Transgenic plants

Transgenic plants – Herbicide resistance in transgenic plants.

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Foreign gene

In genetically engineered plant cells, a bacterium Agrobacterium is mainly involved in transfer of foreign gene.

However, Agrobacterium cannot infect all plants since it has a narrow range of host specificity.

Therefore other techniques have been developed to introduce foreign DNA into plant cells.

Novel methods of ensuring DNA uptake into cells include electroporation and mechanical delivery or biolistics.

Electroporation is a process of creating temporary pores in the cell membrane by application of electric field.

Creation of such pores in a membrane allows introduction of foreign molecules such as DNA, RNA, antibodies, drugs, etc. into cytoplasm.

The development of this technique is due to contribution of biophysics, bioengineering, cell and molecular biology.

While the technique is now used widely to create transgenic microorganisms, plants and animals, it is used increasingly for application of gene therapy.

The mechanical particle delivery or gene gun methods to deliver DNA on microscopic particles into target tissue or cells.

The process is increasingly used to introduce new genes into a range of bacterial, fungal plant and mammalian species.

It is the main method of choice for genetic engineering of many plant species including rice, corn, wheat, cotton and soyabean.

(modified) Transgenic plants:

Presently, more than 50 types of genetically engineered plant species, called transgenic plants have been successfully developed.

These plants were made to resist insect pests, viruses or herbicides through incorporation of foreign gene into DNA of host plant cells.

Initially transgenic plants were developed more in dicotyledons, but now extended to several monocotyledons like wheat, maize, rice and oats.

Transgenic plants have also been developed and are suitable for food industries (delaying ripening in tomato).

Gene pharming, the use of transgenic plants as bioreactors or factories for production of speciality chemicals and pharmaceuticals is being pursued by a number of firms.

Plants have been engineered to produce human proteins, such as hormones, in their seeds.

A weed called mouse-eared cress has been engineered to produce a biodegradable plastic (polyhydroxybutyrate, or PHB) in tissue granules.

Transgenic dicotyledonous plants

1. Nicotiana tabacum

2. Beta vulgaris

3. Glycine max

4. Helianthus annuus

5. Solanum tuberosum

6. Gossypium hirsutum

Transgenic monocotyledonous plants

1. Asparagus sp.

2. Oryza sativa

3. Zea mays

4. Avena sativa

Herbicide resistance in transgenic plants

Under normal circumstances, herbicides affect photosynthesis or biosynthesis of essential amino acids.

Under field conditions, application of herbicides not only kills the unwanted weeds but also greatly affects the field crops.

In order to protect the crops against exposure to herbicides, scientists after intensive research isolated a gene from Streptomyces hygroscopicus which encodes an enzyme, capable of inactivating the herbicide ‘Basta’.

Transgenic plants with this gene have been developed, demonstrating effectiveness of this gene for protection against herbicide ‘Basta’.

Thus, herbicide-tolerant crop plants have now been developed by genetically manipulating plant genomes resistant to specific herbicides.

Improved resistance to insect pests and microbial diseases

Genes from Bacillus thuringiensis (Bt2) have been introduced into several crops, including tomato and cotton, and field-testing has demonstrated impressive results against many pests.

Spore preparation of this bacterium is used as a biological insecticide during the last 20 years.

Insecticidal activity depends on a toxic protein called delta endotoxins.

The toxin gene (Bt2) from Bacillus thuringiensis has been isolated and used for Agrobacterium.

Ti plasmid mediated transformation of tobacco, cotton and tomato plants. The transgenic plants were resistant to the Manducta sexta, a pest of tobacco.

India had acquired technology from U.S.A. for introducing Bt toxin gene in cotton for the development of resistance against pests in this major cash crop of India.

Widespread use of insecticides, fungicides and pesticides for crop protection undoubtedly has damaging effects on the environment and hence it is important to improve the control of pests and diseases by genetic means.

Genetic modification of plants is an attempt for ecofriendly measures against environmental degradation.

Through genetic modification, the oil-producing soya bean was tailored to produce a wide range of industrial lubricants, cosmetic compounds and detergents that are biodegradable.

A whole new area of biotechnology has been opened up and plants are made to synthesize many novel substances including functional human antibody fragments.

