CBSE Class 12 Biology Revision Notes Chapter 11: Organisms and Populations

Population ecology explains how groups of organisms grow, decline and interact with other species in a habitat. This chapter connects population-level changes with natural selection, resource availability and ecological relationships.

Organisms rarely live as isolated individuals in nature. Members of the same species usually occupy a defined geographical area, use similar resources and form a population.

These CBSE Class 12 Biology Revision Notes Chapter 11 follow the current 2026–27 chapter sequence. They explain population attributes, growth equations, life-history strategies and interspecific interactions through concise notes and comparison tables.

Key Takeaways

  • N: The symbol used for population density.
  • r: The intrinsic rate of natural increase.
  • K: The carrying capacity of a habitat.
  • 25%: Nearly one-fourth of all insects are phytophagous and feed on plants.

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Organisms and Populations Class 12 Notes Overview

The Organisms and Populations Class 12 Notes examine ecology at the population level.

The chapter explains how population size changes and how different species affect one another while living in the same biological community.

Chapter Area Main Focus
Population attributes Birth rate, death rate, sex ratio and age structure
Population density Measurement of population size
Population growth Natality, mortality, immigration and emigration
Growth models Exponential and logistic growth
Life-history variation Reproductive strategies that improve fitness
Population interactions Relationships between different species

Class 12 Biology revision infographic explaining sexual reproduction in flowering plants and the flower pollination cycle.

Access Class 12 Biology Chapter 11 Organisms and Populations Notes in 30 Minutes

Use this sequence for rapid revision:

  1. Learn the definition of population.
  2. Revise birth rate, death rate, sex ratio and age distribution.
  3. Understand the three types of age pyramids.
  4. Learn different ways to measure population density.
  5. Memorise the population density equation.
  6. Compare exponential and logistic growth.
  7. Revise the meanings of r and K.
  8. Learn major life-history variations.
  9. Memorise the signs used for population interactions.
  10. Revise examples of predation, competition, parasitism, commensalism and mutualism.

Population Ecology in Class 12 Biology Chapter 11

Ecology studies interactions among organisms and between organisms and their environment.

It mainly examines four levels of biological organisation:

  • Organisms
  • Populations
  • Communities
  • Biomes

The current chapter explores ecology at the population level.

Population ecology connects ecological processes with population genetics, natural selection and evolution.

What Is a Population?

A population is a group of individuals of the same species that:

  • Occupy a defined geographical area
  • Share or compete for similar resources
  • Potentially interbreed
  • Experience similar environmental conditions

A group produced through asexual reproduction may also be treated as a population in ecological studies.

Examples include:

  • Cormorants in a wetland
  • Rats in an abandoned building
  • Teak trees in a forest
  • Bacteria in a culture plate
  • Lotus plants in a pond

Natural selection acts at the population level because inherited traits become more or less common across generations.

Population Attributes in Organisms and Populations Notes

Population attributes are characteristics of a group that cannot be assigned to an individual organism.

An individual may be born or die, but a population has a birth rate and death rate.

Important attributes include:

  • Birth rate
  • Death rate
  • Sex ratio
  • Age distribution
  • Population density

Natality and Mortality in a Population

Natality and mortality describe additions to and losses from a population.

Birth Rate or Natality

Natality is the number of births added to a population during a given period.

Birth rate is expressed as the number of births per individual in the population.

For example:

Initial lotus population = 20 plants
New plants produced = 8

Birth rate = 8/20
Birth rate = 0.4 offspring per lotus per year

Death Rate or Mortality

Mortality is the number of deaths occurring in a population during a given period.

For example:

Initial fruit fly population = 40
Number of deaths = 4

Death rate = 4/40
Death rate = 0.1 individual per fruit fly per week

Birth Rate and Death Rate Compared

Feature Birth Rate Death Rate
Also called Natality Mortality
Effect Increases population Decreases population
Measurement Births per individual per unit time Deaths per individual per unit time
Example New lotus plants Dead fruit flies

Sex Ratio and Age Distribution

An individual organism is male or female, while a population has a sex ratio.

For example, a population may contain:

  • 60% females
  • 40% males

Age distribution represents the percentage of individuals belonging to different age groups.

The major age groups are:

  • Pre-reproductive
  • Reproductive
  • Post-reproductive

Age Pyramids in Class 12 Biology

When the age distribution of males and females is represented graphically, the resulting structure is called an age pyramid.

The shape of the pyramid indicates whether the population is expanding, stable or declining.

Expanding Population

An expanding population has a high proportion of pre-reproductive individuals.

Its main features are:

  • Broad base
  • High birth rate
  • Increasing future population
  • More young individuals than reproductive adults

Stable Population

A stable population has comparable proportions of pre-reproductive and reproductive individuals.

