It is the branch of science which deals with the synthesis of artificial genes, repair of genes and combination of genes of two different organisms. The technique for the combination of genes of two different organisms involves the cutting of a particular gene (or a particular segment of DNA) with the help of certain enzymes and then inserting them into the chromosomes of another organism so that both types of genes can combine with each other and produce a new organism having characteristics of both genes.

The organisms which are genetically modified by inserting the genes from other organisms into them are known as ‘Transgenic organisms’ or ‘genetically modified organisms’ (GMO).

Importance of Genetic Engineering

Following is the importance of genetic engineering:

1. Genetic engineering is used for making various types of medicines. The treatment of diseases which is done with the help of genetic engineered medicines is called ‘gene therapy’.
2. Genetic engineering helps in finding out the characteristics of genetic materials like that of DNA and RNA.
3. With the help of genetic engineering, we can change those genes of a person which are harmful for him.
4. With the help of genetic engineering, the scientists are also trying to insert the genes of nitrogen-fixing bacteria rhizobium in non-leguminous plants so that the non-leguminous plants can also do fixation of free nitrogen gas present in the atmosphere.
5. With the help of genetic engineering, it is possible to make a human with desired characters.

Test Your Understanding and Answer These Questions:

1. What are transgenic organisms?
2. Expand GMO.
3. Explain importance of genetic engineering.

Complementary pairing is such a specific type of pairing in which the sequence of bases in one strand determines the sequence of bases on the opposite strand. For example, in the structure of DNA wherever adenine appears in one strand, thymine appears opposite to it on the other strand. In the same way, wherever cytosine appears in one strand, guanine appears opposite to it on other strands.

Test Your Understanding and Answer These Questions:

1. What is complementary pairing?

The full form of DNA is Deoxyribonucleic Acid. DNA is the genetic material in all the eukaryotic organisms. DNA is present mostly in the nucleus of all cells and is a main component of chromosomes.

Chemical Composition of DNA

The basic structural unit of DNA is nucleotide i.e. DNA is made up of a large number of nucleotide molecules which are joined by covalent bonds to form a long chain. As the DNA is made up of a large number of nucleotide molecules so it is also called ‘polynucleotide’.

A nucleotide molecule is further made up of three different components:

    1. A phosphate group
    2. A pentose sugar
    3. A nitrogen containing base

1. Phosphate group

The phosphate group present in DNA consists of 1 atom of phosphorous and 4 atoms of oxygen. While making the structure of DNA the phosphate group is represented by writing letter P in the circle as  .

2. Pentose sugar

The sugar present in DNA is a five carbon sugar or pentose sugar which is also called as deoxyribose sugar. In the structure of DNA the pentose sugar is represented by writing letter S in the pentagon as  .

3. Nitrogenous base

There are 4 different types of nitrogenous bases in the structure of DNA. The names of these 4 nitrogenous bases are

    1. Adenine
    2. Thymine
    3. Cytosine and
    4. Guanine.

These 4 nitrogenous bases are classified into two types:

    1. Purines: – E.g. Adenine and Guanine.
    2. Pyrimidines: – E.g. Cytosine and Thymine.

Structure of DNA

The structure of DNA was first of all established by two scientists named James Watson and Francis Crick in 1953. These scientists proposed that DNA molecule has a duplex structure i.e. DNA molecule consists of two strands of polynucleotide chains which are coiled around each other in the form of a double helix. That is why; it is also called as double helical structure of DNA. The single helical turn of DNA molecule has a breadth of 20 A^o length of 34 A^o in which 10 units of nucleotides are present. Each strand of DNA consists of alternating sugar and phosphate groups bonded with each other. The 4 nitrogenous bases i.e. adenine, cytosine, thymine and guanine are stacked inside the helix in planes perpendicular to the helical axis.

The nitrogenous base present on two opposite strands are bonded with each other with the help of weak hydrogen bonds. Adenine and Thymine are bonded by two hydrogen bonds whereas cytosine and guanine are bonded by three hydrogen bonds.

This structure of DNA resembles with the structure of a spiral staircase in which the steps are represented by nitrogenous bases and railing of the staircase is represented by the sugar phosphate framework.

Test Your Understanding and Answer These Questions:

1. Write the full form of DNA.
2. Why is DNA called a polynucleotide?
3. Explain chemical composition of DNA.
4. Name two purines and pyrimidines.
5. What are the components of deoxyribo nucleic acid?
6. How many types of nitrogenous bases are present in a molecule of DNA? Name them.
7. Explain double helical structure of DNA proposed by Watson and Crick.
8. What types of bonds are present between two polynucleotide strands of DNA?