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Recombinant DNA technology

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Gene transfer in plants, Cutting of DNA, Advantages of recombinant DNA

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Practical application of genetic transformation

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Plant tissue culture – origin and techniques

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Basic techniques of plant tissue culture

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Status of tissue culture technology in India And application

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Protoplast fusion and Practical applications

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Single cell protein and its uses

Gene transfer in plants, Cutting of DNA, Advantages of recombinant DNA

Gene transfer in plants, Cutting of DNA, Advantages of recombinant DNA.

Gene transfer in plants

Gene transfer in plants

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Agrobacterium tumefaciens is a soil inhabiting bacterium and has Ti (tumor inducing) plasmid.

This bacterium invades crops such as tomato, sunflower, brinjal and cotton and causes crown gall disease which is in the form of tumerous growth.

The Ti plasmid carried by the pathogenic bacterium causes tumours.

For effective cloning of foreign genes by the plant cells, and for introduction of genes into plant system, Agrobacterium strains are modified by the removal of tumour – inducing genes from the bacterium.

T–DNA is the part of Ti plasmid transferred into plant cell DNA.

The T–DNA which holds the desired foreign gene after splicing is introduced into the plant cell.

The bacterial plasmid do not produce tumerous growth since the gene had been deleted.

Once the T–DNA along with the spliced gene is introduced, it combines with the chromosome of the donor cell where it produces copies of itself, by migrating from one chromosomal position to another.

Through tissue culture methods, such plant cells are cultured, induced to multiply and differentiate to form into plantlets.

The plantlets are transplanted to soil, where they are allowed to express the foreign gene introduced into them when they multiply and grow in larger population.

How DNA is cut?

All bacteria produce atleast one type of restriction enzymes.

They are meant to help the recombinant researchers to enable them to cut the DNA but to help in the very survival of the bacterial species against the invading bacterial viruses.

The restriction enzymes can chop up and render harmless invading viral DNA.Restriction enzymes cleave DNA at very specific places along its length.

The restriction enzyme ECORI (E.coli Restriction Enzyme I) produced by the intestinal bacterium E. coli recognizes the following sequence.

Two DNA molecules with sticky ends (ends that are staggered or uneven) tend to join with other molecules with a complementary sequence of nucleotides in the ends.

With the same enzyme, DNA fragments are cut with the matching sequence of nucleotides which complements with the sticky ends.

Action of restriction enzyme

Fragments of DNA from different organisms or even from different species may be joined together at their sticky ends, thereby producing recombinant DNA.

This is made possible by the use of an enzyme called ligase. Hence, ligase is used to join the two DNA fragments.

Restriction enzyme and ligase do not care about the source of DNA.

Whenever the correct sequence of nucleotides are met with by the specific restriction endonuclease, it cuts it.

Similarly whenever the correct sequences of nucleotides at their sticky ends in the two strands are met with, these are ligated (joined) by the enzyme ligase.

Advantages of recombinant DNA

A sample of therapeutic drugs presently manufactured through recombinant DNA is found in the table given below.

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Recombinant DNA technology

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Transgenic plants – Herbicide resistance in transgenic plants

---

Practical application of genetic transformation

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Plant tissue culture – origin and techniques

---

Basic techniques of plant tissue culture

---

Status of tissue culture technology in India And application

---

Protoplast fusion and Practical applications

---

Single cell protein and its uses

Recombinant DNA technology

Recombinant DNA technology Biotechnology Introduction

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In the science of genetics, some sweeping changes are taking place.

A lowly bacterium that is found in the bowels of everyone namely Eschefichia coli is drawing the attention of all scientists and learned people.

This bacterium has become one of the potentially most powerful tools known to science in genetic manipulation.

In this chapter, you will learn that we now have the ability to find specific genes, to cut them away from chromosomes, to insert them into the chromosomes of other species.

Genes have been duplicated countless times to harvest their protein product in large quantities.

There are both advantages and disadvantages in doing recombinant research on plants.

Several important species such as carrot, cabbage, citrus and potatoes can be grown from single cells.

So, once a gene is introduced to a cell, a clone of that cell can produce countless altered progeny.

Most plant characteristics that need improvement such as growth rate, size of edible parts and amino acid balance are polygenic – controlled by many genes.

Most of the genes responsible for such traits were not yet identified. It is also very difficult to clone five or more genes which control a trait.

These inevitable disadvantages are there in recombinant DNA technology.

Recombinant DNA technology

It is a technique where the selected DNA of one organism (Donor) is introduced to combine with the DNA of another organism called recipient organism.