Its main features are:

  • Nearly bell-shaped structure
  • Birth and death rates are balanced
  • Population size remains relatively constant

Declining Population

A declining population has fewer pre-reproductive individuals than reproductive adults.

Its main features are:

  • Narrow base
  • Low birth rate
  • Larger proportion of older individuals
  • Population likely to decrease

Types of Age Pyramids

Age Pyramid Population Status Main Feature
Expanding Growing Broad pre-reproductive base
Stable Stationary Similar young and reproductive groups
Declining Decreasing Narrow pre-reproductive base

Population Density in Class 12 Biology Chapter 11

The size of a population in a habitat is called population density and is represented by N.

Changes in population density help ecologists assess:

  • Competition
  • Predation
  • Disease
  • Pesticide effects
  • Resource availability
  • Habitat quality

Population density can range from fewer than ten individuals to several million.

Methods of Measuring Population Density

Population density does not always mean the number of individuals.

It may be measured through:

  • Total number
  • Biomass
  • Percentage cover
  • Relative density
  • Indirect evidence

Total Number

Counting individuals is suitable when organisms are clearly visible and easy to identify.

For example, the number of lotus plants in a pond may be counted directly.

Biomass

Biomass may be more meaningful when individuals differ greatly in size.

A single banyan tree may have a greater ecological impact than hundreds of small weeds.

Percentage Cover

Percentage cover is useful for plant populations spreading over an area.

It indicates the proportion of the ground occupied by a species.

Relative Density

Relative density may be used when direct counting is difficult.

Examples include:

  • Number of fish caught per trap
  • Number of birds observed per hour
  • Number of insects collected per net sweep

Indirect Population Estimates

Some animal populations are estimated through signs of their presence.

Tiger populations may be assessed using:

  • Pug marks
  • Faecal pellets
  • Other field evidence

Population Growth in Organisms and Populations

Population growth refers to changes in population density over time.

Population size is affected by food availability, predation, weather and movement between habitats.

Four basic processes determine the change:

  • Natality
  • Mortality
  • Immigration
  • Emigration

Natality

Natality adds new individuals through reproduction.

Mortality

Mortality removes individuals through death.

Immigration

Immigration is the arrival of individuals of the same species from another habitat.

It increases population density.

Emigration

Emigration is the departure of individuals from a habitat.

It decreases population density.

Population Growth Equation

Suppose:

Nt = Population density at time t
Nt+1 = Population density at the next time interval
B = Number of births
I = Number of immigrants
D = Number of deaths
E = Number of emigrants

The population growth equation is:

Nt+1 = Nt + [(B + I) − (D + E)]

Population density increases when:

B + I > D + E

Population density decreases when:

B + I < D + E

Births and deaths usually have the greatest influence under normal conditions.

Immigration may become more important when a species is colonising a new habitat.

Factors Affecting Population Density

Process Meaning Effect on Population
Natality Birth of new individuals Increase
Immigration Arrival from another habitat Increase
Mortality Death of individuals Decrease
Emigration Departure to another habitat Decrease

Exponential Growth in Population Ecology

Exponential growth occurs when resources such as food and space are unlimited.

Under ideal conditions, each species can realise its full potential for population increase.

Suppose:

N = Population size
b = Per capita birth rate
d = Per capita death rate

The rate of population change is:

dN/dt = (b − d)N

Let:

r = b − d

Therefore:

dN/dt = rN

Here, r represents the intrinsic rate of natural increase.

Intrinsic Rate of Natural Increase

The value of r indicates the inherent potential of a population to grow.

Higher r values indicate faster potential growth.

Examples given in the chapter include:

Population Approximate r Value
Norway rat 0.015
Flour beetle 0.12
Human population in India in 1981 0.0205

Exponential Growth Equation

The integral form is:

Nt = N0ert

Where:

Nt = Population density after time t
N0 = Population density at time zero
r = Intrinsic rate of natural increase
e = Base of natural logarithms, approximately 2.71828
t = Time

J-Shaped Population Growth Curve

When population density is plotted against time, exponential growth produces a J-shaped curve.

The population increases slowly at first and then rises rapidly.

However, exponential growth cannot continue indefinitely because natural resources are finite.

Logistic Growth and Carrying Capacity

Logistic growth occurs when resources become limited.

A habitat can support only a maximum number of individuals of a species.

This maximum sustainable population is called the carrying capacity and is represented by K.

Phases of Logistic Growth

A population showing logistic growth passes through:

  1. Lag phase
  2. Acceleration phase
  3. Deceleration phase
  4. Asymptote

At the asymptote, population density approaches carrying capacity.

