Characteristics of Cosmids

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Questions and Answers

What is the typical range of DNA that can be contained in a cosmid?

  • 1-5 kb
  • 100-150 kb
  • 200-250 kb
  • 37-52 kb (correct)

What is the function of an origin of replication in a cosmid?

  • To initiate replication (correct)
  • To initiate transcription
  • To provide antibiotic resistance
  • To regulate gene expression

How do cosmids typically replicate in cells?

  • As a virus
  • As a phage
  • As a chromosome
  • As a plasmid (correct)

What is the purpose of a selectable marker in a cosmid?

<p>To identify transformed cells (D)</p> Signup and view all the answers

What is the name of the sequence in a cosmid that is derived from lambda phage?

<p>cos sequence (C)</p> Signup and view all the answers

What is the shape of the cosmid DNA inside the phage capsid?

<p>Linear (B)</p> Signup and view all the answers

What is the primary function of the cosN site in cosmid DNA?

<p>Linearizing the circular cosmid with two 'cohesive' or 'sticky ends' of 12bp (C)</p> Signup and view all the answers

What is the maximum size of DNA that can be carried by a cosmid?

<p>45 kb (A)</p> Signup and view all the answers

What is the advantage of using cosmids in gene cloning?

<p>They can pick up large DNA fragments, allowing for the establishment of gene libraries (C)</p> Signup and view all the answers

What is a limitation of using cosmids as vectors?

<p>They cannot accept more than 50 kb of the insert (A)</p> Signup and view all the answers

What is the role of the cosB site in cosmid DNA?

<p>Holding the terminase while it is cutting and separating the strands (C)</p> Signup and view all the answers

What is a disadvantage of using cosmids compared to plasmids?

<p>They have slower replication (A)</p> Signup and view all the answers

What is the primary function of cos sites in cosmid DNA?

<p>To enable the packaging of DNA into λ phage particles (B)</p> Signup and view all the answers

What is the origin of replication in cosmid DNA?

<p>From plasmid DNA (D)</p> Signup and view all the answers

What is the characteristic of the two single-stranded regions at the ends of cosmid DNA?

<p>They are complementary and form a duplex (B)</p> Signup and view all the answers

What is the purpose of the protein coat of bacteriophage in cosmid DNA?

<p>To form inactive phage particles for transduction (B)</p> Signup and view all the answers

What is the consequence of the lack of structural and regulatory genes of lambda DNA in cosmid DNA?

<p>No lysis and integration of cosmid DNA in the host cell (D)</p> Signup and view all the answers

What is the size range of DNA fragments that can be cloned using cosmids?

<p>25 to 45 kb (A)</p> Signup and view all the answers

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Study Notes

Cosmid Vectors

  • A cosmid is a type of hybrid plasmid that contains a bacterial "ori" sequence and a Lambda phage cos sequence.
  • Cosmids are formed by joining the ends of a linearized plasmid DNA with the cos site of lambda DNA.
  • They are used as a cloning vector in genetic engineering and can be used to build genomic libraries.

Characteristics of Cosmids

  • Cosmids can contain 37 to 52 kb of DNA, with a normal range of 45 kb.
  • They can replicate as plasmids if they have a suitable origin of replication (ori).
  • They frequently contain a gene for selection, such as antibiotic resistance.

Cos Sequences

  • Cos sequences are ~200 base pairs long and essential for packaging.
  • They contain three important sites: cosN, cosB, and cosQ.

Advantages of Cosmids

  • High transformation efficiency and capable of producing a large number of clones from a small quantity of DNA.
  • Can carry up to 45 kb of insert compared to 25 kb carried by plasmids and λ.
  • Used to establish gene libraries and help in the study of non-sense sequences in the genome of organisms.

Disadvantages of Cosmids

  • Cannot accept more than 50 kb of the insert.
  • Packaging enzyme fails to pack recombinant cosmids into the phage head if any one of the two cos-ends is missing.
  • Packaging enzyme fails to pack the DNA into the phage head if more than one recombinant cosmid joins together to form a large DNA.
  • Slower replication.
  • Lack structural and regulatory genes of lambda DNA, hence no lysis and integration of cosmid DNA in the host cell.

Examples of Cosmids

  • Col EI cosmid
  • pHC 79
  • pJB8
  • pWE cosmid

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