NCERT Textbook: Host–Vector System | Biotechnology for Class 12 - NEET PDF Download

 Page 1


Recombinant DNA technology is a two component system 
involving a compatible host and a vector. A variety of hosts 
as well as vectors are available for gene cloning. In this 
chapter, students will be exposed to the fundamentals 
of different kinds of prokaryotic and eukaryotic host and 
vectors used in rDNA technology.
2.1 Two Key Componen Ts of Re Combinan T 
Dna Te Chnology As discussed in the previous chapter (Chapter 1), rDNA 
technology refers to joining two different DNA molecules with 
an aim to isolate, propagate, characterise and manipulate 
the genes for various applications. This technique involves 
two major steps (Fig. 2.1). In the ??rst step, the desired 
DNA molecule, generally termed as insert (target gene), 
is isolated from the source. In the second step, this target 
gene is inserted into a convenient carrier DNA molecule 
called vector. The vector containing the insert is called 
recombinant DNA (rDNA). Subsequently, the rDNA is 
introduced into an organism referred to as host. Using 
Host–Vector System
2.1 Two Key 
Components of 
Recombinant DNA 
Technology 
2.2 Host
2.3 Vector
2.4 Eukaryotic Host 
Vector System
2.5 Expression Vectors
2.6 Shuttle Vectors
2
Chapter 
Chapter 2_Host vector system.indd   9 23-01-2025   11:20:25
Reprint 2025-26
Page 2


Recombinant DNA technology is a two component system 
involving a compatible host and a vector. A variety of hosts 
as well as vectors are available for gene cloning. In this 
chapter, students will be exposed to the fundamentals 
of different kinds of prokaryotic and eukaryotic host and 
vectors used in rDNA technology.
2.1 Two Key Componen Ts of Re Combinan T 
Dna Te Chnology As discussed in the previous chapter (Chapter 1), rDNA 
technology refers to joining two different DNA molecules with 
an aim to isolate, propagate, characterise and manipulate 
the genes for various applications. This technique involves 
two major steps (Fig. 2.1). In the ??rst step, the desired 
DNA molecule, generally termed as insert (target gene), 
is isolated from the source. In the second step, this target 
gene is inserted into a convenient carrier DNA molecule 
called vector. The vector containing the insert is called 
recombinant DNA (rDNA). Subsequently, the rDNA is 
introduced into an organism referred to as host. Using 
Host–Vector System
2.1 Two Key 
Components of 
Recombinant DNA 
Technology 
2.2 Host
2.3 Vector
2.4 Eukaryotic Host 
Vector System
2.5 Expression Vectors
2.6 Shuttle Vectors
2
Chapter 
Chapter 2_Host vector system.indd   9 23-01-2025   11:20:25
Reprint 2025-26
Biotechnology XII 10
Fig. 2.1: Schematic representation of gene cloning
Vector
Restriction
digestion
Target gene
(Insert)
Foreign DNA
Ligation
Vector containing
target gene
(Recombinant DNA)
Recombinant DNA
Transformation
Host cell
Propagation
Chromosomal
DNA
Recombinant
DNA 
multiplied
genetic machinery of the host, the rDNA 
undergoes propagation and expression. 
This whole process of rDNA technology 
is covered under the term ‘gene cloning’. 
Thus, gene cloning may be considered as 
a two component system: a compatible 
host and a vector, where the vector 
provides essential sequences required 
for its replication in a compatible host, 
which provides various replication 
functions (enzymes and proteins).
2.2 h os T
A large number of host organisms, both 
prokaryotic and eukaryotic are used for 
gene cloning (Fig. 2.2). A host should 
allow easy entry of the rDNA into the cell 
and should not consider the recombinant 
DNA as a foreign DNA and degrade it. 
The host must supply all the required 
enzymes and proteins for smooth 
replication of the vector DNA along with 
insert. A wide variety of genetically de??ned strains are 
available as hosts. 
Among prokaryotic hosts, E. coli is the most extensively 
used. Typical E. coli is a rod-shaped Gram-negative 
bacterium commonly found in the lower intestine of warm-
blooded organisms. It is able to reproduce and grow rapidly, 
doubling its population about every 20 mins. K12 strain 
of E. coli is one of the most commonly used hosts in gene 
cloning. Other prokaryotic hosts have also been developed. 