Logistic Growth Equation

The Verhulst-Pearl logistic growth equation is:

dN/dt = rN [(K − N)/K]

Where:

N = Population density
r = Intrinsic rate of natural increase
K = Carrying capacity

S-Shaped Population Growth Curve

When population density is plotted against time, logistic growth produces a sigmoid or S-shaped curve.

Growth slows as the population approaches K because competition for limited resources increases.

The logistic model is considered more realistic for most natural populations.

Exponential and Logistic Growth Compared

The difference between exponential growth and logistic growth is based mainly on resource availability.

Feature Exponential Growth Logistic Growth
Resources Unlimited Limited
Growth curve J-shaped S-shaped
Carrying capacity Not considered Represented by K
Growth pattern Continuous rapid increase Slows near carrying capacity
Ecological realism Idealised More realistic
Main equation dN/dt = rN dN/dt = rN [(K − N)/K]

Life History Variation in Populations

Life history variation refers to differences in the reproductive strategies of organisms.

Populations evolve strategies that maximise reproductive fitness under specific environmental conditions.

Frequency of Reproduction

Some organisms reproduce only once during their lifetime.

Examples include:

  • Pacific salmon
  • Bamboo

Other organisms reproduce several times.

Examples include:

  • Most birds
  • Most mammals

Number and Size of Offspring

Some organisms produce many small offspring.

Examples include:

  • Oysters
  • Pelagic fishes

Others produce fewer, larger offspring.

Examples include:

  • Birds
  • Mammals

Life-History Strategies Compared

Strategy Examples Main Feature
Reproduce once Pacific salmon, bamboo One major reproductive event
Reproduce repeatedly Birds, mammals Several reproductive events
Many small offspring Oysters, pelagic fishes High offspring number
Few large offspring Birds, mammals Greater investment per offspring

Life-history traits evolve according to the abiotic and biotic conditions of the habitat.

Population Interactions in Class 12 Biology

Different species cannot survive in complete isolation.

Plants, animals and microbes interact to form biological communities.

Population interactions between two different species are called interspecific interactions.

The outcome may be:

  • Beneficial
  • Harmful
  • Neutral

Symbols used to represent these outcomes are:

  • + for benefit
  • − for harm
  • 0 for no effect

Types of Interspecific Interactions

The major interspecific interactions are mutualism, competition, predation, parasitism, commensalism and amensalism.

Species A Species B Interaction
+ + Mutualism
− − Competition
+ − Predation
+ − Parasitism
+ 0 Commensalism
− 0 Amensalism

Predation in Organisms and Populations

Predation is an interaction in which the predator benefits and the prey is harmed.

Predation includes:

  • Animals feeding on animals
  • Herbivores feeding on plants
  • Seed-eating birds feeding on plant products

Importance of Predators

Predators:

  • Transfer energy to higher trophic levels
  • Control prey populations
  • Prevent ecosystem instability
  • Maintain species diversity
  • Support biological pest control

Prickly Pear Cactus in Australia

Prickly pear cactus spread rapidly in Australia because it lacked natural predators.

It was controlled after a cactus-feeding moth from its native habitat was introduced.

This demonstrates the ecological basis of biological control.

Pisaster and Species Diversity

The starfish Pisaster is an important predator in rocky intertidal communities.

When it was removed experimentally, more than ten invertebrate species disappeared within one year.

The prey species competed more intensely in the absence of the predator.

Prey Defences Against Predators

Prey species protect themselves through:

  • Camouflage
  • Toxic chemicals
  • Distasteful substances
  • Warning colours
  • Rapid escape

The monarch butterfly is distasteful to birds because it stores chemicals obtained during its caterpillar stage.

Plant Defences Against Herbivores

Plants cannot escape herbivores, so they use structural and chemical defences.

Morphological Defences

Examples include:

  • Thorns in Acacia
  • Spines in cactus

Chemical Defences

Plants may produce substances that:

  • Reduce feeding
  • Inhibit digestion
  • Disrupt reproduction
  • Cause illness
  • Kill herbivores

Calotropis produces poisonous cardiac glycosides.

Other plant defence chemicals include:

  • Nicotine
  • Caffeine
  • Quinine
  • Strychnine
  • Opium

Competition in Population Ecology

Competition occurs when the presence of one species lowers the fitness of another.

The competing species may use the same food, space or other resources.

Competition can occur between:

  • Closely related species
  • Unrelated species
  • Species facing limited resources
  • Species experiencing direct interference

Flamingoes and Fishes

In shallow South American lakes, visiting flamingoes and resident fishes compete for zooplankton.

These organisms are unrelated but use the same food resource.

Interference Competition

Competition may occur even when resources are abundant.

One species may reduce another species’ feeding efficiency through its presence or behaviour.