As a result, the recipient organism acquires the genetic abilities of the donor.

Altering the genome of an organism by introducing genes of interest is known as gene manipulation or DNA recombinant technology.

As this mechanism has the ability to engineer new organisms, it is known as genetic engineering.

Basic techniques of genetic engineering

Bacterial cells have different kinds of enzymes.

Some of these can cut DNA into fragments and others can join such fragments.

For example, restriction endonucleases discovered in 1970 are involved in cutting DNA at specific sites. Hence they are called molecular scissors.

The enzyme DNA ligase discovered in 1966 acts like a paste molecule to join DNA fragments.

Thus the restriction endonuclease and the DNA ligase are the basic tools required for genetic engineering.

Events of Recombinant DNA technology:

The events of recombinant DNA technology are as follows.

1. The DNA of donor organism or gene of interest is isolated and cut into fragments using restriction endonucleases.

2. They are attached to a suitable replicon. Such replicon is known as vector or cloning vehicle, which is nothing but the extra chromosomal circular DNA found in the cytoplasm of Eschrichia coli is called plasmid. The plasmids are the most suitable vectors.

3. The DNA of the vector is cut into fragments using the same restriction endonucleases. Using the enzyme DNA ligase, the DNA fragments of donor and vector are joined together. This process is called splicing. As a result of splicing hybrid DNA or recombinant DNA (rDNA) is obtained.

4. The rDNA is introduced into the host cells such as E.coli, Bacillus subtilis, Streptomyces sp. etc.,

5. For this the host cells are treated with the enzyme cellulase. So that the cell wall of host becomes permeable to the entry of rDNA.

The host organism follows the instructions of “foreign rDNA”.

It continues to multiply with the foreign DNA or gene of interest.

After a short time, this results in a colony of bacteria having rDNA fragments.

Each colony is grown separately to obtain multiplication of rDNA fragments.

At the end we get a number of colonies having identical copies of rDNA fragments.

This is called molecular coloning or gene coloning.

Once the gene for the production of human insulin from pancreatic cells is introduced into E.coli, the recipient cell produces human insulin.

This is the way by which the human insulin is made to be produced by bacterial cell such as E.coli

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Gene transfer in plants, Cutting of DNA, Advantages of recombinant DNA

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Transgenic plants – Herbicide resistance in transgenic plants

---

Practical application of genetic transformation

---

Plant tissue culture – origin and techniques

---

Basic techniques of plant tissue culture

---

Status of tissue culture technology in India And application

---

Protoplast fusion and Practical applications

---

Single cell protein and its uses

Structure of RNA and Types of RNA

Structure of RNA and Types of RNA

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The Ribonucleic acid is otherwise known as RNA.

This is universally present in all organisms except in DNA viruses.

It is made up of nucleotides called ribonucleotides.

There are four types of nucleotides having four different nitrogenous bases.

But sugar and phosphate are common for all nucleotides.

The four nucleotides are adenine, cytosine, guanine and uracil.

The RNA plays an important role in protein synthesis.

Now we will know more about their types and their role in the biology of an organism.

Types of RNA

There are three major types of RNA which occur in all organisms.

They are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

Messenger RNA

As the name suggests mRNA carries the genetic information from DNA to the ribosomes.

Genetic informations on the DNA are transcribed into the mRNA by a process called transcription.

Here the “message” is translated into action

i.e. based on the genetic information different types of proteins are synthesised.

The type of gene that is involved in protein synthesis depends upon their length, kinds and sequence of nucleotides.

It is about 3 to 5 per cent of the RNA content of the cell.

The mRNA is always single stranded.

The mRNA is produced as a complementary copy of the DNA, which is involved in protein synthesis.

Transfer RNA

Transfer RNA is also known as soluble RNA (sRNA).

The tRNA is a small molecule compared with other types of RNAs.

It amounts to about 15 per cent of total RNA of the cell.

The tRNA molecule performs a number of functions.

The most important one is to act as a carrier of aminoacid to the site of protein synthesis.

There are about more than 20 types of tRNAs. Each tRNA is

specific for a particular amino acid. In bacterial cell, there are more than 70 tRNAs and in eukaryotic cells the number is even greater.

There are four or five tRNAs specific for a particular amino acid and these are called isoacceptor tRNAs.

Structure of RNA

The tRNA has a cloverleaf like structure.

