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Vectors a.k.saha

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Page 2: Vectors a.k.saha

Dr. A.K. SahaProfessor

Department of ZoologyUniversity of Rajshahi

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Vector is an agent that can carry a DNA fragment into

a host cell. If it is used for reproducing the DNA

fragment, it is called a “cloning vector.” If it is used

for expressing certain gene in the DNA fragment, it is

called an “expression vector”.

Commonly used vectors include Plasmid, Lambda

phage, Cosmid and Yeast artificial chromosome

(YAC).

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The cosmid can be defined as the hybrid derived from

plasmids which contain cos site of phage λ (cosmid =

cos site + plasmid).

A cosmid, first described by Collins and Hohn in 1978,

is a type of hybrid plasmid (often used as a cloning

vector) that contains cos sequences, DNA sequences

originally from the Lambda phage.

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Characteristics of Cosmid

Cosmids are able to contain 37 to 52 kbp of DNA, while normal plasmids are able to carry only 1-20 kbp

They can replicate as plasmids if they have a suitable origin of replication.

They frequently also contain a gene for selection such as antibiotic resistance.

Those cells which did not take up the cosmid would be unable to grow.

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1. Origin of replication.

2. A marker gene coding for antibiotic resistance.

3. A special cleavage site for the insertion of foreign DNA and

4. Small in size.

Special Features

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Figure. A typical plasmid vector. It contains a polylinker site, an ampicillin resistance gene (ampr) and a replication origin (ORI).

Polylinker

pUC192686 bp

ORIampr

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The YAC vector is capable of carrying a large DNA fragment (up to 2 Mb), but its transformation efficiency is very low.

Yeast Artificial Chromosome (YAC)

CEN= Centromeres

TEL= Telomeres

ARS=Autonomous replicating sequence

Figure. Cloning by the yeast artificial chromosome (YAC) vector.

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Centromers (CEN), telomeres (TEL) and autonomous replicating sequence (ARS) for proliferation in the host cell.ampr for selective amplification and markers such as TRP1 and URA3 for identifying cells containing the YAC vector.Recognition sites of restriction enzymes (e.g., EcoRI and BamHI).

Procedure :

1. The target DNA is partially digested by EcoRI and the YAC vector is cleaved by EcoRI and BamHI.2. Ligate the cleaved vector segments with a digested DNA fragment to form an artificial chromosome.3. Transform yeast cells to make a large number of copies.

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Phages are viruses that can infect bacteria. The major advantage of the phage vector is its high transformation efficiency, about 1000 times more efficient than the plasmid vector.

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Plasmids are extrachromosomal circular DNA molecules found in most bacterial species and in some species of eukaryotes.Many important bacterial genes are not part of the main chromosome but are on separate circles of DNA called plasmids.

A plasmid is a molecule which can be stably inherited without being linked to the chromosome.

Plasmids were originally defined as ‘‘extrachromosomal, hereditary determinants’’(Lederberg,1952). They are generally covalently closed circular (CCC) molecules of double-stranded deoxyribonucleic acid (DNA) that very in length from 1 kbp to 700 kbp. And occur in the majority of bacterial cells.

Plasmids

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Fig. The structure of pBR322 showing the unique cleavage sites.

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CHARACTERSTICS OF PLASMID

Plasmids are important in medicine and in agriculture

because they confer antibiotic resistance on pathogens of

animals and man, and because they can code for toxins

and other proteins which increase the virulence of these

pathogens.

Nitrogen Fixation:

Plasmids enable species of Rhizobium to fix nitrogen in the

nodules of leguminous plants.

Antibiotic Production:

Plasmid also code for antibiotics which can be used to

control pathogenic bacteria.

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Biodegradation:

Plasmids genes code for a wide range of metabolic

activities and enable bacteria to degrade compounds

which would accumulates as pollutants.

Recombinant DNA:

Plasmid can be isolated quite easily from bacterial cells.