Gause’s Competitive Exclusion Principle

Gause’s Competitive Exclusion Principle states that two closely related species competing for the same limited resources cannot coexist indefinitely.

The competitively inferior species may eventually be eliminated.

Abingdon Tortoise and Goats

The Abingdon tortoise became extinct after goats were introduced to the Galapagos Islands.

Goats were more efficient browsers and competed with the tortoise for vegetation.

Competitive Release

Competitive release occurs when a species expands its distribution after a superior competitor is removed.

Connell’s experiments showed that:

  • The larger barnacle Balanus dominated the intertidal region.
  • It excluded the smaller barnacle Chthamalus.
  • Removing Balanus allowed Chthamalus to expand its range.

Resource Partitioning and Species Coexistence

Competition does not always lead to extinction.

Species may evolve ways to divide resources and coexist.

This process is called resource partitioning.

Species may avoid competition by using:

  • Different feeding times
  • Different food sources
  • Different parts of a habitat
  • Different foraging methods

MacArthur observed five closely related warbler species living on the same tree.

They avoided direct competition by feeding in different parts of the tree and following different foraging patterns.

Parasitism in Organisms and Populations

Parasitism is an interaction in which the parasite benefits while the host is harmed.

Parasites obtain shelter and food from the host.

Many parasites are host-specific and evolve along with their hosts.

Adaptations of Parasites

Parasites may possess:

  • Adhesive organs
  • Suckers
  • Reduced sensory organs
  • Reduced digestive systems
  • High reproductive capacity
  • Complex life cycles

Intermediate Hosts and Vectors

The human liver fluke requires two intermediate hosts:

  • Snail
  • Fish

The malarial parasite uses a mosquito as a vector.

Effects of Parasites on Hosts

Parasites may:

  • Reduce host survival
  • Lower host growth
  • Reduce reproduction
  • Decrease population density
  • Make the host more vulnerable to predators

Ectoparasites

Ectoparasites live on the outer surface of the host.

Examples include:

  • Lice on humans
  • Ticks on dogs
  • Copepods on marine fish

Cuscuta is a parasitic plant that obtains nutrition from its host.

It has lost its chlorophyll and leaves during evolution.

Endoparasites

Endoparasites live inside the host body.

They may occupy:

  • Liver
  • Kidneys
  • Lungs
  • Red blood cells
  • Intestines

Their body structure is often simplified, while reproductive capacity is highly developed.

Brood Parasitism

Brood parasitism occurs when a bird lays its eggs in another bird’s nest.

The host bird incubates the eggs and raises the young.

The parasitic bird’s eggs may resemble the host’s eggs in size and colour.

The koel laying eggs in a crow’s nest is a common example.

Commensalism in Class 12 Biology

Commensalism is an interaction in which one species benefits while the other is neither helped nor harmed.

Examples include:

  • Orchid growing on a mango tree
  • Barnacles attached to a whale
  • Cattle egret feeding near grazing cattle
  • Clownfish living among sea anemone tentacles

Cattle Egret and Grazing Cattle

Grazing cattle disturb insects hidden in vegetation.

The cattle egret catches these insects and gains food.

The cattle remain unaffected.

Clownfish and Sea Anemone

The clownfish receives protection from predators by living among the sea anemone’s stinging tentacles.

The anemone receives no clear benefit or harm.

Mutualism in Organisms and Populations

Mutualism benefits both interacting species.

It often involves the exchange of food, nutrients, shelter, pollination or seed dispersal.

Lichens

Lichens are formed by an association between:

  • A fungus
  • An alga or cyanobacterium

The photosynthetic partner supplies food, while the fungus provides water, minerals and protection.

Mycorrhiza

Mycorrhiza is an association between fungi and plant roots.

The fungus:

  • Improves mineral absorption
  • Helps absorb phosphorus
  • Receives carbohydrates from the plant

Plant-Pollinator Mutualism

Plants offer rewards such as:

  • Nectar
  • Pollen
  • Nutritious fruits

Animals provide services such as:

  • Pollination
  • Seed dispersal

These interactions may lead to co-evolution.

Fig Tree and Wasp

A fig species is often pollinated by a specific wasp species.

The wasp:

  • Pollinates the fig flowers
  • Uses the fruit as an egg-laying site
  • Feeds its larvae on some developing seeds

Both species benefit from the interaction.

Ophrys Orchid and Sexual Deceit

The Mediterranean orchid Ophrys resembles a female bee.

A male bee attempts to mate with the flower and carries pollen to another orchid.

The orchid gains pollination, while the bee receives no reward.

This interaction shows how closely linked evolutionary changes can become.

Amensalism in Population Interactions

Amensalism is an interaction in which one species is harmed while the other remains unaffected.

