Mullis invented the Polymerase Chain reaction
  5. 5. The first cloning experiment done by Boyer and Cohen
  6. 6. Cloning Vector • It is the central component of a gene cloning process. • A small piece of DNA into which a foreign DNA fragment can be inserted. • The insertion of the fragment is carried out by treating the vector and the foreign DNA with a restriction enzyme that creates the same overhang, then ligating the fragments together.
  7. 7. Characteristics of a cloning vector • Ori (Origin of replication) is a specific sequence of nucleotide from where replication starts • It should have selectable marker gene • It should have restriction sites: a synthetic multiple cloning site (MCS) can be inserted into the vector • Replicate inside the host cell to form multiple copies of the recombinant DNA molecule. • Less than 10kb in size.
  8. 8. Contd…. Origin of Replication: Allow the vector as well as the foreign DNA to amplify in the host cell Selectable Marker: Antibiotic resistance genes- Allow the host to grow on selective media; Can selectively amplify this specific vector in the host cell Multiple Cloning Sites: Allow insertion of foreign DNA
  9. 9. Types of Cloning Vectors • They allow the exogenous DNA to be inserted, stored, and manipulated mainly at DNA level. • Types 1. Plasmid vectors 2. Bacteriophage vectors 3. Cosmids 4. Phagemids 5. Fosmids 6. BACs & YACs
  10. 10. Plasmid Vector • Plasmid vectors are double-stranded, extra-chromosomal DNA molecules, circular, self-replicating. • Advantages: – Small, easy to handle – Easy purification – Straightforward selection strategies – Useful for cloning small DNA fragments (< 10kbp) • Disadvantages: – Less useful for cloning large DNA fragments (> 10kbp)
  11. 11. 1. Contains an origin of replication, allowing for replication independent of host’s genome. 2. Contains Selective markers: Selection of cells containing a plasmid  twin antibiotic resistance  blue-white screening 3. Contains a multiple cloning site (MCS) 4. Easy to be isolated from the host cell. 5. Plasmids range in size from 1.0kb to 250kb e.g. pUC8 is 2.1 kb and TOL is 117 kb in size. A plasmid vector for cloning
  12. 12. pBR322 • It was one of the first vectors to be developed in 1977. • The ‘p’ indicates that it is plasmid, ‘BR’ indicates Bolivar and Rodriguez • ‘322’ distinguishes it from the other plasmids produced in the same laboratory e.g. pBR325, pBR327, pBR328. • It is 4363bp in size i.e. less than 10kb • It carries two sets of antibiotic resistance genes i.e. either ampicillin or tetracycline can be used as a selectable marker. • Each of the marker genes carries unique restriction sites and insertion of DNA into these sites inactivates the specific marker site. e.g. insertion of new DNA with Pst1, Puv1, Ppa1 or Sca1 inactivates the ampR gene. • It has a high copy number. They are about 15 molecules present in transformed cells but it can be increased to 1000 to 3000 by plasmid amplification in the presence of protein synthesis inhibitor i.e. chloramphenicol. • The vector comprises DNA derived from three different naturally occurring plasmids: the ampR gene is from R1 plasmid, tetR from R6-5 plasmid and the ori gene from pMB1 plasmid.
  13. 13. Features of pBR322
  14. 14. Ampicillin resistant? yes yes Tetracycline resistant? No yes B X B B B X Ampr ori Ampr Tcr ori pBR322 Ampr Tcr ori Screening by insertional inactivation of a resistance gene
  15. 15. Replica plating: transfer of the colonies from one plate to another using absorbent pad or Velvet transfer of colonies +ampicillin + ampicillin + tetracycline these colonies have bacteria with recombinant plasmid
  16. 16. Screening bacteria by replica plating
  17. 17. pBR327 • It was produced by removing a 1089bp segment from pBR322. • The ampR and tetR genes are intact. • It has high copy number than pBR322 i.e. 30-45 molecules per E. coli cell. Thus, more the copies of the cloned genes more will be the effect of the cloned gene on the host cell detectable. • The deletion destroys the conjugative ability of the vector which is important for biological containment.
