NCERT Textbook: Basic Processes | Biotechnology for Class 11 - NEET PDF Download

 Page 1


Basic 
Processes
7.1 DNA as the Genetic 
Material
7.2 Prokaryotic and 
Eukaryotic Gene 
Organisation
7.3 DNA Replication
7.4 Gene Expression
7.5 Genetic Code
7.6 Translation
7.7  Gene Mutation
7.8 DNA Repair
7.9 Regulation of Gene 
Expression
7.1 DNA As the Ge Netic MAteri Al You have studied in previous chapter that characters or 
traits are inherited from parents to offspring through 
genes. You are also aware that these genes are present 
on chromosomes which are made up of nucleic acids and 
proteins. However, understanding the nature of gene which 
is responsible for expression of trait was one of the biggest 
challenges before the scienti??c community. Answer to this 
question came after a few experimental evidences that 
deoxyribonucleic acid (DNA) determines the trait or feature 
of any organism except a few viruses.
Credit of discovery of DNA goes to Johann Friedrich 
Miescher, who for the ??rst time isolated an acidic substance 
from nuclei of pus cells and named nuclein having DNA 
and protein. Due to presence in chromosome and nucleus 
these two chemical components; nucleic acid (mainly DNA) 
and protein became possible candidates to be the genetic 
material. Still, the nature of genetic material remained 
unknown for a long time. Gradually, experiments with 
microorganisms by different investigators yielded results 
that provided evidences in favour of DNA as genetic material.
Chapter 7
Chapter 7.indd   166 09/01/2025   15:20:05
Reprint 2025-26
Page 2


Basic 
Processes
7.1 DNA as the Genetic 
Material
7.2 Prokaryotic and 
Eukaryotic Gene 
Organisation
7.3 DNA Replication
7.4 Gene Expression
7.5 Genetic Code
7.6 Translation
7.7  Gene Mutation
7.8 DNA Repair
7.9 Regulation of Gene 
Expression
7.1 DNA As the Ge Netic MAteri Al You have studied in previous chapter that characters or 
traits are inherited from parents to offspring through 
genes. You are also aware that these genes are present 
on chromosomes which are made up of nucleic acids and 
proteins. However, understanding the nature of gene which 
is responsible for expression of trait was one of the biggest 
challenges before the scienti??c community. Answer to this 
question came after a few experimental evidences that 
deoxyribonucleic acid (DNA) determines the trait or feature 
of any organism except a few viruses.
Credit of discovery of DNA goes to Johann Friedrich 
Miescher, who for the ??rst time isolated an acidic substance 
from nuclei of pus cells and named nuclein having DNA 
and protein. Due to presence in chromosome and nucleus 
these two chemical components; nucleic acid (mainly DNA) 
and protein became possible candidates to be the genetic 
material. Still, the nature of genetic material remained 
unknown for a long time. Gradually, experiments with 
microorganisms by different investigators yielded results 
that provided evidences in favour of DNA as genetic material.
Chapter 7
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167
Basic Processes 7.1.1 Discovery of the transforming principle
In 1928, a British medical of??cer, Frederick Grif??th made 
an observation in the course of developing a vaccine 
against pneumonia caused by bacterium Streptococcus 
pneumoniae (also called Diplococcus pneumoniae) in 
mammals, which causes pneumonia in humans and is 
normally lethal in mice. He identi??ed two different strains 
(varieties) of the bacterium i.e. virulent (disease causing) 
and non-virulent (harmless). In virulent strain, each 
bacterium is surrounded by a polysaccharide capsule 
because of which the bacterial colony when grown on an 
agar plate appear smooth and are referred to as smooth 
strain (S). The non-virulent strain lack polysaccharide 
coat and produce rough looking colony and are referred 
to as rough strain (R). The S type bacteria kill mice by 
causing pneumonia.
