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Genetic Materials- Introduction

Gene

The substance which controls the inheritance of the trait from one generation to the next generation and can express the effect through the formation and functioning of the traits are called genetic materials.

Gene

Gene is the small segment of DNA/ RNA which determines a particular character of the organism. It is the hereditary unit that is transmitted from one generation to the next generation.

Characteristics of Gene:

1. Genes are present in a linear fashion on the chromosome.

2. There are several genes in each chromosome.

3. Gene occupies the specific position on the chromosome ( Gene locus).

4. Genes undergo self-replication.

5. Genes are the one that determines the phenotype and genetic constitution of organisms.

6. Each gene has at least two alleles. For example, the height is determined by TT, Tt, or tt.

Cistron

It is the functional unit of DNA that contains the information for coding a specific polypeptide chain.

Recon

It is the smallest unit of DNA that could undergo crossing over and recombination.

Muton

It is the smallest unit of DNA that could undergo mutation.

Central Dogma

It is the flow of genetic information from DNA to RNA by transcription and from RNA to protein by translation. This is unidirectional.



Fig: Central Dogma

Reverse Central Dogma

It is the flow of genetic information from RNA to DNA by reverse transcription, DNA to RNA by transcription, and RNA to protein by translation. It occurs in organisms with RNA as genetic material.



Fig: Reverse Central Dogma

DNA as Genetic Material

Many scientists did experiments regarding the concept that DNA is a genetic material.

a) Bacterial Transformation (F. Griffith -1928)

b) Bacterial Transduction ( Hershey and Chase-1952)

c) Tobacco Mosaic Virus (Frankel Cornot and Singer-1957)

Griffith's Experiment (Transformation)

In1928, British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus pneumoniae bacteria and mice. Griffith used two related strains of bacteria, known as R and S.

R strain (Non -capsulated - RI, RII, RIII)

R strain of bacteria was non -capsulated and had a rough appearance. They were non -virulent and didn't cause disease so they were non-pathogenic.

S strain ( capsulated -SI, SII, SIII )

S strain of bacteria was capsulated and had a smooth appearance. They were virulent and caused disease so pathogenic.

He used sIII and RII bacteria. He injected these bacteria into mice.

1.Living S type ------------------> mice died

2. Living R type-------------------> mice survived

3. Heat killed S-type-------------> mice survived

4. Heat killed S -type + living R type------------>   mice died


Griffith concluded that the R -strain bacteria must have taken up what he called a "transforming principle " from the heat-killed S-type bacteria, which allowed them to "transform" into smooth-coated bacteria and become virulent.

Although this experiment was completed, Griffith did not prove the transformation of material. Therefore, Avery, MacCarty, and MacLeod made further studies in 1944. They isolated three components -polysaccharides, protein, and DNA from the heat-killed S- type bacteria. Then they made a series   of experiments, and obtained the following results:

1.The polysaccharide of heat-killed S type +  R- type living cell------------> mice survived

2.Protein of heat-killed S type+Living R-type  -------------->mice survived

3.DNA of heat-killed S type+  R type living cell -------------->mice died

4. DNA of heat-killed S type    +R -type living cell +DNAase----------------> mice survived

Thus, this experiment supported Griffith's experiments and gave molecular explanations to their experiments. They concluded that the DNA isolated from the heat-killed S- type when added to the  R- type cells changed their surface from rough to smooth, and hence made them virulent. Thus, from this experiment, it was shown that DNA was the genetic material as the DNA component of the heat-killed S-type transformed living R_type into virulent S- type cells.

Nucleic Acid

Nucleic acids are the polymers of nucleotides. So, the nucleic acid is polynucleotides.

Nucleotide

A nucleotide is a molecule formed by the combination of one nucleoside and one phosphate group.


Nucleoside

It is a molecule formed by one pentose sugar and one nitrogen base.

Pentose sugar

Pentose sugars are the five-carbon monosaccharides found in nucleic acids. RNA consists of ribose sugar while the DNA consists of Deoxyribose sugar. Ribose and deoxyribose sugar differ in structure at C2. Deoxyribose has one oxygen less at C2 compared to a ribose sugar.