  18. 18. FROM pBR322 to pUC • pBR322 requires double screening • pBR322 has limited number of restriction site For these reasons pUC (on the left) was engineered
  19. 19. pUC8- Lac selection plasmid • It is 2750bp in size and is one of the most popular E. coli cloning vectors. • Derived from pBR322 in which only the ori and the ampR genes remain. • The nucleotide sequence of ampR gene has been changed so that it no longer contains the unique restriction sites. • The restriction sites are clustered into the lac Z’ gene. • It has a high copy number of 500-700 molecules per cell even before amplification. • The identification of the recombinant cells can be achieved by a single step process i.e. by plating onto agar medium containing ampicillin and X-gal.
  20. 20. Conti- • The clustering of the restriction sites allows a DNA fragment with two different sticky ends to be cloned without resorting to additional manipulations. • pUC18/pUC19 is a vector having different combinations of restriction sites and provide greater flexibility for the DNA to be cloned. • The restriction site clusters in these vectors are the same as the clusters in M13mp series of vectors. Thus, the DNA cloned in pUC vectors can be transferred directly to its M13mp counterpart enabling the cloned gene to be obtained as a single stranded DNA.
  21. 21. Plasmid vectorsPlasmid vectors
  22. 22. The pUC18 cloning vector
  23. 23. Next Major Advance in Plasmid(ology)  The inclusion of polylinkers into plasmid vectors  Polylinker is a tandem array of restriction endonuclease sites in a very short expanse of DNA  For example, pUC18’s polylinker  Sites for 13 RE’s  Region spans the equivalent of 20 amino acids or 60 nucleotides
  24. 24. The Polylinker Advantage  Unique sites (usually)  Insert excision facilitated  Restriction endonuclease mapping and Subcloning made easier  Directional cloning
  25. 25. Blue-White screening for pUC18 • Colonies with recombinant plasmids are white, and colonies with nonrecombinant plasmids are blue. • Resistant to ampicillin, has (ampr gene) • Contains portion of the lac operon which codes for beta-galactosidase. • X-gal is a substrate of beta- galactosidase and turns blue in the presence of functional beta- galactosidase is added to the medium. • Insertion of foreign DNA into the polylinker disrupts the lac operon, beta-galactosidase becomes non- functional and the colonies fail to turn blue, but appear white.
  26. 26. Another Major Advance: Blue-White Screening
  27. 27. Alpha complementation LacZ Beta galactosidase (Homotetramer) • 1021aa 3,1 kbp • Bacteria carry mutant allele (LacZΔM15) lacking N- terminal domain  inactive protein • Alpha peptide carried by vector plasmid • MCS inserted into LacZ alpha peptide  With insert = white colonies Without insert = blue colonies • Exploits X-Gal (5-bromo-4- cloro-3-indonyl- Betagalactoside), a chromogenic substrate analog to galactose
  28. 28. What are advantages of pUC over pBR322? • Single step screening • MCS increases the number of potential cloning strategies available  X-gal (also abbreviated BCIG for bromo-chloro-indolyl-galactopyranoside) is an organic compound consisting of galactoside linked to indole.  X-gal is cleaved by β-galactosidase yielding galactose and 5-bromo-4- chloro-3-hydroxyindole.  The latter is then oxidized into 5,5'-dibromo-4,4'-dichloro-indigo, an insoluble blue product.  Thus, if X-gal and an inducer of β-galactosidase (usually IPTG) is contained within an agar medium on a culture plate, colonies which have a functional lacZ gene can easily be distinguished.
  29. 29. pGEM3Z • It is very similar to pUC vector and is of same size (2750bp). • It carries the ampR and Lac Z gene. The cluster of restriction sites is present in the Lac Z gene. • It has two promoter sequences i.e. T7 promoter (RNA polymerase of T7 bacteriophage) and SP6 (RNA polymerase of SP6 phage) promoter sequences that lie on the either side of the cluster of restriction sites. • These promoters act as the sites for the attachment of RNA polymerase. Thus if a recombinant pGEM3Z is mixed with RNA polymerase in a test tube, transcription occurs and RNA copies of the cloned fragment are synthesized.
  30. 30. Bacteriophage • These are the viruses that specifically infect bacteria and during infection inject the phage DNA into the host cell where it undergoes replication. • The phages are simple in structure and consist of DNA molecule having several genes for phage replication which is surrounded by a capsid made up of proteins.