Grif??th made a series of experiments with S and R type 
bacteria (Fig. 7.1). When he injected live S bacteria into 
mice, the mice developed pneumonia and died. However, 
when he infected mice with R type bacteria mice showed 
no ill effects. The results of these two experiments 
con??rmed that the polysaccharide coat present in S type 
bacteria was apparently necessary for virulence.
In order to understand further, Grif??th killed some 
Fig. 7.1: Grif??th’s transformation experiment
Rough strain Rough strain Smooth strain Heat-killed
smooth strain
Mouse survives Mouse dies Mouse survives Mouse dies
Heat-killed
smooth strain
+
Chapter 7.indd   167 09/01/2025   15:20:06
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Page 3


Basic 
Processes
7.1 DNA as the Genetic 
Material
7.2 Prokaryotic and 
Eukaryotic Gene 
Organisation
7.3 DNA Replication
7.4 Gene Expression
7.5 Genetic Code
7.6 Translation
7.7  Gene Mutation
7.8 DNA Repair
7.9 Regulation of Gene 
Expression
7.1 DNA As the Ge Netic MAteri Al You have studied in previous chapter that characters or 
traits are inherited from parents to offspring through 
genes. You are also aware that these genes are present 
on chromosomes which are made up of nucleic acids and 
proteins. However, understanding the nature of gene which 
is responsible for expression of trait was one of the biggest 
challenges before the scienti??c community. Answer to this 
question came after a few experimental evidences that 
deoxyribonucleic acid (DNA) determines the trait or feature 
of any organism except a few viruses.
Credit of discovery of DNA goes to Johann Friedrich 
Miescher, who for the ??rst time isolated an acidic substance 
from nuclei of pus cells and named nuclein having DNA 
and protein. Due to presence in chromosome and nucleus 
these two chemical components; nucleic acid (mainly DNA) 
and protein became possible candidates to be the genetic 
material. Still, the nature of genetic material remained 
unknown for a long time. Gradually, experiments with 
microorganisms by different investigators yielded results 
that provided evidences in favour of DNA as genetic material.
Chapter 7
Chapter 7.indd   166 09/01/2025   15:20:05
Reprint 2025-26
167
Basic Processes 7.1.1 Discovery of the transforming principle
In 1928, a British medical of??cer, Frederick Grif??th made 
an observation in the course of developing a vaccine 
against pneumonia caused by bacterium Streptococcus 
pneumoniae (also called Diplococcus pneumoniae) in 
mammals, which causes pneumonia in humans and is 
normally lethal in mice. He identi??ed two different strains 
(varieties) of the bacterium i.e. virulent (disease causing) 
and non-virulent (harmless). In virulent strain, each 
bacterium is surrounded by a polysaccharide capsule 
because of which the bacterial colony when grown on an 
agar plate appear smooth and are referred to as smooth 
strain (S). The non-virulent strain lack polysaccharide 
coat and produce rough looking colony and are referred 
to as rough strain (R). The S type bacteria kill mice by 
causing pneumonia.
Grif??th made a series of experiments with S and R type 
bacteria (Fig. 7.1). When he injected live S bacteria into 
mice, the mice developed pneumonia and died. However, 
when he infected mice with R type bacteria mice showed 
no ill effects. The results of these two experiments 
con??rmed that the polysaccharide coat present in S type 
bacteria was apparently necessary for virulence.
In order to understand further, Grif??th killed some 
Fig. 7.1: Grif??th’s transformation experiment
Rough strain Rough strain Smooth strain Heat-killed
smooth strain
Mouse survives Mouse dies Mouse survives Mouse dies
Heat-killed
smooth strain
+
Chapter 7.indd   167 09/01/2025   15:20:06
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168
Biotechnology virulent S bacteria by boiling them and injected the said 
heat-killed bacteria into mice. As per his expectations, 
mice survived. However, quite unexpectedly, mice died 
due to pneumonia when it was injected with a mixture of 
heat-killed S bacteria and live R bacteria. Examination 
of blood and tissue ??uid of the dead mice revealed the 
presence of live S type bacteria. Based on the above 
observation, Grif??th concluded that the R-strain bacteria 
must have taken up what he called a ‘transforming 
principle’ from the heat-killed S bacteria, which 
allowed them to ‘transform’ into smooth-coated bacteria 
and become virulent. He called the phenomenon as 
transformation, which means transfer of genetic material 
from one cell to another that alter the genetic makeup 
of the recipient cell. But the nature of transforming 
substance still needed to be determined.