The full formation of RNA is ribonucleic acid. RNA is the genetic material of some viruses i.e. it is responsible for the transmission of hereditary information from one generation to other in viruses. RNA is present in the cytoplasm of prokaryotes as nucleus is absent in prokaryotes. In eukaryotes RNA is found in the nucleolus of nucleus. A molecule of RNA consists of a single polynucleotide chain. It also consists of 4 nitrogenous bases namely

1.  Adenine
2.  Cytosine
3.  Guanine and
4.  Uracil

Retrovirus

A virus which has RNA as the genetic material is called ‘retrovirus’. For example the AIDS virus HIV (Human Immunodeficiency Virus) and TMV (Tobacco Mosaic Virus) are retrovirus.

Types of RNA

RNA is of three types:

1. mRNA – (messenger RNA)
2. tRNA – (transfer RNA)
3. rRNA – (ribosomal RNA)

Test Your Understanding and Answer These Questions:

1. Write the full form of RNA.
2. Define retrovirus. Give examples.
3. Name the various types of RNA.

Nucleic acid was first of all isolated by a scientist Fredrick Meisher in 1870 from the nucleus of the pus cells. Nucleic acids are substances which are responsible for the transmission of hereditary information from one generation to other. Actually nucleic acids are strong acids which were found in the nucleus that is these are called nucleic acids.

Types of Nucleic Acids

There are two types of nucleic acids in different organisms

    1. DNA
    2. RNA

The fact that DNA is the genetic material of eukaryotic organisms was confirmed by the scientists Griffith, Avery, Macleod and McCarty.

Test Your Understanding and Answer These Questions:

1. Define nucleic acids.
2. Name the types of nucleic acids.
3. Name the scientists who provided the evidence of DNA as a genetic material.

The process by which the sex of a person is determined is called sex determination. The sex of a person is determined by sex chromosomes. The sex chromosomes in human beings are present in 23 pair. Both the sexes either male or female contain 23 pairs of chromosomes. The first 22 pairs of chromosomes are autosomes which are similar in both the males and females.

The sex chromosomes are similar in females and are called XX. This means that females can produce only one kind of eggs each having X chromosome. On the other hand sex chromosomes of males are dissimilar and are called as XY. The chromosome Y is much smaller in size as compared to chromosome X. So a male can produce two types of sperms out of which half the sperms will have X chromosomes while the other half will have the Y chromosome.

Determination of Sex of Human Child

The sex of a child depends on the fusion of different kinds of sperms with eggs during fertilization. As sperms are of two types, so the following two cases may arise

Case 1 – If the sperm containing X chromosome fuses with the egg containing X chromosome then the child born will be a girl having XX combination of sex chromosomes.

Case 2 – If the sperm containing Y chromosome fuses with the egg containing X chromosome then the child born will be a boy having XY combination of sex chromosomes.

Sex Determination in Reptiles

The sex of reptiles can be determined with temperature. For example, if the temperature of eggs of lizards is raised then male lizards are formed on the other hand if the temperature of eggs of lizards is lowered then female lizards are produced. In the same way, if the temperature of eggs of tortoise is increased then it results in the formation of female tortoise, on the other hand, if the temperature of eggs of tortoise is lowered then it results in the formation of a male tortoise.

Test Your Understanding and Answer These Questions:

1. What is sex determination?
2. Explain how sex is determined in human beings.
3. Explain sex determination in reptiles.
4. Explain the significance of temperature in determination of sex in reptiles.

Genotype

Genotype is the particular set of genes present in an organism’s cell. In other words, the genotype is the genetic constitution of an organism.

Phenotype

All the observable characteristics of an organism, such as shape, size, colour and behaviour are called phenotype.

Test Your Understanding and Answer These Questions:

1. What is genotype?
2. What is phenotype?

Gene is the hereditary unit which is responsible for the inheritance of characters from one generation to other. Actually gene is a part of DNA and is made up of nucleotides. The term gene was first of all coined by scientist Johanssen in 1909.

Genes are responsible for our characteristics i.e. all our characters such as our complexion, appearance, height, colour of hair, colour of eyes, shape of nose etc. are determined by genes. Genes are located on the chromosomes which are present in the nucleus of every cell. Chemically genes contain DNA as primary component. The total number of genes in human beings is 3, 00,000 and the number of genes present on one set of chromosomes range in between 10,000 to 15,000. From the above discussion we can now define the gene as gene is a hereditary unit present on a chromosome which can be transmitted from one generation to another.