  31. 31. Types of Phages • On the basis of structure Head and Tail phages: e.g. λ phage Filamentous phages: e.g. M13 • On the basis of phage infection cycle Lytic Phage: The infection cycle is completed very quickly and the release of new phage particles is associated with the lysis of the host cell Lysogenic phage: The phage DNA gets integrated into the bacterial DNA known as PROPHAGE and after many cell divisions released by the lysis of the host cell e.g. λ phage. Some phages do not form prophages and the new phage particles are continuously assembled and released from the host cell without the lysis of the host cell e.g. M13 phage.
  32. 32. λ Phage • It is 49kb in size and is used as a cloning vector because: The genes related in terms of function are clustered together in the genome and allows the genes to be switched on and off as a group rather than individually. The linear double stranded DNA molecule has a stretch of 12 nucleotides at its either ends which act as sticky ends or cohesive ends (cos sites) They can base pair to form a circular DNA molecule which is important for insertion into the bacterial genome. Another role of cos sites is in the formation of large number of λ DNA molecules by rolling circle mechanism of replication (Catenane).
  33. 33. λ Phage Cloning Vector • “Head and Tail” phage, very well-studied • Large, linear genome of ~49.0 kb • Two lifestyle modes – Lytic: replicative mode – Lysogenic: latent mode • It has a large size genome (49kb) and only 3kb new DNA can be inserted because if the size of the molecule is more than 52kb then it can not be packaged into the head of the phage. • The phage has more than one recognition sequence for almost all the restriction endonucleases. So the use of any restriction enzyme will break the phage DNA into number of small fragments. • Despite these disadvantages, λ phages are used to clone large DNA (5kb to 25kb) molecules.
  34. 34. Lytic and Lysogenic cycle
  35. 35. Recombination and Lysogeny Cos site: At the ends short (12bp) ss- complementary region “cohesive or sticky” ends--- circulation after infection Left Arm: Structural genes for head and tail Central Region: genes for lysogenic growth and recombination/insertion of genome into baterial genome Right Arm: genes involved in DNA replication and lytic growth Only 30 kb is required for lytic growth. Thus, one could clone 19 kb of “foreign” DNA. Packaging efficiency 78%-105% of the lambda genome.
  36. 36. Engineered version of bacteriophage λ (infects E. coli). Central region of the λ chromosome (linear) is cut with a restriction enzyme and digested DNA is inserted. DNA is packaged in phage heads to form virus particles. Phages with both ends of the λ chormosome and a 37-52 kb insert replicate by infecting E. coli. Phages replicate using E. coli and the lytic cycle. Produces large quantities of 37-52 kb cloned DNA. Like plasmid vectors, large number of restriction sites available; phage λ cloning vectors are useful for larger DNA fragments than pUC19 plasmid vectors. Phage λ cloning vectors:
  37. 37. But bacterial transformation with recombiant lambda phages is very ineffective In Vitro Packaging
  38. 38. DNA can be packaged into phage particle in vitro cos sequences
  39. 39. The packaged phage particles are infectious How to transfer recombinant lambda into cells?
  40. 40. The infection process is about thousand times more efficienct than transformation with plasmid vectors. 106 tansformed colonies per microgram of plasmid vector 109 plaques per microgram of recombinant Lambda vector
  41. 41. Strategies • For the construction of the cloning vector, the non-essential region (between positions 20 and 35) in the genome of the phage was removed which decreased the size of the molecule by 15kb. Thus, upto 18kb of new DNA can be inserted • By in vitro mutagenesis and by natural selection one or two restriction sites are removed e.g. EcoR1.
  42. 42. Insertion Vectors • In these vectors the non-essential region is deleted and the two arms ligated together. The vector possesses one unique restriction site. • λgt10: It can clone up to 8kb of new DNA inserted in the EcoR1 site located in the c1 gene. • λZAP11: In this vector up to 10kb of DNA can be inserted in to any of the 6 restriction sites within the polylinker. This inactivates the lac Z gene of the vector
  43. 43. Replacement Vectors • This vector has two recognition sites for the restriction endonuclease used for cloning. • These sites replace the segment of DNA (Stuffer fragment) from the vector genome by the DNA to be cloned. • These vectors can carry large pieces of DNA than insertion vectors. • λEMBL4: This vector can be used to insert up to 20kb of DNA.