Nitrogenous Bases

The nitrogenous bases found in the nucleotides are aromatic heterocyclic compounds. There are two categories of nitrogenous bases- purines and pyrimidines.

i) Purine
It is a double-ringed structure.

They are Adenine (A) and Guanine (G).

ii) Pyrimidine

It is a single-ringed structure.

They are Cytosine (C), Thymine (T), and Uracil (U).



Phosphate Group

It is present as Phosphoric acid (H3PO4).





DNA: Deoxyribonucleic acid

DNA is the macromolecule formed by polymerization of repeated units called deoxyribonucleotide.

DNA is located in the nucleus, mitochondria, and plastids.

Structure of DNA

There are various types of DNA based on structure. They are:

1. A-DNA

It has 11 base pairs in a turn and is a right-handed duplex.

2. B-DNA

It has 10 base pairs in a turn and is a right-handed duplex.

3. C-DNA

It has 9 base pairs in a turn and is a right-handed duplex.

4. D-DNA

It has 8 base pairs in a turn and is a right-handed duplex.

5. Z-DNA

It has 12 base pairs in a turn and is a left-handed duplex.

Structure of B-DNA

The structure of B-DNA was proposed by James Watson and Francis Crick in 1953 for which they were awarded Nobel Prize in 1962. The B-DNA has the following structure:

1. The DNA is a double helix consisting of two polydeoxyribonucleotide strands twisted around each other on a common axis.

2. Each nucleotide is made up of sugar, phosphate, and nitrogenous bases. Many such nucleotides are linked by phosphodiester bonds to form a polynucleotide chain.

3. The two strands are anti-parallel, i.e. one strand runs in the 3' to 5' direction while the other in the 5' to 3' direction.

4. The width between two strands of a double helix is 20 angstrom.

5. The distance of one turn is 34 angstrom. The distance between two base pairs is 3.4 angstrom. There are 10 base pairs on a complete turn.

6. The two polynucleotide chains are not identical but complementary to each other due to base pairing.

7.  The two strands are held together by hydrogen bonds between the complementary base pairing. The A and T are linked by 2 hydrogen bonds and G and C are linked by 3 hydrogen bonds.

8. The complementary base pairing in the DNA helix proves Chargaff's rule. The content of adenine equals that of Thymine and Guanine equals that of Cytosine.

9. The nucleotide in the helix is joined together by phosphodiester bonds. Sugar and Phosphate molecules form the backbone of the DNA strand.

10. The sugar and phosphate backbone do not conceal the bases inside. There are two grooves along the surface of the DNA molecule. One is wide and deep and is called the major groove and the other is narrow and shallow and is called a minor groove.


Functions of DNA

1. It is a genetic material that carries all the hereditary information from one generation to other.

2. It controls all the biological activities of the cells. DNA synthesizes proteins, enzymes, and other biochemicals.

3. It synthesizes RNA by the process of transcription.

4. It guides the process of protein synthesis in the cell.



RNA: Ribonucleic acid

It is found as the genetic material of some viruses such as animal viruses and bacteriophages. It is a single-stranded long-chain macromolecule of ribonucleotide.

Ribonucleotide is made up of ribose sugar, phosphate, and nitrogen bases like Adenine, Guanine, Cytosine, and Uracil.


Functions of RNA

1.It plays important role in protein synthesis.

2. It is the hereditary material in some viruses.

Types of RNA

1. Messenger RNA (mRNA)

It is linear and occurs in the nucleus. It constitutes 2-5% of total RNA. It acts as a template for protein synthesis. It is unstable and short-lived.


2. Ribosomal RNA (r- RNA)

It is the most stable and abundant RNA about 70-80% of total RNA. It is associated with the ribosome. It assembles the amino acids to form protein.

3. Transfer RNA (t-RNA)

It is soluble RNA and is unstable. It carries amino acids. It is a cloverleaf structure in 2-Dimensional form. It has 5 arms:

i) Acceptor arm

ii) Anticodon arm

iii) DHU arm

iv) Ribosomal binding site

v)  Extra arm