7.1.2 Biochemical characterisation of 
transforming principle
Three scientists, Oswald T. Avery, Colin Macleod and 
Maclyn McCarty conducted a series of experiments 
to identify the Grif??th’s transforming principle, and 
it was con??rmed in 1944 that the transforming agent 
is DNA (Fig. 7.2). In the design of experiment, they 
focused on three main components of smooth strain 
of bacteria, i.e., DNA, RNA and protein. They prepared 
an extract of heat-killed smooth strain of the bacteria 
from which lipids and carbohydrates were removed. 
Remaining components of the extract having proteins, 
RNA and DNA were retained for further experiment by 
dividing the extract into three parts. These extracts 
were separately treated with hydrolytic enzymes like 
ribonuclease (RNase), deoxyribonuclease (DNase) and 
protease to degrade RNA, DNA and protein, respectively, 
for their transforming ability by transferring each of the 
enzyme treated extracts into three different cultures of 
rough strain of bacteria. Transformation of rough strain 
into the smooth strain was observed in those colonies 
in which RNase and protease treated extract were added 
and not in the colony to which DNase treated extract 
was added. These results established beyond doubt that 
it is DNA which acts as a likely transforming principle.
Chapter 7.indd   168 09/01/2025   15:20:06
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Page 4


Basic 
Processes
7.1 DNA as the Genetic 
Material
7.2 Prokaryotic and 
Eukaryotic Gene 
Organisation
7.3 DNA Replication
7.4 Gene Expression
7.5 Genetic Code
7.6 Translation
7.7  Gene Mutation
7.8 DNA Repair
7.9 Regulation of Gene 
Expression
7.1 DNA As the Ge Netic MAteri Al You have studied in previous chapter that characters or 
traits are inherited from parents to offspring through 
genes. You are also aware that these genes are present 
on chromosomes which are made up of nucleic acids and 
proteins. However, understanding the nature of gene which 
is responsible for expression of trait was one of the biggest 
challenges before the scienti??c community. Answer to this 
question came after a few experimental evidences that 
deoxyribonucleic acid (DNA) determines the trait or feature 
of any organism except a few viruses.
Credit of discovery of DNA goes to Johann Friedrich 
Miescher, who for the ??rst time isolated an acidic substance 
from nuclei of pus cells and named nuclein having DNA 
and protein. Due to presence in chromosome and nucleus 
these two chemical components; nucleic acid (mainly DNA) 
and protein became possible candidates to be the genetic 
material. Still, the nature of genetic material remained 
unknown for a long time. Gradually, experiments with 
microorganisms by different investigators yielded results 
that provided evidences in favour of DNA as genetic material.
Chapter 7
Chapter 7.indd   166 09/01/2025   15:20:05
Reprint 2025-26
167
Basic Processes 7.1.1 Discovery of the transforming principle
In 1928, a British medical of??cer, Frederick Grif??th made 
an observation in the course of developing a vaccine 
against pneumonia caused by bacterium Streptococcus 
pneumoniae (also called Diplococcus pneumoniae) in 
mammals, which causes pneumonia in humans and is 
normally lethal in mice. He identi??ed two different strains 
(varieties) of the bacterium i.e. virulent (disease causing) 
and non-virulent (harmless). In virulent strain, each 
bacterium is surrounded by a polysaccharide capsule 
because of which the bacterial colony when grown on an 
agar plate appear smooth and are referred to as smooth 
strain (S). The non-virulent strain lack polysaccharide 
coat and produce rough looking colony and are referred 
to as rough strain (R). The S type bacteria kill mice by 
causing pneumonia.