Its symbolic representation is:

Species A = −
Species B = 0

Unlike competition, both species do not experience harm.

Population Interactions Quick Comparison

Interaction Species Outcome Example
Mutualism +/+ Lichen
Competition −/− Balanus and Chthamalus
Predation +/− Starfish and invertebrate prey
Parasitism +/− Lice and humans
Commensalism +/0 Orchid and mango tree
Amensalism −/0 One species harmed, other unaffected

Organisms and Populations Class 12 Quick Revision Tables

The following tables summarise the major concepts from the Class 12 Biology Chapter 11 Notes.

Important Population Terms

Term Meaning
Population Individuals of one species in a defined area
Population density Size of a population in a habitat
Natality Addition through birth
Mortality Loss through death
Immigration Entry from another habitat
Emigration Departure to another habitat
Sex ratio Proportion of males and females
Age pyramid Graphical representation of age distribution

Population Growth Symbols

Symbol Meaning
N Population density
B Births
I Immigration
D Deaths
E Emigration
r Intrinsic rate of natural increase
K Carrying capacity
t Time

Important Population Growth Equations

Concept Equation
Change in population density Nt+1 = Nt + [(B + I) − (D + E)]
Exponential growth dN/dt = rN
Integrated exponential growth Nt = N0ert
Logistic growth dN/dt = rN [(K − N)/K]

Major Ecological Examples

Concept Example
Biological control Cactus-feeding moth controlling prickly pear
Predator maintains diversity Pisaster starfish
Chemical plant defence Cardiac glycosides in Calotropis
Competition Flamingoes and fishes
Competitive exclusion Balanus excluding Chthamalus
Resource partitioning Five warbler species
Brood parasitism Koel and crow
Commensalism Cattle egret and cattle
Mutualism Fig tree and wasp
Sexual deceit Ophrys orchid and bee

Useful Links for Class 12 Biology

Section Useful Links
Syllabus CBSE Class 12 Biology Syllabus
Revision Notes CBSE Class 12 Biology Revision Notes
Biology Notes CBSE Class 12 Biology Revision Notes Chapter 1
NCERT Solutions NCERT Solutions for Class 12 Biology
Sample Papers CBSE Sample Papers for Class 12 Biology
Important Questions Important Questions Class 12 Biology
NCERT Books NCERT Books for Class 12 Biology
Class 12 Support CBSE Class 12 Syllabus

Q.1 What happens to a DNA fragment that is transferred into an alien organism?

Ans

Two situations arise:
a. The alien DNA doesn’t integrate to the host’s genome and is lost after the cell containing the alien DNA dies.
b. The alien DNA integrates to the host’s genome and multiplies as the cell reproduces.

Q.2 What is insertional inactivation? Give an example.

Ans

The inactivation of a gene by inserting a fragment of DNA into the middle of its coding sequence is called insertional inactivation. Any products from the inactivated gene will not work because of the codes added to it. Cromogenic selectable markers show insertional inactivation. A recombinant DNA is inserted within the coding sequence of an enzyme, -galactosidase inactivating the enzyme. The presence of a substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. Presence of insert results into insertional inactivation of the -galactosidase and the colonies do not produce any colour.

Q.3 Define Biotechnology.

Ans

Biotechnology is defined as the industrial use of living organisms (or their parts) to produce various products and services. It is the fusion of biology and technology.

Q.4 Expand EFB. What is the definition of Biotechnology according to EFB?

Ans

EFB stands for European Federation of Biotechnology The definition of Biotechnology as given by EFB: ‘The integration of natural science and organisms, cells, parts thereof, and molecular analogues for products and services’.

Q.5 What are the two core techniques that enabled birth of modern biotechnology?

Ans

The two core techniques are:
a. Genetic engineering: The ability to alter genetic material of an organism according to ones need and get the desired phenotypic expression
b. Sterile microbial culturing techniques: The ability to grow desired organism in a contamination free environment

Q.6 What do you understand by the term genetic engineering? What are the basic three steps involved in genetic modification of an organism?

Ans

Genetic engineering is the scientific alteration of genes or genetic material to produce desirable new traits in organisms or to eliminate undesirable ones. It aims at introducing new characteristics or attributes physiologically or physically, such as introducing a novel trait, enhancing existing ones, or producing a new protein. It involves:
a. Isolation of DNA
b. Manipulation of the DNA
c. Reintroduction of DNA into cells/organisms.

Q.7 What is the difference between modern biotechnology and biotechnology in general?

Ans

Modern biotechnology involves highly sophisticated techniques such as genetic engineering, protein and enzyme engineering and in vitro fertilisation and reproduction techniques whereas biotechnology in general also includes non-sophisticated techniques like agriculture, genetic breeding, curd and bread making etc. In brief, modern biotechnology is biotechnology at the molecular level.