It is synthesized in the nucleus on a small part of DNA.

In 1965, R.W. Holley suggested the cloverleaf model of tRNA.

Though tRNA molecule consists of a single strand, it assumes clover leaf like structure through folding.

There are three folds in the clover leaf tRNA. It has four arms namely anticodon arm, D arm, T C arm and aminoacid acceptor arm.

The tRNA molecules are made up of 73 to 93 ribonucleotides.

The acceptor arm carries an aminoacid.

The anticodon arm has three anticodon nucleotides, which will join with the complementary codon in mRNA during protein synthesis

i.e. three nucleotides in the tRNA pairs with three nucleotides of mRNA.

In certain tRNAs in addition to these four arms an extra arm called variable arm occurs as shown in the figure.

The aminoacid acceptor and the anticodon arms are oriented in opposite directions.

Ribosomal RNA

This is found in the ribosomes.

The rRNA represents about 40 to 60 per cent of the total weight of the ribosomes.

Relatively it constitutes about 80 per cent of the total RNA of the cells.

They are produced in the nucleus.

They are the most stable forms of RNA.

They consist of single strand of nucleotides.

At some regions, the strand is folded.

Comparison between DNA and RNA are explained below.

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

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Types of chromosomes with special types

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Gene and genome

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Linkage and mechanism of linkage

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Crossing over, gene mapping and recombination of chromosome

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Mutation and classification of mutation

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Mutagenic agents and its significance

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Structural Chromosomal aberrations

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Numerical chromosomal aberrations

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Role of DNA 

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Structure of DNA and Function of DNA

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Replication of DNA

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Replication of DNA

Replication of DNA is explained fully with detail.

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DNA is the genetic material of almost all the organisms.

One of the active functions of DNA is to make its copies which are transmitted to the daughter cells.

Replication is the process by which DNA makes exact copies of itself.

Replication is the basis of life and takes place during the interphase stage.

Watson and Crick suggested the semiconservative method of replication of DNA.

This has been proved by Messelson and Stahl’s in their experiments on Escherichia coli using radioactive isotopes.

The replication of chromosome in E. coli is completed in 40 minutes.

During replication of DNA, the two complementary strand of DNA uncoil and separate from one end in a zipper like fashion.

The enzyme helicase unwinds the two strands and as a result replication fork is formed.

As the DNA unwinds, the part of the DNA that is found above the replication fork becomes supercoils.

These are called positive supercoils.

An enzyme called topoisomerase releases these supercoils.

Based on separated DNA strands, new strands grow by the addition of nucleotides.

DNA polymerase I, II and III are involved in this elongation.

However, these enzymes are not capable of initiating DNA synthesis.

For the synthesis of new DNA, two things are required.

One is RNA primer and the enzyme primase.

The DNA polymerase moves along the newly formed RNA primer nucleotides, which leads to the elongation of DNA.

In the other strand, DNA is synthesized in small fragments called Okazaki fragments.

These fragments are linked by the enzyme called ligase.

In the resulting DNA, one of the strand is parental and the other is the newer strand which is formed discontinuously.

Hence, it is called semi discontinuous replication.

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

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Types of chromosomes with special types

---

Gene and genome

---

Linkage and mechanism of linkage

---

Crossing over, gene mapping and recombination of chromosome

---

Mutation and classification of mutation

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Mutagenic agents and its significance

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Structural Chromosomal aberrations

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Numerical chromosomal aberrations

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Role of DNA 

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Structure of DNA and Function of DNA

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Structure of RNA and Types of RNA

Role of DNA

Role of DNA – Hereditary, and Bacterial transformation are explained in detail.

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It is evident that chromosomes are the carriers of genetic material. Chromosomes contain proteins, DNA and RNA.

Role of DNA as a genetic material

It is universally accepted that DNA is the genetic material in most of the organisms and higher organisms.

In most of the plant viruses, RNA is the genetic material.

There are many direct evidences for DNA being the genetic material.

Here, we will discuss one of the evidences illustrated by Frederick Griffith.

Hereditary role of DNA – Bacterial transformation

Hereditary role of DNA – Bacterial transformation in 1928, the bacteriologist Frederick Griffith conducted an experiment using Diplococcus pneumoniae.

He studied two strains of virulent Diplococcus causing pneumonia.

The virulent strain synthesized a smooth polysaccharide coat and produces smooth colonies.