Grif??th made a series of experiments with S and R type 
bacteria (Fig. 7.1). When he injected live S bacteria into 
mice, the mice developed pneumonia and died. However, 
when he infected mice with R type bacteria mice showed 
no ill effects. The results of these two experiments 
con??rmed that the polysaccharide coat present in S type 
bacteria was apparently necessary for virulence.
In order to understand further, Grif??th killed some 
Fig. 7.1: Grif??th’s transformation experiment
Rough strain Rough strain Smooth strain Heat-killed
smooth strain
Mouse survives Mouse dies Mouse survives Mouse dies
Heat-killed
smooth strain
+
Chapter 7.indd   167 09/01/2025   15:20:06
Reprint 2025-26
168
Biotechnology virulent S bacteria by boiling them and injected the said 
heat-killed bacteria into mice. As per his expectations, 
mice survived. However, quite unexpectedly, mice died 
due to pneumonia when it was injected with a mixture of 
heat-killed S bacteria and live R bacteria. Examination 
of blood and tissue ??uid of the dead mice revealed the 
presence of live S type bacteria. Based on the above 
observation, Grif??th concluded that the R-strain bacteria 
must have taken up what he called a ‘transforming 
principle’ from the heat-killed S bacteria, which 
allowed them to ‘transform’ into smooth-coated bacteria 
and become virulent. He called the phenomenon as 
transformation, which means transfer of genetic material 
from one cell to another that alter the genetic makeup 
of the recipient cell. But the nature of transforming 
substance still needed to be determined.
7.1.2 Biochemical characterisation of 
transforming principle
Three scientists, Oswald T. Avery, Colin Macleod and 
Maclyn McCarty conducted a series of experiments 
to identify the Grif??th’s transforming principle, and 
it was con??rmed in 1944 that the transforming agent 
is DNA (Fig. 7.2). In the design of experiment, they 
focused on three main components of smooth strain 
of bacteria, i.e., DNA, RNA and protein. They prepared 
an extract of heat-killed smooth strain of the bacteria 
from which lipids and carbohydrates were removed. 
Remaining components of the extract having proteins, 
RNA and DNA were retained for further experiment by 
dividing the extract into three parts. These extracts 
were separately treated with hydrolytic enzymes like 
ribonuclease (RNase), deoxyribonuclease (DNase) and 
protease to degrade RNA, DNA and protein, respectively, 
for their transforming ability by transferring each of the 
enzyme treated extracts into three different cultures of 
rough strain of bacteria. Transformation of rough strain 
into the smooth strain was observed in those colonies 
in which RNase and protease treated extract were added 
and not in the colony to which DNase treated extract 
was added. These results established beyond doubt that 
it is DNA which acts as a likely transforming principle.
Chapter 7.indd   168 09/01/2025   15:20:06
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169
Basic Processes 7.1.3 The Hershey – Chase experiment
Later on, yet another experiment conducted by Alfred 
Hershey and Martha Chase (1952) with T2 bacteriophages 
provided evidence in favour of DNA as genetic material. 
The virus T2 bacteriophage that infects Escherichia coli 
bacteria contains DNA surrounded by a protein coat. 
When it infects a bacterial cell, it attaches onto the outer 
surface followed by injecting its DNA into the cell. In a 
series of their experiments with T2 bacteriophage and 
E. coli, the purpose was to establish as to which component 
is responsible for multiplication of phage particles, DNA or 
protein. To identify easily, T2 bacteriophages were initially 
grown with the colonies of E. coli in medium containing 
radioactive phosphorous (
32
P) and radioactive sulfur (
35
S) 
separately (Fig. 7.3). This led to labelling of one set of 
bacteriophages with radioactive phosphorous (
32
P) and the 
other set with radioactive sulphur (
35
S). 