New genes from other species can be inserted into

isolated plasmids and the modified plasmid then can be

put back into its normal host cell. Such “recombinant

DNAs” are transcribed, and translated into potentially

useful products.

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Figure: Two super-coiled plasmid DNA molecules

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TRANSFER OF PLASMID DNA

1. Formation of specific donar -recipient pairs (effective contact)

2. Preparation for DNA transfer (mobilization)

3. DNA transfer

4. Formation of a replicative functional plasmid in the recipient (replication)

The Plasmid transfer process can be divided into four stages:

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Figure: Plasmid transfer by conjugation between bacterial cells. The donor and recipient cells attach to each other by a pilus, an appendage present on the surface of the donor cell. A copy of the plasmid is then passed through the pilus to the recipient cell.

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TYPES OF PLASMIDS

Stringent or Low copy number plasmid: Which may range from 1 or 2.

Relaxed or high copy number plasmids: Range from 10-100.

Many types of plasmids are found in a variety

of E. coli strains but three main types- F, R,

and Col plasmids have been studied.

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1. F, the sex plasmid: Ability to transfer chromosomal genes (that is, genes not carried on the plasmid) and the ability to transfer F itself to a cell lacking the plasmid.

2. R, the drug-resistance plasmid: Resistance to one or more antibiotics and often the ability to transfer the resistance to cells lacking R.

3. Col, the colcigenic plasmid: Ability to synthesize colicins- that is, proteins capable of killing closely related bacterial strains that lack the Col plasmid.

On the basis of Co-existence:

1. Compatible plasmid: Bacteria often contain two or more different plasmid which can co-exist, and are said to be compatible

2. Incompatible plasmid: Can not co-exist together.

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(a)

Bacterial chromosome

Plasmids

Cell division

Bacterial chromosome

Plasmids

Cell division

Chromosome carrying integrated plasmid

(b)

Figure: Replication strategies for (a) a non-integrative plasmid, and (b) an episome

B

Bacterial chromosome

Plasmids

Cell division

Bacterial chromosome

Plasmids

Cell division

Chromosome carrying integrated plasmid

B

A

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Donor Recipient

Pilus

Pilus

DNA polymerase

Copying of transferred strand in recipient

Chromosomal DNA

Chromosomal DNAF- plasmid

F- plasmid

New DonorOld Donor

F- plasmid

Figure: A model for transfer of F plasmid DNA from an F+ cell by a looped rolling-circle mechanism

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A new copy of F, generated by replication, is transferred to a recipient cell

A copy of F remains in the donor cell cell

(c)

(a) (b)

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A. For-high-copy number plasmids

B. For-low-copy number plasmids

BA

Plasmid replication

Cell division and random

segregation daughter cells

Cell division and partitioning into plasmid daughter

cells

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Chromosomal DNA is much larger than the DNA of

plasmids.

Chromosomal DNA extracted from cells is

obtained as broken, linear molecules.

Most plasmid DNA is extracted in a covalently

closed, circular form.

Differences between chromosomal and plasmid DNA

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Plasmids as Tools for Molecular Biology

They are used as a vectors to clone DNA. A variety of different enzymes can be used to insert pieces of DNA, from animals, plants or prokaryotes, into plasmids.

Circular molecules consisting partly of plasmid DNA and partly of inserted DNA can then be put back into a suitable bacterium.

The plasmids replicate during bacterial growth so that many copies of the cloned DNA can easily be obtained.

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The Agrobacterium Plasmid Ti

This plasmid has recently become very important in plant

breeding because specific genes can be inserted into Ti

plasmid by recombinant DNA techniques, and sometimes

these genes can become integrated into plant

chromosome, thereby permanently changing the genotype

and phenotype of the plant. New plant varieties having

desirable and economically valuable characteristics

derived from unrelated species can be developed in this

way.

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The Agrobacterium Plasmid Ti

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Role of Ti plasmid

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Transgenic plant by Ti plasmid

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