For example, Bacillus subtilis constitutes an important 
alternative host, where the aim is secretion of a protein 
encoded by a cloned gene. Among eukaryotic hosts, the 
most widely used is yeast.
2.3 Ve CTo R
In principle, any molecule of DNA, which can replicate itself 
inside a host cell, can work as a vector for gene cloning. 
However, in order for a plasmid to act as vector, it must 
Chapter 2_Host vector system.indd   10 23-01-2025   11:20:26
Reprint 2025-26
Page 3


Recombinant DNA technology is a two component system 
involving a compatible host and a vector. A variety of hosts 
as well as vectors are available for gene cloning. In this 
chapter, students will be exposed to the fundamentals 
of different kinds of prokaryotic and eukaryotic host and 
vectors used in rDNA technology.
2.1 Two Key Componen Ts of Re Combinan T 
Dna Te Chnology As discussed in the previous chapter (Chapter 1), rDNA 
technology refers to joining two different DNA molecules with 
an aim to isolate, propagate, characterise and manipulate 
the genes for various applications. This technique involves 
two major steps (Fig. 2.1). In the ??rst step, the desired 
DNA molecule, generally termed as insert (target gene), 
is isolated from the source. In the second step, this target 
gene is inserted into a convenient carrier DNA molecule 
called vector. The vector containing the insert is called 
recombinant DNA (rDNA). Subsequently, the rDNA is 
introduced into an organism referred to as host. Using 
Host–Vector System
2.1 Two Key 
Components of 
Recombinant DNA 
Technology 
2.2 Host
2.3 Vector
2.4 Eukaryotic Host 
Vector System
2.5 Expression Vectors
2.6 Shuttle Vectors
2
Chapter 
Chapter 2_Host vector system.indd   9 23-01-2025   11:20:25
Reprint 2025-26
Biotechnology XII 10
Fig. 2.1: Schematic representation of gene cloning
Vector
Restriction
digestion
Target gene
(Insert)
Foreign DNA
Ligation
Vector containing
target gene
(Recombinant DNA)
Recombinant DNA
Transformation
Host cell
Propagation
Chromosomal
DNA
Recombinant
DNA 
multiplied
genetic machinery of the host, the rDNA 
undergoes propagation and expression. 
This whole process of rDNA technology 
is covered under the term ‘gene cloning’. 
Thus, gene cloning may be considered as 
a two component system: a compatible 
host and a vector, where the vector 
provides essential sequences required 
for its replication in a compatible host, 
which provides various replication 
functions (enzymes and proteins).
2.2 h os T
A large number of host organisms, both 
prokaryotic and eukaryotic are used for 
gene cloning (Fig. 2.2). A host should 
allow easy entry of the rDNA into the cell 
and should not consider the recombinant 
DNA as a foreign DNA and degrade it. 
The host must supply all the required 
enzymes and proteins for smooth 
replication of the vector DNA along with 
insert. A wide variety of genetically de??ned strains are 
available as hosts. 
Among prokaryotic hosts, E. coli is the most extensively 
used. Typical E. coli is a rod-shaped Gram-negative 
bacterium commonly found in the lower intestine of warm-
blooded organisms. It is able to reproduce and grow rapidly, 
doubling its population about every 20 mins. K12 strain 
of E. coli is one of the most commonly used hosts in gene 
cloning. Other prokaryotic hosts have also been developed. 
For example, Bacillus subtilis constitutes an important 
alternative host, where the aim is secretion of a protein 
encoded by a cloned gene. Among eukaryotic hosts, the 
most widely used is yeast.
2.3 Ve CTo R
In principle, any molecule of DNA, which can replicate itself 
inside a host cell, can work as a vector for gene cloning. 
However, in order for a plasmid to act as vector, it must 
Chapter 2_Host vector system.indd   10 23-01-2025   11:20:26
Reprint 2025-26
Host-Vector System
11
ful??l the following features:
1. For easy incorporation into the host cell, a vector 
should itself be small in size and be able to integrate a 
large size of the insert.
2. The vector should have an origin of replication or ori, 
so that the vector is capable of autonomous replication 
inside the host organism. 