Q.8 What is the benefit of sexual reproduction over asexual reproduction?

Ans

Sexual reproduction provides opportunities for variations in the genetic makeup of an organism, which may be beneficial to the organism as well as the population, especially under environmental stress.

Q.9 Traditional hybridization procedures used in plants and animal breeding, very often lead to inclusion and multiplication of undesirable genes. How can you overcome this problem?

Ans

We can overcome the problem by the use of genetic engineering where we can isolate and introduce a single or a set of specific genes without introducing undesired genes into the target organism.

Q.10 What is origin of replication?

Ans

The origin of replication is a particular DNA sequence in the chromosome or a plasmid at which DNA replication is initiated.

Q.11 What is cloning?

Ans

Cloning collectively refers to the process used to create copies of DNA fragments (molecular cloning), cells (cell cloning), or organisms. The term also encompasses situations whereby organisms reproduce asexually.

Q.12 What are ‘molecular scissors’?

Ans

Molecular scissors are enzymes that cut molecules at a specific point.
– Restriction enzymes are molecular scissors that cut double-stranded DNA.
– Ribozymes are molecular scissors that cut RNA.
Molecular scissors are used as tools of modern biotechnology for cutting the desired fragment of DNA, RNA etc.

Q.13 What are vectors? Write a short note on cloning vectors.

Ans

A vector is an organism that does not cause disease itself but which acts as a vehicle for the disease causing pathogens. A cloning vector is a small DNA vehicle that carries a foreign DNA fragment. It is used to transfer the desired gene into the host during genetic engineering experiments. The insertion of the fragment into the cloning vector is carried out by treating the vehicle and the foreign DNA with the same restriction enzyme, then ligating the fragments together. There are many different types of cloning vectors.
Eg.: Plasmids, Bacteriophages (λ phage), Cosmid, Bacterial Artificial Chromosome (BAC), Yeast Artificial Chromosome (YAC) etc.


Plasmid pBR322 is an E. coli cloning vector, showing restriction sites (HindIII, EcoRI, BamHI, SalI, PvuII, PstI, ClaI), ori and antibiotic resistance genes (ampR and tetR).

Q.14 Write a short note on DNA ligase.

Ans

DNA ligase is a type of enzyme that joins double stranded DNA fragments. DNA ligase is used in vivo in DNA repair and DNA replication and in vitro in recombinant DNA technologies. DNA ligase forms covalent phosphodiester bonds between 3′ hydroxyl ends of one nucleotide and the 5′ phosphate end of the another.

Q.15 Why are plasmids and bacteriophages used as cloning vectors?

Ans

It is so because:

  • Plasmids and bacteriophages have the ability to replicate inside bacterial cells without being controlled by the chromosomal DNA.
  • Bacteriophages and many plasmids are present in high number of copies per cell and thus high number of the engineered gene can be obtained.

Q.16 What are the key tools required for genetic engineering?

Ans

The key tools required for genetic engineering are:
a. Enzymes: Restriction endonuclease, DNA ligase, DNA polymerase etc.
b. Vectors: Plasmids, phasemids, cosmids, BACs, YACs etc.
c. Transformation tools like gene gun, microinjection etc.
Other tools required are those used for gene isolation, separation and screening of recombinants.

Q.17 Write a short note on the followings:
a. Ori
b. Selectable marker
c. Cloning sites

Ans

a. Ori: Ori or origin of replication is a particular DNA sequence at which DNA replication is initiated. The ori binds the pre-replication complex, a protein complex that recognizes, unwinds, and begins to copy DNA. Prokaryotes have single origin of replication per circular chromosome while eukaryotes have multiple origin of replication per chromosome (to help speedup the replication of the usually large genetic content).
b. Selectable marker: Selectable markers are genes coding for a certain known phenotypic characteristics that helps in identifying and eliminating non-transformed organisms and selectively permitting the growth of the transformed (having the recombinant gene) organisms. Genes that code for resistance to antibiotic such as ampicillin, chloramphenicol, tetracycline or kanamycin or impart specific colours to the organism/ microbial colonies are used as selectable markers.
c. Cloning sites: Cloning site is that region of a vector in which the gene to be transferred to the host is inserted. The site contains recognition sequences specific to various restriction endonuclease so that the gene-containing DNA fragment can be inserted into the vector. Multiple recognition sequences may be present but generally one is preferred to keep thing simple.

Q.18 From which organism restriction endonuclease was first extracted?

Ans

Escherichia coli

Q.19 Write a short note on Agrobacterium tumifaciens.

