Gene:

It is a hereditary unit which is transmitted from one generation to another generation.

Properties and function of gene:

1.Genes are present in a lengthwise fashion on the chromosomes which has at least two alleles.

2.It occupies a specific locus on a chromosomes.

3.Gene determine the phenotype and genetic constitution of organism.

4.Genes express themselves through the production of chemical substances.

Griffith’s experiment:

In 1928 Frederick Griffith was interested in developing a vaccines against the Pneumococcus which causes pneumonia. One strain has a capsules and can causes pneumonia which is smooth. The harmless strain is rough. Griffith experiment obtained the following results.

Experiment 1: Living rough Pneumococcus bacteria was injected. It did not causes pneumonia. Mice remained healthy. Rough Pneumococcus bacteria are not injective.

Experiment 2: Living smooth Pneumococcus bacteria was injected. It caused pneumonia. Mice died.  Smooth  Pneumococcus bacteria are injective.

Experiment 3: Heat killed smooth Pneumococcus bacteria was injected. It did not causes pneumonia. Mice remained healthy. Smooth Pneumococcus bacteria that are killed by heat are not injective.

Experiment 4: Living rough and heat killed smooth Pneumococcus bacteria were injected together. Mice suffer from pneumonia and died.

On the basis of these results, he concluded that ‘ something must be passing from the heat killed capsulated forms which caused them to develop capsules and become virulent.

Fig. Griffith’s experiment on transformation of bacteria.

Further in 1944, Oswald Avery, Colin Macleod and Maclyn McCarty elaborated Griffith’s experiment in order to identify transferring substances which obtained the following results;

1.Living non-capsulated Pneumococcus + capsule or heat killed capsulated bacteria → Mice remain alive.

2.Living non-capsulated Pneumococcus + cell wall of heat killed capsulated bacteria → Mice remain alive.

3.Living non-capsulated Pneumococcus + DNA of heat killed capsulated bacteria → Mice get infected by pneumonia and died.

4.Living non-capsulated Pneumococcus + DNA of heat killed capsulated bacteria + enzyme deoxyribonuclease → Mice remain alive.

From the experiment, they strongly recommended that DNA is the genetic material and is responsible for the transformation in bacteria.

Deoxyribonucleic Acid (DNA):

DNA is mainly found in the nucleus of eukaryotic cell, but also found in mitochondria  and chloroplast in little amount. It is formed by the end to end polymerization of a large number of repeated units called nucleotides. Those nucleotides are formed by i) deoxy-ribose sugar ii) Phosphate and iii) nitrogenous base.

i) Deoxy-ribose sugar: It is a pentose sugar having 5 carbon atoms. Due to the deoxyribose sugar it is called deoxyribose nucleic acid.

ii) Phosphate: the phosphate in the DNA is present as phosphoric acid (H3PO4).

iii) Nitrogenous Bases: the nitrogenous bases are of two types- Purine and pyrimidine. Purine bases comprise mainly adenine (A) and guanine (G) while pyrimidine bases comprise cytosine (C) and thymine (T).

Nucleoside: nitrogenous base + sugar

Nucleotide: nucleoside + phosphate

Watson and crick’s model of DNA

J.D. Watson and F. crick (1953) combined the physical and chemical bata generated by early work and purposed a double helix model for DNA molecule. This model is widely accepted. According to this model, the DNA molecule consists of two strand which are connected together by hydrogen bonds and helically twisted. Each step on the one strand consist of a nucleotide of purine base which alternate with that of pyrimidine base. Thus, a strand of a DNA molecule is a polymer of four nucleotide i.e. A, G, T, C. the two strand join together to form double helix. Bases of two nucleotide form hydrogen bond i.e. A combines with T by two H-bond (A=T) and G combines with C by three hydrogen bond. However, the sequence of bonding is such that for every A.T.G.C. On one strand there would be T.A.C.G. on the other strand. Therefore, the two strand are complementary to each other. The two strands of double helix run in antiparallel direction, i.e. they have opposite polarity.

                     Fig: Structure of DNA (Watson and Crick Model)

The hydrogen bonds between the two strands are such that maintain a distance of 20 A0. The double helix coils in right hand direction. The turning of double helix results in the formation of a deep and wide groove called major groove. The major groove is the site if bonding of specific protein. The distance between two strands forms a minor groove. The DNA model also suggested a copying mechanism of the genetic material. DNA replication is the fundamental and unique event underlying growth and reproduction in all living organism ranging from the smallest viruses to the most complex of all creatures including man. DNA replicates by semiconservative mechanism.