3. The vector needs to have unique restriction sites. If it 
possesses too many restriction sites, then it would be 
fragmented into several pieces.
4. The vector needs to have at least one selectable 
marker. The selectable markers are required to screen 
out transformants [For example: resistance to 
antibiotics such as tetracycline (tet
R
) and ampicillin 
(amp
R
)].
A variety of both prokaryotic as well as eukaryotic vectors are 
given in (Fig. 2.2).
2.3.1 Plasmid as a vector
Plasmids are circular, double-stranded (ds) and extra 
chromosomal DNAs capable of autonomous replication. 
They occur naturally in many bacteria, archaea, and even in 
eukaryotes, such as yeast. Size of plasmids ranges from a few 
thousand base pairs to more than 100 kilobase pairs (kbp). 
Like the host-cell chromosomal DNA, plasmid DNA is replicated 
before every cell division. During cell division, atleast one copy of 
the plasmid DNA is segregated to each daughter cell, assuring 
Two component system
of gene cloning
Yeast
Plant Cells
Animal Cells
E. coli
Prokaryotic Prokaryotic Eukaryotic Eukaryotic
Yeast (YAC)
Plants (pTi)
Animals (Vaccinia)
Plasmid (pBR322)
Bacteriophage () ?
Host Vector
Fig. 2.2: Two component system of gene cloning showing various kinds of eukaryotic and 
prokaryotic hosts and vectors
Chapter 2_Host vector system.indd   11 23-01-2025   11:20:26
Reprint 2025-26
Page 4


Recombinant DNA technology is a two component system 
involving a compatible host and a vector. A variety of hosts 
as well as vectors are available for gene cloning. In this 
chapter, students will be exposed to the fundamentals 
of different kinds of prokaryotic and eukaryotic host and 
vectors used in rDNA technology.
2.1 Two Key Componen Ts of Re Combinan T 
Dna Te Chnology As discussed in the previous chapter (Chapter 1), rDNA 
technology refers to joining two different DNA molecules with 
an aim to isolate, propagate, characterise and manipulate 
the genes for various applications. This technique involves 
two major steps (Fig. 2.1). In the ??rst step, the desired 
DNA molecule, generally termed as insert (target gene), 
is isolated from the source. In the second step, this target 
gene is inserted into a convenient carrier DNA molecule 
called vector. The vector containing the insert is called 
recombinant DNA (rDNA). Subsequently, the rDNA is 
introduced into an organism referred to as host. Using 
Host–Vector System
2.1 Two Key 
Components of 
Recombinant DNA 
Technology 
2.2 Host
2.3 Vector
2.4 Eukaryotic Host 
Vector System
2.5 Expression Vectors
2.6 Shuttle Vectors
2
Chapter 
Chapter 2_Host vector system.indd   9 23-01-2025   11:20:25
Reprint 2025-26
Biotechnology XII 10
Fig. 2.1: Schematic representation of gene cloning
Vector
Restriction
digestion
Target gene
(Insert)
Foreign DNA
Ligation
Vector containing
target gene
(Recombinant DNA)
Recombinant DNA
Transformation
Host cell
Propagation
Chromosomal
DNA
Recombinant
DNA 
multiplied
genetic machinery of the host, the rDNA 
undergoes propagation and expression. 
This whole process of rDNA technology 
is covered under the term ‘gene cloning’. 
Thus, gene cloning may be considered as 
a two component system: a compatible 
host and a vector, where the vector 
provides essential sequences required 
for its replication in a compatible host, 
which provides various replication 
functions (enzymes and proteins).
2.2 h os T
A large number of host organisms, both 
prokaryotic and eukaryotic are used for 
gene cloning (Fig. 2.2). A host should 
allow easy entry of the rDNA into the cell 
and should not consider the recombinant 
DNA as a foreign DNA and degrade it. 
The host must supply all the required 
enzymes and proteins for smooth 
replication of the vector DNA along with 
insert. A wide variety of genetically de??ned strains are 
available as hosts. 
Among prokaryotic hosts, E. coli is the most extensively 
used. Typical E. coli is a rod-shaped Gram-negative 
bacterium commonly found in the lower intestine of warm-
blooded organisms. It is able to reproduce and grow rapidly, 
doubling its population about every 20 mins. K12 strain 
of E. coli is one of the most commonly used hosts in gene 
cloning. Other prokaryotic hosts have also been developed. 