  44. 44. λ Replacement vectors • Left arm: – head & tail proteins • Right arm: – DNA synthesis – regulation – host lysis • Deleted central region: – integration & excision – regulation
  45. 45. M13 Phage • It is 6407 nucleotides in length, circular and consists of a single stranded DNA molecule and is used as a cloning vector because It is less than 10kb in size. The double stranded replicative form of the genome acts like a plasmid. Genes cloned can be obtained in the form of single stranded DNA which is helpful in gene sequencing and in vitro mutagenesis. It is easily prepared from the culture of infected E. coli cells. Single-stranded, circular genome, 6.4 kb Infect only F+ bacteria, using pilus F- coded Can clone pieces of DNA up to 6X the M13 genome size (36 kb) -- but the larger the DNA, the less stable the clone is….. • Drawback: foreign DNA can be unstable (slows down host cell growth, so deletions confer a selective advantage)
  46. 46. M13 Phage Cloning Vector • The M13 genome is 6.4kb in length • Consists of ten closely packed genes for replication of the phage. • There is a single 507 nucleotide intergenic sequence (IS) into which new DNA can be inserted • This region includes the ori gene Useful for – Sequencing – Site-directed mutagenesis (later) – Any other technique that requires single stranded DNA
  47. 47. M13 structure Used in ‘phage display’ techniques
  48. 48. M13 life cycle: an overview ss ss ds Isolate for cloning
  49. 49. M13 doesn’t lyse cells, but it does slow them down M13 infections form plaques, but they are “turbid” “lawn” of E. coli
  50. 50. M13 Vectors • M13mp1-The lac Z gene was introduced in the IS and it does not have any unique restriction site in the gene. • M13mp2- This vector was constructed by including the EcoR1 site in the lac Z gene. This was done by a single nucleotide change in GGATTC near the start of the lac Z gene to GAATTC by in vitro mutagenesis. The β-galactosidase enzyme remains functional in the vector. • M13mp7- It was formed by inserting a polylinker into the EcoR1 site of lac Z gene of M13mp2. A polylinker is a short nucleotide sequence that consists of number of restriction sites (EcoR1, BamH1, Sal1 and Pst1) and has a EcoR1 sticky ends. This polylinker does not disrupt the function of Lac Z gene.
  51. 51. M13 mp18: engineered for alpha complementation
  52. 52. Bacteriophage vectors Advantages: – Useful for cloning large DNA fragments (10 - 23 kb) – Inherent size selection for large inserts Disadvantages: – Less easy to handle Uses of Bacteriophages: Lambda phage cloning vectors: • For gene cloning of large DNA fragments (eukaryotic genes) • Excellent selection capability (stuffer stuff) • Clone lots of precisely-sized DNA fragments for library construction M13 based cloning vectors: • Single-stranded DNA • Sequencing • Site-directed mutagenesis
  53. 53. Phagemids/Phasmids: plasmid/F1M13 hybrids A phagemid or phasmid is a plasmid that contains an f1 origin of replication from an f1 phage.  It can be used as a type of cloning vector in combination with filamentous phage M13.  A phagemid can be replicated as a plasmid, and also be packaged as single stranded DNA in viral particles.  Phagemids contain an origin of replication (ori) for double stranded replication, as well as an f1 ori to enable single stranded replication and packaging into phage particles.  Many commonly used plasmids contain an f1 ori and are thus phagemids.  Similarly to a plasmid, a phagemid can be used to clone DNA fragments and be introduced into a bacterial host by a range of techniques, such as transformation and electroporation.  However, infection of a bacterial host containing a phagemid with a 'helper' phage, for example VCSM13 or M13K07, provides the necessary viral components to enable single stranded DNA replication and packaging of the phagemid DNA into phage particles.  The 'helper' phage infects the bacterial host by first attaching to the host cell's pilus and then, after attachment, transporting the phage genome into the cytoplasm of the host cell.  Inside the cell, the phage genome triggers production of single stranded phagemid DNA in the cytoplasm. This phagemid DNA is then packaged into phage particles.
  54. 54. Phagemids/Phasmids: plasmid/M13 hybrids The phage particles containing ssDNA are released from the bacterial host cell into the extracellular environment.  Filamentous phages retard bacterial growth but, contrasting with the lambda phage and the T7 phage, are not generally lytic.  Helper phages are usually engineered to package less efficiently (via a defective phage origin of replication) than the phagemid so that the resultant phage particles contain predominantly phagemid DNA.  F1 Filamentous phage infection requires the presence of a pilus so only bacterial hosts containing the F-plasmid or its derivatives can be used to generate phage particles.