DNA replication:

The process of synthesis of DNA molecule from preexisting DNA is called DNA replication.DNA replication may occur by following process: 

1.Semi-conservative replication 

2.Conservative replication 

3.Dispersive replication 

Semi-conservative mode of DNA replication

Enzymes involved in semi conservative DNA replication: 

Helicase- helicase enzyme breaks the hydrogen bonds formed between bases of nucleotides
Topoisomerase- topoisomerase enzyme breaks and reseals one strand of DNA
DNA polymerase- it connects the nucleotides to form DNA polymer nucleotides. It forms RNA primer in DNA strand which is like a knot formed o thread. Due to its formation, newly formed DNA strands cannot separate.
Ligase- ligase binds the fragments of DNA strand together
Repair enzyme- this enzyme cuts off wrong base of nucleotide and rejoin correct base in newly formed DNA

Process:

DNA replication in eukaryotes may begin at several points. In replication ,the helicase enzyme breaks the hydrogen bond between the bases of nucleotides. due to breaking of hydrogen bonds of nucleotides, the two strands separate. The process of separation of DNA strands also supported by enzyme topoisomerase. This enzyme breaks and reseals one strand of DNA molecule. 

Now the bases of DNA strands become exposed and joins with the free nucleotides present in nucleoplasm only in 5’-3’ direction. This process or formation of polynucleotide chain takes place in the presence of enzyme DNA polymerase. The four types of nucleotides are present in nucleus in the form of dATP,  dGTP,  dTTP and dCTP. These  molecules are triphosphates and break down into monophosphates dAMP, dTMP, dCMP and dGMP with the release of energy. These monophosphates nucleotides make hydrogen bonds with the complementary nucleotides of DNA strand. 

According to base pairing rule the nucleotides are arranged. As DNA strand formation occurs only in 5’-3’ direction , in on strand of DNA a continuous strand is formed called leading strand ehereas in other strand of DNA, fragments of strands are formed, these fragments are called Okazaki fragments named after Japanese scientist Okazaki. The strand with Okazaki fragments is called lagging strand. 

Here, both DNA strands take part in replication process, so they are called as templates. The segments of strands are joined by the help of enzyme ligase. At the beginning of strand formation, RNA primer is formed at the DNA strand and it is due to the enzyme primase. Sometime a wrong base maybe linked with the DNA nucleotide. When it occurs, the repair enzyme replaces the wrong base by correct base. 

In the above process, newly formed DNA have one strand of old DNA and one of new formed polynucleotide chain. Hence the process is called as semi conservative process of DNA replication. It means half of parent DNA is conserved in new formed DNA.

Structure of RNA:

The RNA is usually single stranded except viruses such as  TMV, yellow mosaic virus, reovirus etc. RNA is found both in the nucleus and in the cytoplasm. The single strand of the RNA is folded either at certain regions or entirely to form hairpin shaped structure. The RNA dose not possess equal purine-pyrimidine ratio, as is found in the DNA.

                                               Fig: Structure of RNA.

Like DNA, the RNA is also the polymer of four nucleotides each one contains D-ribose, phosphoric acid and a nitrogenous base. The bases are two purines (A,G) and two pyrimidines (C,U). Thyamine is not found in RNA. Pairing between bases occurs as A-U and G-C.

 If the RNA is involved in genetic mechanism, it is called genetic RNA as found in plant, animal, and bacterial viruses. The DNA acts as genetic material and RNA follows the order of DNA, In such cells the RNA dose not have genetic role. Therefore, it is called non-genetic RNA. The non- genetic RNA is of three types:

1. m-RNA(messenger RNA): it constitutes about 5% - 10% of the total RNA present in the cell, m-RNA  carries the genetic information from DNA for Protein synthesis.

2. r- RNA(Ribosomal RNA): It makes about 80% of the total RNA in the cell. It is the major component of ribosomal.

3. t RNA (Transfer RNA): it is also known as soluble RNA. It forms about 10% - 15% of the total RNA. It used to transfer amino acid molecule to the site of protein synthesis.

A nucleotide is made up of three molecules. These are

a) Pentose sugar: the pentose sugar is ribose.

b) Nitrogenous bases:  i) Purine = Adenine And Guanine ii) Pyrimidine=Cytosine And Uracil

c) Phosphoric Acid

              Fig. Structure of t-RNA

Transcription ( RNA synthesis):

Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. The transcript RNA is complementary to DNA in base sequence. Transcription involves the following steps;

1.A portion of DNA strand uncoils and the two strand separate at a specific points.

2.One strand of DNA( 3’-5’) DNA act as master strand for the formation of RNA, while other strand ( 5’-3’) does not participate in RNA formation.

3.The template strand has a promoter or initiation site and a terminator site.

4.RNA polymerase initially binds at the promoter site of the template DNA strand and proceeds the formation of RNA strand.

5.RNA synthesis initiates from 5’ to 3’ direction on the master strand.

6.Free ribonucleotides slot into the place opposite the exposed bases on the template DNA strand.

7.RNA polymerase moves alone template DNA causing elongation of RNA.

8.The sugars and phosphate of the m-RNA nucleotides are then linked together to form a RNA molecule.

9.RNA gets detached and leaves the nucleus through nuclear pore into the cytoplasm.

10.The DNA strand coil up again and formed the double helix.