18 Questions
What is the typical range of DNA that can be contained in a cosmid?
37-52 kb
What is the function of an origin of replication in a cosmid?
To initiate replication
How do cosmids typically replicate in cells?
As a plasmid
What is the purpose of a selectable marker in a cosmid?
To identify transformed cells
What is the name of the sequence in a cosmid that is derived from lambda phage?
cos sequence
What is the shape of the cosmid DNA inside the phage capsid?
Linear
What is the primary function of the cosN site in cosmid DNA?
Linearizing the circular cosmid with two 'cohesive' or 'sticky ends' of 12bp
What is the maximum size of DNA that can be carried by a cosmid?
45 kb
What is the advantage of using cosmids in gene cloning?
They can pick up large DNA fragments, allowing for the establishment of gene libraries
What is a limitation of using cosmids as vectors?
They cannot accept more than 50 kb of the insert
What is the role of the cosB site in cosmid DNA?
Holding the terminase while it is cutting and separating the strands
What is a disadvantage of using cosmids compared to plasmids?
They have slower replication
What is the primary function of cos sites in cosmid DNA?
To enable the packaging of DNA into λ phage particles
What is the origin of replication in cosmid DNA?
From plasmid DNA
What is the characteristic of the two single-stranded regions at the ends of cosmid DNA?
They are complementary and form a duplex
What is the purpose of the protein coat of bacteriophage in cosmid DNA?
To form inactive phage particles for transduction
What is the consequence of the lack of structural and regulatory genes of lambda DNA in cosmid DNA?
No lysis and integration of cosmid DNA in the host cell
What is the size range of DNA fragments that can be cloned using cosmids?
25 to 45 kb
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
Learn about the features and properties of cosmids, a type of vector used in molecular biology. Understand their structure, function, and applications in gene cloning. Discover how cosmids are used to incorporate large DNA fragments into λ phage particles.
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