  55. 55.  A cosmid is  a  type  of  hybrid plasmid that  contains  a Lambda phage  cos sequence.   Cosmids (cos sites + plasmid = cosmids) DNA sequences are originally from  the lambda phage.  They are often used as a cloning vector in genetic engineering.   Cosmids can be used to build genomic libraries.   They were first described by Collins and Hohn in 1978.  Cosmids  can  contain  37  to  52  (normally  45) kb of  DNA,  limits  based  on  the  normal bacteriophage packaging size.   They can replicate as plasmids if they have a suitable origin of replication: for  example SV40 ori in  mammalian  cells, ColE1 ori for  double-stranded  DNA  replication or f1 ori for single-stranded DNA replication in prokaryotes.   They frequently also contain a gene for selection such as antibiotic resistance ,  so  that  the  transformed  cells  can  be  identified  by  plating  on  a  medium  containing the antibiotic. Those cells which did not take up the cosmid would  be unable to grow.[3]  Unlike  plasmids,  they  can  also  be  packaged  in phage capsids,  which  allows  the foreign genes to be transferred into or between cells by transduction.   Plasmids become unstable after a certain amount of DNA has been inserted  into them, because their increased size is more conducive to recombination.  To circumvent this, phage transduction is used instead. This is made possible  by the cohesive ends, also known as cos sites. In this way, they are similar to  using the lambda phage as a vector, except all the lambda genes have been  deleted with the exception of the cos sequence. Cosmids
  56. 56. Features  of  both  plasmid  and  lambda phage cloning vectors. Circular. Do not occur naturally Origin (ori) sequence for E. coli. Selectable marker, e.g. ampR . Restriction sites (for cloning). Contain Phage λ (lambda) cos sites  which  permits  packaging  into  λ  phage  heads  and  therefore  introduction to E. coli cells. Packaging only occurs with 37-52 kb  fragments  -  selection  for  large  fragments Useful for 37-52 kb. Packaged DNA is inserted into cells  and  then  replicates  as  a  very  large  plasmid Cosmids
  57. 57. Cloning in a cosmid Desired ligation Products-- these are packaged
  58. 58. Cloning in a cosmid Instead of transformation, desired ligation products are packaged and then transfected into cells Selection for colonies, not screening of plaques (not infectious)
  59. 59. 1. Based  on  the  E. coli bacterial F-plasmid. 2. Can  insert  40  kb  fragment of DNA. 3. Low  copy  number  in  the  host (e.g., 1 fosmid). 4.Fosmids  offer  higher  stability than comparable  high  copy  number  cosmids.  5. Contain  other  features  similar  to  plasmids/cosmids such as  origin  sequence  and  polylinker. Fosmid:
  60. 60. Vectors  that  enable  artificial  chromosomes  to  be  created  and  cloned into E. coli. Features: Useful  for  cloning  up  to  200-300  kb,  but  can  be  handled  like  regular bacterial plasmid vectors. Useful  for  sequencing  large  stretches  of  chromosomal  DNA;  frequently used in genome sequencing projects. Like other vectors, BACs contain: Origin (ori) sequence derived from an E. coli plasmid called the  F factor. Multiple cloning sites (restriction sites). Selectable markers (antibiotic resistance). Bacterial Artificial Chromosomes (BACs):
  61. 61. BACs: Bacterial Artificial Chromosomes  Based on the F factor of E. coli: 100 kb plasmid, propagates through conjugation low copy number (1-2 copies per cell) 2  genes  (parA  and  parB):  accurate  partitioning  during  cell  division   BACs:  just  have  par  genes,  replication  ori,  cloning  sites,  selectable marker  Can propagate very large pieces of DNA: up to 300 kb   Relatively  easy  to  manipulate:  move  into  cells  by  transformation (electroporation)
  62. 62. General BAC vector replication selection Cloning, etc 7 kb
  63. 63. • Vectors  that  enable  artificial  chromosomes  to  be  created and cloned into yeast. • Based on the chromosome of Yeast Features: • CEN1, centromere sequencesegregation • TEL, telomere sequencesextremity protection • ARS1, autonomous replicating sequencereplication • Selectable  marker  (amino  acid  dependence,  etc.)  on  each arm. • Amp • ori,  origin  of  replication  for  propagation  in  an  E. coli  host. • Restriction sites (for DNA ligation). • Acquiring 150kbp it acquires chromosome like features • SUP4 gene, a suppressor tRNA gene which overcomes  the  effect  of  the  ade-2  ochre  mutation  and  restores  wild-type activity, resulting in colorless colonies.  • The host cells are also designed to have recessive  trp1  and  ura3  alleles  which  can  be  complemented  by  the  corresponding  TRP1  and  URA3  alleles  in  the  vector,  providing  a  selection  system  for  identifying  cells  containing the YAC vector.  • Useful for cloning very large DNA fragments up to 500  kb; useful for very large DNA fragments. YACs: Yeast Artificial Chromosomes