This strain was called strain-S.

Another strain which lacked the proper polysaccharide coat is harmless and produces rough colonies.

This strain was called strain-R.

When Griffith injected S-type of cells into the mouse, the mouse died.

When R-type cells were injected into the mouse, the mice did not die.

He injected heat killed S-type cells into the mouse. The mouse did not die.

Griffith killed some smooth strain bacteria and mixed it with live rough strain bacteria.

When the mixture of heat killed S-type cells and R-type cells was injected into the mouse, the mouse was dead.

The living rough strain of Diplococcus had been transformed into S-type cells.

That is the hereditary material of heat killed S-type cells had transformed R-type cells into virulent smooth strains.

Thus the phenomenon of changing the character of one strain by transferring the DNA of another strain into the former is called transformation.

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

---

Types of chromosomes with special types

---

Gene and genome

---

Linkage and mechanism of linkage

---

Crossing over, gene mapping and recombination of chromosome

---

Mutation and classification of mutation

---

Mutagenic agents and its significance

---

Structural Chromosomal aberrations

---

Numerical chromosomal aberrations

---

 

Structure of DNA and Function of DNA

---

Replication of DNA

---

Structure of RNA and Types of RNA

Numerical chromosomal aberrations

Numerical chromosomal aberrations are explained fully in detail.

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Description about Numerical chromosomal aberrations

Each species of an organism has a specific number of chromosomes in its somatic cells.

These chromosomes are found in pairs. At the time of formation of gametes the chromosome number is reduced.

Hence, the gemetes carry haploid set of chromosomes.

Alterations in the number of chromosomes from the diploid set is called numerical chromosomal aberrations.

It is also known as ploidy. There are two types of ploidy they are euploidy and aneuploidy.

Euploidy

Euploidy is the variation in the chromosome number that occurs due to increase or decrease of full set of chromosomes.

Monoploidy, diploidy and polyploidy are the types in euploidy.

Diploidy

In most of the plants and animals, the somatic cells contain two sets of chromosome.

Diploidy is formed by the union of two gametes during fertilization.

Polyploidy

Addition of one or more sets of chromosomes to the diploid set results in polyploidy.

It is commonly noticed in plants and rare in animals.

They are of two kinds – autopolyploidy and allopolyploidy.

Autopolyploidy

Addition of one or more haploid set of its own genome in an organism results in autopolyploidy.

Watermelon, grapes and banana are autotriploids, whereas apple is an autotetraploid.

Allopolyploidy

Increase in one or more haploid set of chromosomes from two different species result in allopolyploidy.

Triticale is the first man made cereal.

It is obtained by crossing a wheat Triticum durum (2n = 4x = 28) and a rye Secale cereale (2n = 2x = 14).

The Fl hybrid (2n = 3x = 21) is sterile.

Then the chromosome number is doubled using colchicine and it becomes an hexaploid.

Aneuploidy

Variation that involves one or two chromosomes within the diploid set of an organism results in aneuploidy.

It is of two types – hypoploidy and hyperploidy.

Hypoploidy

Decrease in one or two chromosomes from the diploid set is described as hypoploidy.

There are two types of hypoploidy – monosomy and nullisomy. Monosomy is due to loss of a chromosome from the diploid set

i.e. 2n – 1.

Nullisomy is the condition in which a pair of homologous chromosomes is lost from the diploid set i.e. 2n – 2.

Hyperploidy

Addition of one or two chromosomes to the diploid set of chromosome results in hyperploidy.

There are two types of hyperploidy – trisomy and tetrasomy.

Trisomy results due to the addition of one chromosome to diploid set of chromosomes.

It is represented by 2n + 1. Trisomics are observed in Datura stramonium.

Tetrasomy results due to the addition of two chromosomes to diploid set of chromosome.

It is represented by 2n+2.

Significance of ploidy

Polyploidy plays an important role in plant breeding and horticulture.

0 Polyploidy has more vigorous effect than the diploids and results in
the production of large sized flowers and fruits.

Hence, it has economical significance.

It plays significant role in the evolution of new species.

Polyploidy results in the changes in the season of flowering and fruiting.

Polyploids are vigorous invaders of new habitats.

It leads to the formation of new varieties which show high resistance to disease and increase in yield.

Tetraploid cabbages and tomatoes contain more ascorbic acid whereas tetraploid corn contains more vitamin A.