Types of Genes

Genes can be classified into two types:

1. Autosomal Genes

The genes which are present on autosomes are known as autosomal genes.

2. Sex Linked Genes

The genes which are present on sex chromosomes are called sex linked genes.

Test Your Understanding and Answer These Questions:

1. Define genes.
2. Explain various types of genes.
3. Where are the genes located?
4. What is importance of genes in living organisms?
5. What is chemical nature of genes?
6. How many genes have been estimated to be present in one human chromosome?

The organisms can be classified into two types depending on the fact that whether their nucleus is enclosed by a nuclear membrane or not. These two types are

Prokaryotes

The organism in which nucleus is not bound by nuclear membrane are called prokaryotes e.g. bacteria are prokaryotic organisms. Prokaryotes are unicellular. In prokaryotes only one circular chromosome is present, which is made up of DNA and some proteins. In prokaryotes nucleolus and other membrane bound organelles such as mitochondria, chloroplast and golgi bodies are absent.

Eukaryotes

The organism in which nucleus is bound by nuclear membrane are called eukaryotes e.g. algae, fungi, protozoa, plants and animals. Eukaryotes may be both unicellular and multicellular. In eukaryotes more than one chromosome are present. In eukaryotes nucleolus and other membrane bound organelles such as mitochondria, chloroplasts and golgi bodies are also present.

Test Your Understanding and Answer These Questions:

1. Define prokaryotes and eukaryotes.
2. Give differences between prokaryotes and eukaryotes.
3. Give one example each of prokaryotes and eukaryotes.

Any variation in number of chromosomes (increase or decrease in the number of chromosomes) from normal number, or a change in the shape of a chromosome during the fertilization which leads to severe abnormality or disorder in the child after birth is called genetic disorders. The treatment of genetic diseases is not still possible, but the scientists are doing their best to find out their treatments. The important examples of genetic disorder are:

    1. Down’s syndrome
    2. Turner’s syndrome
    3. Klinefelter’s syndrome
    4. Supermales
    5. Superfemales

1. Down’s Syndrome

Down’s syndrome is a genetic disorder. It is caused by the presence of an extra chromosome in 21st pair. This means that a person suffering from Down’s syndrome possesses 3 chromosomes in its 21st pair of chromosome, which can be seen clearly in the figure of the karyotype of the person suffering from Down’s syndrome.

Symptoms

A person suffering from Down’s syndrome shows mental retardation, rounded face, broad forehead, flattened nose, widely separated eyes, permanently open mouth, protruding tongue, short neck, flat hands and stubby fingers.

2. Turner’s Syndrome

This genetic disease is found in women. The women suffering from Turner’s syndrome have only 45 chromosomes as compared to the normal number of chromosomes of 46.

Symptoms

The women suffering from Turner’s syndrome are sterile with reduced ovaries, underdeveloped breasts, loose skin of the neck and abnormal intelligence. Such women may not menstruate or ovulate.

3. Klinefelter’s Syndrome

This disorder is found in males. In this disease the concerned person possesses one more chromosome. Thus he possesses 47 chromosomes as compared to the normal number of chromosomes of 46.

Symptoms

The person who suffers from klinefelter’s syndrome is a sterile male with small testes, long legs, mental retardation, and obesity. Such persons also have characteristics of females such as breasts and less hair on the body.

4. Supermales

This disorder is also found in males. In this disorder the male possesses one additional Y chromosome. Their genotype can be represented as 44 autosomes + XYY.

Symptoms

Such males usually suffer form unusual height, mental retardation and more aggressiveness than normal males.

5. Superfemales

This disorder is found in females. In this disorder the females possess one additional X chromosome. Their genotype can be represented as 44 autosomes + XXX.

Symptoms

Such females usually suffer from mental retardation and abnormal sexual development.

Test Your Understanding and Answer These Questions:

1. What are genetic disorders? Name some genetic disorders.
2. What is Down’s syndrome? What are symptoms of Down’s Syndrome?
3. What is Turner’s syndrome? What are symptoms of Turner’s Syndrome?
4. What is Klinefelter’s syndrome? What are symptoms of Klinefelter’s Syndrome?
5. What are supermales? What are symptoms of supermales?
6. What are superfemales? What are symptoms of superfemales?