35
S and 
32
P labelled T2 phages were now inoculated 
into two separate cultures of unlabelled E. coli bacterial 
colony. After infection, the bacterial colonies were agitated 
Fig. 7.2: Con??rmation of transforming principle
Chapter 7.indd   169 09/01/2025   15:20:06
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Page 5


Basic 
Processes
7.1 DNA as the Genetic 
Material
7.2 Prokaryotic and 
Eukaryotic Gene 
Organisation
7.3 DNA Replication
7.4 Gene Expression
7.5 Genetic Code
7.6 Translation
7.7  Gene Mutation
7.8 DNA Repair
7.9 Regulation of Gene 
Expression
7.1 DNA As the Ge Netic MAteri Al You have studied in previous chapter that characters or 
traits are inherited from parents to offspring through 
genes. You are also aware that these genes are present 
on chromosomes which are made up of nucleic acids and 
proteins. However, understanding the nature of gene which 
is responsible for expression of trait was one of the biggest 
challenges before the scienti??c community. Answer to this 
question came after a few experimental evidences that 
deoxyribonucleic acid (DNA) determines the trait or feature 
of any organism except a few viruses.
Credit of discovery of DNA goes to Johann Friedrich 
Miescher, who for the ??rst time isolated an acidic substance 
from nuclei of pus cells and named nuclein having DNA 
and protein. Due to presence in chromosome and nucleus 
these two chemical components; nucleic acid (mainly DNA) 
and protein became possible candidates to be the genetic 
material. Still, the nature of genetic material remained 
unknown for a long time. Gradually, experiments with 
microorganisms by different investigators yielded results 
that provided evidences in favour of DNA as genetic material.
Chapter 7
Chapter 7.indd   166 09/01/2025   15:20:05
Reprint 2025-26
167
Basic Processes 7.1.1 Discovery of the transforming principle
In 1928, a British medical of??cer, Frederick Grif??th made 
an observation in the course of developing a vaccine 
against pneumonia caused by bacterium Streptococcus 
pneumoniae (also called Diplococcus pneumoniae) in 
mammals, which causes pneumonia in humans and is 
normally lethal in mice. He identi??ed two different strains 
(varieties) of the bacterium i.e. virulent (disease causing) 
and non-virulent (harmless). In virulent strain, each 
bacterium is surrounded by a polysaccharide capsule 
because of which the bacterial colony when grown on an 
agar plate appear smooth and are referred to as smooth 
strain (S). The non-virulent strain lack polysaccharide 
coat and produce rough looking colony and are referred 
to as rough strain (R). The S type bacteria kill mice by 
causing pneumonia.
Grif??th made a series of experiments with S and R type 
bacteria (Fig. 7.1). When he injected live S bacteria into 
mice, the mice developed pneumonia and died. However, 
when he infected mice with R type bacteria mice showed 
no ill effects. The results of these two experiments 
con??rmed that the polysaccharide coat present in S type 
bacteria was apparently necessary for virulence.
In order to understand further, Grif??th killed some 
Fig. 7.1: Grif??th’s transformation experiment
Rough strain Rough strain Smooth strain Heat-killed
smooth strain
Mouse survives Mouse dies Mouse survives Mouse dies
Heat-killed
smooth strain
+
Chapter 7.indd   167 09/01/2025   15:20:06
Reprint 2025-26
168
Biotechnology virulent S bacteria by boiling them and injected the said 
heat-killed bacteria into mice. As per his expectations, 
mice survived. However, quite unexpectedly, mice died 
due to pneumonia when it was injected with a mixture of 
heat-killed S bacteria and live R bacteria. Examination 
of blood and tissue ??uid of the dead mice revealed the 
presence of live S type bacteria. Based on the above 
observation, Grif??th concluded that the R-strain bacteria 
must have taken up what he called a ‘transforming 
principle’ from the heat-killed S bacteria, which 
allowed them to ‘transform’ into smooth-coated bacteria 
and become virulent. He called the phenomenon as 
transformation, which means transfer of genetic material 
from one cell to another that alter the genetic makeup 
of the recipient cell. But the nature of transforming 
substance still needed to be determined.