Both euploidy and aneuploidy in man cause congenital diseases.

Polyploidy varieties like apple, pear, grape and watermelons are cultivated because of their large size.

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For more details about Numerical chromosomal aberrations click here

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

---

Types of chromosomes with special types

---

Gene and genome

---

Linkage and mechanism of linkage

---

Crossing over, gene mapping and recombination of chromosome

---

Mutation and classification of mutation

---

Mutagenic agents and its significance

---

Structural Chromosomal aberrations

---

 

Role of DNA 

---

Structure of DNA and Function of DNA

---

Replication of DNA

---

Structure of RNA and Types of RNA

Structural chromosomal aberrations

Structural chromosomal aberrations is explained with full details.

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In an organism, any visible abnormality in chromosome number or structure from the diploid set is known as chromosomal aberration.

The chromosomal aberrations based on the structure of the chromosome are of four types – deletion, duplication, inversion and transversion.

Structural chromosomal aberrations

Deletion – Structural chromosomal aberrations

The loss of a segment of the genetic material in a chromosome is called deletion.

It may be terminal or intercalary. When the deletion occurs near the end of the chromosome, then it is called terminal deletion.

Eg. Drosophila and Maize.

When the deletion occurs in the middle of the chromosome then, it is called intercalary deletion.

Most of the deletions lead to death of an organism.

Duplication – Structural chromosomal aberrations

When a segment of a chromosome is present more than once in a chromosome then, it is called duplication.

For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h.

Due to aberration, the genes ‘g’ and ‘h’ are duplicated and the sequence of genes becomes a, b, c, d, e, f, g, h, g and h.

In Drosophila, corn and peas a number of duplications are reported.

Some duplications are useful in the evolution of the organism.

Inversion – Structural chromosomal aberrations

It is another chromosomal abnormality in which, the order of genes in a chromosomal segment is reversed by an angle of 180°.

For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h.

Due to aberration, the sequence of genes becomes, a, b, c, d, g, f, e and h.

There are two types of inversion – pericentric and paracentric inversion.

In pericentric inversion, the inverted segment of the chromosome contains centromere.

Sometimes, it is responsible for evolution of the organism.

For example the 17t1, human chromosome is acrocentric, while in Chimpanzee the corresponding chromosome is metacentric.

In paracentric inversion, the inverted segment of the chromosome has no centromere.

Translocation – Structural chromosomal aberrations

It is a kind of a chromosomal abnormality in which the interchange of the chromosomal segments occurs.

When translocation occurs between two non-homologous chromosomes, then it is called reciprocal translocation or illegitimate crossingover.

It is of two kinds ¬ heterozygous translocation and homozygous translocation.

In heterozygous translocation, one member of each pair of chromosomes is normal and the other member is with interchanged segment.

But in homozygous translocation, both the members of paired chromosomes have translocated segments.

They play an important role in species differentiation. Translocations causes hereditary disorders.

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

---

Types of chromosomes with special types

---

Gene and genome

---

Linkage and mechanism of linkage

---

Crossing over, gene mapping and recombination of chromosome

---

Mutation and classification of mutation

---

Mutagenic agents and its significance

---

 

Numerical chromosomal aberrations

---

Role of DNA 

---

Structure of DNA and Function of DNA

---

Replication of DNA

---

Structure of RNA and Types of RNA

Mutagenic agents

Mutagenic agents and its significance are explained in detail.

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In a species, variations are caused by changes in the chemical substances and environmental conditions which cause mutations in the organisms are called mutagens or mutagenic agents.

There are two kinds of mutagenic agents – physical and chemical mutagenic agents.

Physical mutagenic agents

Electromagnetic radiation, radiations like ultraviolet rays, temperature, etc. are some of the examples for physical mutagens.

X-rays and gamma rays are ionizing radiations which induce mutation in seeds. UV rays are nonionizing radiations.

Pollen can be treated with UV since pollen has germinal nucleus in which mutation can be caused.

Chemical mutagenic agents

Chemicals can also be used for inducing mutations in the organisms. Such chemicals are called chemical mutagenic agents.

eg. Nitrous acid, Methyl methane sulphonate (MMS) and ethyl methane sulphonate (EMS).

Ethyl methane sulphonate has been extensively used for inducing mutations in microorganisms, higher plants and animals.

Significance of mutation

Mutations play an important role in the origin of new species and serves as a tool for evolution.
Induced mutations are useful in agriculture, animal husbandry and biotechnology to produce new strains.