  64. 64. DESADVANTAGES OF YAC • Very fragile and prone to breakage,  • Unstable, with their foreign DNA inserts often being deleted • Loss of the entire YAC during mitotic growth • Difficult to separate the YAC from the other host chromosomes • The yield of DNA is not high • Chimaerism
  65. 65. 1. Capable of replicating in two or more types of hosts. 2. Replicate autonomously, or integrate into the host genome and replicate  when the host replicates. 3. Commonly used for transporting genes from one organism to another (i.e.,  transforming animal and plant cells). http://cwx.prenhall.com/bookbind/pubboo ks/horton3/medialib/media_portfolio/23.ht ml Example: *Insert firefly luciferase gene into plasmid and transform Agrobacterium. *Grow Agrobacterium in large quantities and infect tobacco plant. Shuttle vectors:
  66. 66. What determines the choice of vector • insert size vector sizevector size restriction sitesrestriction sites copy numbercopy number cloning efficiencycloning efficiency ability to screen for insertsability to screen for inserts
  67. 67. Not all vectors permit the identification of the desired clones by simple selection or color based strategies. In the majority of cases we need alternative approaches!!!!
  68. 68. Thanks Acknowledgement: All the material/presentations available online on the subject are duly acknowledged. Disclaimer: The author bear no responsibility with regard to the source and authenticity of the content. Questions???

Editor's Notes

  • In molecular biology and microbiology, replica plating is a technique in which one or more secondary Petri plates containing different solid (agar-based) selective growth media (lacking nutrients or containing chemical growth inhibitors such as antibiotics) are inoculated with the same colonies of microorganisms from a primary plate (or master dish), reproducing the original spatial pattern of colonies. The technique involves pressing a velvet-covered disk to a primary plate, and then imprinting secondary plates with cells in colonies removed by the velvet from the original plate. Generally, large numbers of colonies (roughly 30-300) are replica plated due to the difficulty in streaking each out individually onto a separate plate.
    The purpose of replica plating is to be able to compare the master plate and any secondary plates to screen for a selectable phenotype. For example, a colony which appeared on the master plate but failed to appear at the same location on a secondary plate shows that the colony was sensitive to a substance on that particular secondary plate. Common screenable phenotypes include auxotrophy and antibiotic resistance.
    Replica plating is especially useful for negative selection. For example, if one wanted to select colonies that were sensitive to ampicillin, the primary plate could be replica plated on a secondary Amp+ agar plate . The sensitive colonies on the secondary plate would die but the colonies could still be deduced from the primary plate since the two have the same spatial patterns from ampicillin resistant colonies. The sensitive colonies could then be picked off from the primary plate.
    By increasing the variety of secondary plates with different selective growth media, it is possible to rapidly screen a large number of individual isolated colonies for as many phenotypes as there are secondary plates.
    This technique was first described in the context of the Luria-Delbruck experiment in 1943.
    Retrieved from &amp;quot;http://en.wikipedia.org/wiki/Replica_plating&amp;quot;
  • X-gal (also abbreviated BCIG for bromo-chloro-indolyl-galactopyranoside) is an organic compound consisting of galactoside linked to indole.
    X-gal is cleaved by β-galactosidase yielding galactose and 5-bromo-4-chloro-3-hydroxyindole.
    The latter is then oxidized into 5,5&amp;apos;-dibromo-4,4&amp;apos;-dichloro-indigo, an insoluble blue product. Thus, if X-gal and an inducer of β-galactosidase (usually IPTG) is contained within an agar medium on a culture plate, colonies which have a functional lacZ gene can easily be distinguished.
  • ×