7.1.2 Biochemical characterisation of 
transforming principle
Three scientists, Oswald T. Avery, Colin Macleod and 
Maclyn McCarty conducted a series of experiments 
to identify the Grif??th’s transforming principle, and 
it was con??rmed in 1944 that the transforming agent 
is DNA (Fig. 7.2). In the design of experiment, they 
focused on three main components of smooth strain 
of bacteria, i.e., DNA, RNA and protein. They prepared 
an extract of heat-killed smooth strain of the bacteria 
from which lipids and carbohydrates were removed. 
Remaining components of the extract having proteins, 
RNA and DNA were retained for further experiment by 
dividing the extract into three parts. These extracts 
were separately treated with hydrolytic enzymes like 
ribonuclease (RNase), deoxyribonuclease (DNase) and 
protease to degrade RNA, DNA and protein, respectively, 
for their transforming ability by transferring each of the 
enzyme treated extracts into three different cultures of 
rough strain of bacteria. Transformation of rough strain 
into the smooth strain was observed in those colonies 
in which RNase and protease treated extract were added 
and not in the colony to which DNase treated extract 
was added. These results established beyond doubt that 
it is DNA which acts as a likely transforming principle.
Chapter 7.indd   168 09/01/2025   15:20:06
Reprint 2025-26
169
Basic Processes 7.1.3 The Hershey – Chase experiment
Later on, yet another experiment conducted by Alfred 
Hershey and Martha Chase (1952) with T2 bacteriophages 
provided evidence in favour of DNA as genetic material. 
The virus T2 bacteriophage that infects Escherichia coli 
bacteria contains DNA surrounded by a protein coat. 
When it infects a bacterial cell, it attaches onto the outer 
surface followed by injecting its DNA into the cell. In a 
series of their experiments with T2 bacteriophage and 
E. coli, the purpose was to establish as to which component 
is responsible for multiplication of phage particles, DNA or 
protein. To identify easily, T2 bacteriophages were initially 
grown with the colonies of E. coli in medium containing 
radioactive phosphorous (
32
P) and radioactive sulfur (
35
S) 
separately (Fig. 7.3). This led to labelling of one set of 
bacteriophages with radioactive phosphorous (
32
P) and the 
other set with radioactive sulphur (
35
S). 
35
S and 
32
P labelled T2 phages were now inoculated 
into two separate cultures of unlabelled E. coli bacterial 
colony. After infection, the bacterial colonies were agitated 
Fig. 7.2: Con??rmation of transforming principle
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170
Biotechnology in a blender for removing any remaining phage and phage 
parts from the outside of the bacterial cells. The mixture 
of the blender was then centrifuged to separate the 
bacteria (present in pellet) from the phage debris (present 
in supernatant). Pellets of bacterial culture which showed 
radioactivity were infected with phages having radioactive 
DNA, whereas, radioactivity was observed in the 
supernatant which was infected with 
35
S bacteriophage. 
This indicates that proteins did not enter the bacteria from 
the phage. It was therefore, concluded that the material 
which enters into bacterial cell, i.e., the DNA can be the 
genetic material. 
Though the above experiments provided strong evidence 
in favour of DNA as the genetic material, it was not clear 
DNA molecule is the repository of genetic information.  
Subsequent studies made by Erwin Chargaff, Maurice 
Wilkins, Rosalind Franklin, James Watson and Francis 
Crick led to the discovery of DNA structure, clarifying how 
DNA can encode large amounts of information (described 
in Chapter 3).
Fig. 7.3:  Hershey-Chase experiment
BACTERIOPHAGES
sulfur labeled
c protein apsule (red)
After centrifugation
s no ulfur in cells
cell
phosphorus labeled
DNA (green)
After centrifugation
phosphorus in cells
cell
1. Infection
3. Centrifugation
2. Blending
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