For example, Bacillus subtilis constitutes an important 
alternative host, where the aim is secretion of a protein 
encoded by a cloned gene. Among eukaryotic hosts, the 
most widely used is yeast.
2.3 Ve CTo R
In principle, any molecule of DNA, which can replicate itself 
inside a host cell, can work as a vector for gene cloning. 
However, in order for a plasmid to act as vector, it must 
Chapter 2_Host vector system.indd   10 23-01-2025   11:20:26
Reprint 2025-26
Host-Vector System
11
ful??l the following features:
1. For easy incorporation into the host cell, a vector 
should itself be small in size and be able to integrate a 
large size of the insert.
2. The vector should have an origin of replication or ori, 
so that the vector is capable of autonomous replication 
inside the host organism. 
3. The vector needs to have unique restriction sites. If it 
possesses too many restriction sites, then it would be 
fragmented into several pieces.
4. The vector needs to have at least one selectable 
marker. The selectable markers are required to screen 
out transformants [For example: resistance to 
antibiotics such as tetracycline (tet
R
) and ampicillin 
(amp
R
)].
A variety of both prokaryotic as well as eukaryotic vectors are 
given in (Fig. 2.2).
2.3.1 Plasmid as a vector
Plasmids are circular, double-stranded (ds) and extra 
chromosomal DNAs capable of autonomous replication. 
They occur naturally in many bacteria, archaea, and even in 
eukaryotes, such as yeast. Size of plasmids ranges from a few 
thousand base pairs to more than 100 kilobase pairs (kbp). 
Like the host-cell chromosomal DNA, plasmid DNA is replicated 
before every cell division. During cell division, atleast one copy of 
the plasmid DNA is segregated to each daughter cell, assuring 
Two component system
of gene cloning
Yeast
Plant Cells
Animal Cells
E. coli
Prokaryotic Prokaryotic Eukaryotic Eukaryotic
Yeast (YAC)
Plants (pTi)
Animals (Vaccinia)
Plasmid (pBR322)
Bacteriophage () ?
Host Vector
Fig. 2.2: Two component system of gene cloning showing various kinds of eukaryotic and 
prokaryotic hosts and vectors
Chapter 2_Host vector system.indd   11 23-01-2025   11:20:26
Reprint 2025-26
Biotechnology XII 12
continued propagation of the plasmid through successive 
generations of the host cell. Some plasmids can integrate into 
chromosomes. Such plasmids are called episomes.
Several naturally occurring plasmids contain genes 
that provide some bene??t to the host cell. For example, 
some bacterial plasmids encode enzymes that deactivate 
antibiotics, such as ampicillin, tetracycline and chloram-
phenicol. Such drug-resistance providing plasmids, called 
R-plasmids constitute a major category of plasmids used 
for gene cloning. Another category of plasmids produce a 
variety of toxins called ‘colicins’ which kill other bacteria 
(called Col plasmids). Some plasmids contain ‘transfer 
genes’ encoding proteins that can form a macromolecular 
tube, or pilus, through which a copy of the plasmid can 
be transferred to other host cells of the same or related 
bacterial species called F-plasmids. Similarly, based on 
the copy number plasmids can also be classi??ed as follows:
(a) High or multi copy plasmids: Replication and 
segregation of these plasmids are free from control of 
replication of the bacterial chromosomal DNA that allows 
for many copies of these plasmids per cell (10–30 copies) 
[Fig. 2.3(a)]. 
Fig. 2.3: (a) High copy plasmid (b) Low copy plasmid 
DNA replication, cell division and random segregation DNA of
into daughter cells
Chromosomal DNA
(a) (b)
Plasmid
Chapter 2_Host vector system.indd   12 23-01-2025   11:20:26
Reprint 2025-26
Page 5


Recombinant DNA technology is a two component system 
involving a compatible host and a vector. A variety of hosts 
as well as vectors are available for gene cloning. In this 
chapter, students will be exposed to the fundamentals 
of different kinds of prokaryotic and eukaryotic host and 
vectors used in rDNA technology.