For example, mutant strains of Penicillium produces more penicillin.

It is one of the best approaches for improvement of crops.

Induced mutants are reported in paddy, wheat, soyabeans, tomatoes, oats, and barley.

Mutant varieties of wheat are early maturing, disease resistance and they are enriched with protein.

Mutant varieties of paddy produce many tillers with long grains.

The study of mutant strains of viruses helps us to know the fine structure of gene.

The genes are made up of small functional units such as cistron, recon and muton.

Cistron is an unit of function, recon is the unit of recombination and muton is the unit of mutation.

Many types of mutations cause heritable diseases and cancer in human beings.

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STRUCTURE OF CHROMOSOME – CELL BIOLOGY

---

Types of chromosomes with special types

---

Gene and genome

---

Linkage and mechanism of linkage

---

Crossing over, gene mapping and recombination of chromosome

---

Mutation and classification of mutation

---

 

Structural Chromosomal aberrations

---

Numerical chromosomal aberrations

---

Role of DNA 

---

Structure of DNA and Function of DNA

---

Replication of DNA

---

Structure of RNA and Types of RNA

Mutation

Mutation and classification of mutations

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In a species, variations are caused by changes in the environment or any changes in the innate genetic setup of an organism or by the combination of both.

Sudden change in the genetical set up of an organism is defined as mutation.

In 1901, Hugo de Vries first used the term mutation based on his observation on Oenothera lamarckiana.

Charles Darwin termed these sudden change as ‘sports’. According to Bateson, mutation is a discontinuous change.

Based on molecular basis of heredity, mutations is defined as sudden change in the sequence of nucleotides of gene.

The mutation brings about a change in the organism. The organism which undergoes mutation, is called a mutant.

eg. Oenothera lamarckiana.

Mutations that affect the biochemical reactions are called biochemical mutations.

For example, biochemical mutants of Neurospora failed to synthesize certain amino acids.

Some mutations drastically influence the genes and cause death to the individual.

Such mutations are described as lethal mutation.

For example, in the plant Sorghum, recessive mutant fails to produce chlorophyll and therefore they die in the seedling stage.

Thus, most of the mutations are harmful, because they disturb the genic balance of the organism.

Although most of the mutations are useless and even harmful, and some of the mutations play a significant role in the evolution of new species.

Many new strains of cultivated crops and new breeds of domesticated animals are the products of gene mutations.

Small seeded Cicer arietinum (bengal gram) suddenly get mutated to large seeded Cicer gigas is the case of gene mutations.

Classification of mutation

 

Mutations have been classified in various ways based on different criteria.

Depending on the kind of cell in which mutations occur, they are classified into somatic and germinal mutations.

They may be autosomal or sex chromosomal according to their type of chromosome in which they occur.

They may be spontaneous or induced according to their mode of origin.

They may be forward or backward according to their direction.

They may be dominant or recessive according to their phenotypic expression of mutated genes.

Point or gene mutation

Point mutations is sudden change in small segment of DNA either a single nucleotide or a nucleotide pair.

Gross mutations is a change involving more than one or a few nucleotides of a DNA.

The gene mutation may be caused by loss or deletion of a nucleotide pair.

This is called deletion mutations and reported in some bacteriophages.

Addition of one or more nucleotides into a gene results in addition mutations.

Replacement of certain nitrogen bases by another base in the structure of DNA results in substitution mutations.

The deletion and addition mutation alter the nucleotide sequence of genes and ultimately result in the production of defective protein and this leads to the death of the organism.

The substitution mutations can alter the phenotype of the organism and have great genetic significance.

There are two types of substitution mutations – transition and transversion.

When a purine or a pyrimidine is replaced by another purine or pyrimidine respectively this kind of substitution is called transition.

When a mutation involves the replacement of a purine for pyrimidine or viceversa this is called transversion.

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Other links 

STRUCTURE OF CHROMOSOME – CELL BIOLOGY

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Types of chromosomes with special types

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Gene and genome

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Linkage and mechanism of linkage

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Crossing over, gene mapping and recombination of chromosome

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Mutagenic agents and its significance

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Structural Chromosomal aberrations

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Numerical chromosomal aberrations

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Role of DNA 

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Structure of DNA and Function of DNA

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Replication of DNA

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Structure of RNA and Types of RNA