Ans

Agrobacterium tumifaciens is a rod shaped, gram-negative bacteria that is able to deliver a piece of its DNA known as ‘T-DNA’ to transform normal plant cells into a tumor and direct these tumor cells to produce the chemicals required by it. This tumor inducing (Ti) plasmid has been modified into a cloning vector which can deliver genes into a variety of plants without causing the disease.

Q.20 What is known as recognition sequence of a restriction endonuclease? Give an example.

Ans

Many restriction endonucleases exhibit binding specificity and function only after binding to a specific DNA sequence. This specific base sequence is known as the recognition sequence for that particular restriction endonuclease.

Enzyme
Source

Recognition Sequence
Cut
EcoRI
Escherichia coli
5’GAATTC

3’CTTAAG
5’-G AATTC-3’
3’-CTTAA G-5’
HindIII
Haemophillus
influenzae

5’AAGCTT

3’TTCGAA
5’-A AGCTT-3’
3’-TTCGA A-5’

Q.21 Describe the convention for naming restriction endonucleases.

Ans

Naming of restriction enzymes is based on the bacteria from which they are isolated in the following manner:-

  • The first letter of the name comes from the genus.
  • The second two letters come from the species.
  • The next letter comes from the strain.
  • Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria.
    E.g., Naming of EcoRI
E Escherichia genus
co coli species
R RY 13 strain
I First identified order of identification
in the bacterium

 

Q.22 What are nucleases? What is the difference between endonucleases and exonucleases?

Ans

A nuclease is an enzyme capable of cleaving the phosphodiester bonds between the nucleotide subunits of nucleic acids. Restriction endonuclease cleaves the double-stranded DNA at a specific recognition site after binding to it at that point whereas restriction exonuclease removes nucleotides sequentially from the end of the DNA.

Q.23 Explain the working of a restriction endonuclease.

Ans

A restriction endonuclease functions by “scanning” the length of a DNA molecule. Once it finds its specific recognition sequence it binds to the DNA molecule and makes one cut in each of the two sugar-phosphate backbones of the double helix at the specific points.

Q.24 What is a palindromic sequence? What is its significance in genetic engineering?

Ans

A palindromic sequence is a sequence that has the property of reading the same in either direction. Restriction endonucleases used in genetic engineering to cut double-stranded DNA have various palindromic sequences as their recognition sequence. The endonucleases identify these specific palindromic sequences, binds and cleaves the DNA at that point.

5’-GAATTC-3’
3’-CTTAAG-5’
 
5'-G     AATTC-3'
3'-CTTAA     G-5'

Action of EcoRI

Q.25 What are sticky ends? What is its significance?

Ans

A sticky end also called overhangs which is a stretch of unpaired nucleotides at the end of a double-stranded DNA molecule. These unpaired nucleotides can be in either strand, creating either 3′ or 5′ overhangs.

Sticky ends have the ability to form hydrogen bonds with their complementary cut counterparts and facilitate the action of DNA ligases.

5′—G AATTC—3′
3′—CTTAA G—5′

Q.26 How is a DNA fragment amplified?

Ans

A DNA fragment is amplified either by polymerase chain reaction or by microbial cloning method.

Q.27 What is the sole purpose of recombinant DNA technology?

Ans

The sole purpose of the recombinant DNA technology is to express a certain phenotypic character or a protein in an organism in which it is not naturally found. This, in turn, has many applications like in development of better varieties of animals; pest resistant, drought resistant, high yielding plants; drug production and gene therapy etc.

Q.28 With a diagram explain the steps involved in recombinant DNA technology.

Ans

The steps involved in recombinant DNA technology are:

  1. Identification and isolation of desired gene.
  2. Selection of a vector and inserting the DNA fragment containing the desired gene into it. This is done with the help of restriction endonuclease and DNA ligase enzymes.
  3. Inserting the engineered vector into the host cell/organism. This is done by the use of transformation techniques like biolistic (use of gene gun), microinjection or disarmed pathogens.
  4. Screening of recombinant organisms by exploiting the selectable marker introduced along with the desired gene.
  5. Maintenance and multiplication of the recombinants obtained.

Steps involved in recombinant DNA technology

Q.29 How do you separate DNA fragments?

Ans

DNA is negatively charged and can be separated using gel electrophoresis. Here, the negatively charged DNA fragments are forced to move towards the positively charged anode under an electric field through a matrix, generally agarose gel. The fragments separate according to their size, the smallest moving the fastest and reaching the fastest from the point of loading.

Q.30 What is agarose?

Ans

Agarose is a polysaccharide extracted from seaweeds and is a constituent of agar. It is widely used as a medium of gel-electrophoresis.

Q.31 Write a short note on gel-electrophoresis?