2.1 Two Key Componen Ts of Re Combinan T 
Dna Te Chnology As discussed in the previous chapter (Chapter 1), rDNA 
technology refers to joining two different DNA molecules with 
an aim to isolate, propagate, characterise and manipulate 
the genes for various applications. This technique involves 
two major steps (Fig. 2.1). In the ??rst step, the desired 
DNA molecule, generally termed as insert (target gene), 
is isolated from the source. In the second step, this target 
gene is inserted into a convenient carrier DNA molecule 
called vector. The vector containing the insert is called 
recombinant DNA (rDNA). Subsequently, the rDNA is 
introduced into an organism referred to as host. Using 
Host–Vector System
2.1 Two Key 
Components of 
Recombinant DNA 
Technology 
2.2 Host
2.3 Vector
2.4 Eukaryotic Host 
Vector System
2.5 Expression Vectors
2.6 Shuttle Vectors
2
Chapter 
Chapter 2_Host vector system.indd   9 23-01-2025   11:20:25
Reprint 2025-26
Biotechnology XII 10
Fig. 2.1: Schematic representation of gene cloning
Vector
Restriction
digestion
Target gene
(Insert)
Foreign DNA
Ligation
Vector containing
target gene
(Recombinant DNA)
Recombinant DNA
Transformation
Host cell
Propagation
Chromosomal
DNA
Recombinant
DNA 
multiplied
genetic machinery of the host, the rDNA 
undergoes propagation and expression. 
This whole process of rDNA technology 
is covered under the term ‘gene cloning’. 
Thus, gene cloning may be considered as 
a two component system: a compatible 
host and a vector, where the vector 
provides essential sequences required 
for its replication in a compatible host, 
which provides various replication 
functions (enzymes and proteins).
2.2 h os T
A large number of host organisms, both 
prokaryotic and eukaryotic are used for 
gene cloning (Fig. 2.2). A host should 
allow easy entry of the rDNA into the cell 
and should not consider the recombinant 
DNA as a foreign DNA and degrade it. 
The host must supply all the required 
enzymes and proteins for smooth 
replication of the vector DNA along with 
insert. A wide variety of genetically de??ned strains are 
available as hosts. 
Among prokaryotic hosts, E. coli is the most extensively 
used. Typical E. coli is a rod-shaped Gram-negative 
bacterium commonly found in the lower intestine of warm-
blooded organisms. It is able to reproduce and grow rapidly, 
doubling its population about every 20 mins. K12 strain 
of E. coli is one of the most commonly used hosts in gene 
cloning. Other prokaryotic hosts have also been developed. 
For example, Bacillus subtilis constitutes an important 
alternative host, where the aim is secretion of a protein 
encoded by a cloned gene. Among eukaryotic hosts, the 
most widely used is yeast.
2.3 Ve CTo R
In principle, any molecule of DNA, which can replicate itself 
inside a host cell, can work as a vector for gene cloning. 
However, in order for a plasmid to act as vector, it must 
Chapter 2_Host vector system.indd   10 23-01-2025   11:20:26
Reprint 2025-26
Host-Vector System
11
ful??l the following features:
1. For easy incorporation into the host cell, a vector 
should itself be small in size and be able to integrate a 
large size of the insert.
2. The vector should have an origin of replication or ori, 
so that the vector is capable of autonomous replication 
inside the host organism. 
3. The vector needs to have unique restriction sites. If it 
possesses too many restriction sites, then it would be 
fragmented into several pieces.
4. The vector needs to have at least one selectable 
marker. The selectable markers are required to screen 
out transformants [For example: resistance to 
antibiotics such as tetracycline (tet
R
) and ampicillin 
(amp
R
)].
A variety of both prokaryotic as well as eukaryotic vectors are 
given in (Fig. 2.2).
2.3.1 Plasmid as a vector
Plasmids are circular, double-stranded (ds) and extra 
chromosomal DNAs capable of autonomous replication. 
They occur naturally in many bacteria, archaea, and even in 
eukaryotes, such as yeast. Size of plasmids ranges from a few 
thousand base pairs to more than 100 kilobase pairs (kbp). 
Like the host-cell chromosomal DNA, plasmid DNA is replicated 
before every cell division. During cell division, atleast one copy of 
the plasmid DNA is segregated to each daughter cell, assuring 
Two component system
of gene cloning
Yeast
Plant Cells
Animal Cells
E. coli
Prokaryotic Prokaryotic Eukaryotic Eukaryotic
Yeast (YAC)
Plants (pTi)
Animals (Vaccinia)
Plasmid (pBR322)
Bacteriophage () ?