Ans

Gel-electrophoresis is a technique for separating a mixture of molecules on the basis of their size under an electric field. DNA, RNA and proteins can be separated using this procedure. The larger molecules takes more time to pass through the matrix, generally made of agarose, compared to the smaller molecules. It can be used both for analysis and purification (small scale) of samples.

Q.32 What is an antibiotic resistance gene? What is its significance in recombinant DNA technology?

Ans

Antibiotic resistance gene is a gene which is expressed in a microorganism. It is able to withstand the effect of that specific antibiotic. In recombinant DNA technology, antibiotic resistant genes are used in the vectors as selectable markers to identify transformed hosts.

Q.33 Define Transformation.

Ans

Transformation is defined as the genetic alteration of a cell resulting from the uptake and expression of foreign genetic material.

Q.34 What are the various methods available for inserting an engineered vector into the host?

Ans

The various methods are:
a. Use of competent cells: These have the ability to take up extracellular DNA. Competence can be induced by treating the cells with divalents like calcium and subjecting them to heat shocks.
b. Microinjection: Use of a micro-needle to insert the vector into the cell.
c. Gene Gun: Biolistic particle delivery system injects the vector (coated onto a gold particle) into the host cell, generally plant cells.
d. Use of disarmed pathogens: Pathogens which are devoid of the virulent gene but retain the ability to transfer the recombinant DNA into the host.

Q.35 What do you understand by the term competent host with reference to recombinant DNA technology?

Ans

A competent host is a cell that has the ability to take up extracellular DNA from the environment. In recombinant DNA technology, competent microbial cells are used in order to avoid expensive and complex transformation techniques like biolistics and microinjection.

Q.36 Write short note on the following:
a. Micro-injection
b. Biolistics
c. Disarmed pathogen

Ans

a. Micro-injection: Microinjection is a transformation technique where engineered vectors/genes are injected into the host cell using a micro-needle. The process is carried out under an optical microscope called the micromanipulator and the DNA molecules can not only be delivered inside the cell membrane but also the nuclear envelope when needed.
b. Biolistics: Biolistics is a transformation technique involving the use of a gene-gun or the Biolistic Particle Delivery System. Generally used to transform plant cells, the system uses a heavy metal particle coated with the engineered vector. This coated particle is fired into the host cells using a specialized airgun. Specialized biolistics can not only transform genetic material but also cell organelles.
c. Disarmed pathogen: Pathogens/viruses that are devoid of their virulent genes are called disarmed pathogens. These pathogen vectors when allowed to infect a host are able to transfer the recombinant DNA into the cell without causing any disease. Such vectors are however not completely safe as there are chances of such vectors reverting to virulent forms.

Q.37 Expand PCR. Explain the steps involved in the technique. What is the PCR machine called?

Ans

Polymerase Chain Reaction.
The steps involved are: – A PCR starts with a denaturing step. The DNA sample is heated to 94-960C to break the hydrogen bonds between the bases of the two strands.
– When the two strands separate out, the temperature is lowered to 50-560C, which allows the primers (short DNA fragments that acts as starting point for replication) to anneal (base pair) with the denatured strands.
– The temperature is raised to 720C at which the Taq polymerase (a thermostable polymerase) elongates the primer by adding nucleotides.

The PCR machine is called a thermocycler.

Q.38 What is a primer? Why is it required?

Ans

A primer is a short DNA sequence that pairs to a larger DNA fragment at a complementary sequence and acts as a starting point for DNA replication. DNA polymerase elongates a sequence by adding nucleotides to it and thus requires a primer strand.

Q.39 What is a recombinant protein?

Ans

A recombinant protein is a protein expressed artificially in an organism with the help of recombinant DNA technology, e.g., artificial human insulin expressed in E. coli.

Q.40 What is a bioreactor?

Ans

A bioreactor is a device, natural or artificial (bioreactor vessels) that supports biologically active environment and in which chemical reactions involving organisms and/or biologically active substances are carried out.

Q.41 Write a short note on pBR322.

Ans

pBR322 is a commonly used E. coli plasmid cloning vector. The molecule is a double-stranded circular DNA, 4361 base pairs in length. It was the first artificial plasmid created by Bolivar and Rodriguez and was named after them.

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FAQs (Frequently Asked Questions)

Natural habitats have limited food, space and other resources. Population growth therefore slows as density approaches the habitat’s carrying capacity.

Yes. Ecologists may use biomass, percentage cover, fish caught per trap, pug marks or faecal pellets when direct counting is difficult or misleading.

Predators control dominant prey populations. This reduces competition among prey species and prevents one species from excluding the others.

Competing species use different feeding times, habitat zones or foraging methods. This reduces direct competition and allows them to coexist.

A predator usually kills and consumes its prey. A parasite lives on or inside a host and obtains resources while generally keeping the host alive for some time.