Host Vector
Fig. 2.2: Two component system of gene cloning showing various kinds of eukaryotic and 
prokaryotic hosts and vectors
Chapter 2_Host vector system.indd   11 23-01-2025   11:20:26
Reprint 2025-26
Biotechnology XII 12
continued propagation of the plasmid through successive 
generations of the host cell. Some plasmids can integrate into 
chromosomes. Such plasmids are called episomes.
Several naturally occurring plasmids contain genes 
that provide some bene??t to the host cell. For example, 
some bacterial plasmids encode enzymes that deactivate 
antibiotics, such as ampicillin, tetracycline and chloram-
phenicol. Such drug-resistance providing plasmids, called 
R-plasmids constitute a major category of plasmids used 
for gene cloning. Another category of plasmids produce a 
variety of toxins called ‘colicins’ which kill other bacteria 
(called Col plasmids). Some plasmids contain ‘transfer 
genes’ encoding proteins that can form a macromolecular 
tube, or pilus, through which a copy of the plasmid can 
be transferred to other host cells of the same or related 
bacterial species called F-plasmids. Similarly, based on 
the copy number plasmids can also be classi??ed as follows:
(a) High or multi copy plasmids: Replication and 
segregation of these plasmids are free from control of 
replication of the bacterial chromosomal DNA that allows 
for many copies of these plasmids per cell (10–30 copies) 
[Fig. 2.3(a)]. 
Fig. 2.3: (a) High copy plasmid (b) Low copy plasmid 
DNA replication, cell division and random segregation DNA of
into daughter cells
Chromosomal DNA
(a) (b)
Plasmid
Chapter 2_Host vector system.indd   12 23-01-2025   11:20:26
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Host-Vector System
13
Replication of these plasmids is referred to as relaxed 
replication. Use of a high copy number plasmid gives 
rise to higher yields and increased strength of the signals 
during screening of recombinants.
(b) Low or single-copy plasmids: Replication and 
segregation of these plasmids are under the same control as 
replication of the bacterial chromosomal DNA. As a result 
of this, the copy number of these plasmids are restricted 
to only one or few copies per cell [Fig. 2.3(b)]. Replication 
of these plasmids is referred to as stringent replication.
Development of plasmid based vectors
In order to use plasmids as a vector and to enhance their 
utility either for general purposes or to suit particular 
experimental designs, plasmids originally 
found in nature, are modi??ed, shortened, 
reconstructed and recombined under both — 
in vivo as well as in vitro to incorporate the 
properties indicated above into them. For 
example, the plasmid based vector pBR313, 
which replicates in relaxed fashion (giving 
rise to high copy number), contains two 
selectable markers tet
R
 and amp
R
 (selectable 
markers were derived from other two naturally 
occurring plasmids) and carried a number of 
unique restriction sites (cloning sites). 
However, the size of this plasmid was too 
large (9 kb). It was observed that more than 
half of its DNA was not essential for its role 
as a vector. Therefore, the ??rst phase of this 
plasmid vector development led to the 
construction of vector pBR322, whose size 
was reduced to 4,361 bp by deleting most of 
the non-essential sequences of pBR313. The 
vector pBR322 became the most widely used 
vector for gene cloning (Fig. 2.4).
Subsequent development included the introduction of 
synthetic cloning sites termed as multiple cloning site 
(MCS) or poly cloning site or polylinker. MCS is a short 
synthetic DNA fragment containing a large number of unique 
restriction sites. This enables the insertion of a foreign DNA 
cleaved at any of these restriction sites into that region  
EcoRI
PvuII
BamHI
PstI
amp
R
tet
R
pBR322
(4,361 bp)
Origin of
Replication
(ori)
SalI
Fig. 2.4: Vector map of pBR322 showing 
the origin of replication (ori), 
unique cloning sites  for restriction 
enzymes (PstI, EcoRI, BamHI, SalI, 
PvuII) and antibiotic selectable 
marker genes (amp
R
 and tet
R
)
Chapter 2_Host vector system.indd   13 23-01-2025   11:20:26
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