DNA(Deoxyribose Nucleic Acid) :
Describe the watson and cricket model of DNA?
In this article we will discuss about the Deoxyribose Nucleic Acid (DNA) :-(1) Introduction, (2)Molecular Organization, (3) Structure of DNA, (4) Double Helical Structure of Components, (5) Single stranded DNA, (6) Chemical components, (7) Biological Significance of DNA, (8) Function of DNA.
(2) Molecular Organization
(3) Structure of DNA
(4) Double Helical Structure of DNA
(5) Single stranded DNA
(6) Chemical components
(7) Biological Significance of DNA
(8) Function of DNA
DNA(Deoxyribonucleic acid) is the molecule of heridity. DNA is present in all cell except plant virus. In eukaryotic cells, DNA is present in the chromosome of nucleus. In addition, the mitochondria and plastid aslo contains DNA. In eukaryotic Nucleus, the DNA is an the form of a double helix. In Bacteria, mitochondria and plastid the DNA molecules are circular. In virus and bacteriophage, they are coiled.
DNA is a unique molecule found in the cell. It is the hereditary material in most of the organisms. In general, eukaryotes contain more DNA than the prokaryotes. Though mainly concentrated in the nucleus, DNA is also present in chloroplasts and mitochondria in the cytoplasm. Avery and his co-workers (1994) first demonstrated the direct role of DNA in the process of inheritance. Chargaff (1947) reported the presence of purine and pyrimidine bases in equimolar ratios. Through X-ray diffraction technique, Wilkins and co workers (1950) established that purine and pyrimidine bases are repeated at a distance of 3.4 Aº. They also emphasized that DNA molecule is not planar but helical i.e., twisted in the form of a helix. A complete twist occurs on every 34 Aº. DNA is associated with histone protein to constitute the genetic apparatus or the chromosomes.
DNA is a giant molecule. Its molecular weight is very high (up to 2 x 10%). DNA is a natural polymer. It is made up of repeating units called nucleotides. A
nucleotide of DNA consists of a deoxyribose sugar molecule, a phosphoric acid molecule and one of the four nitrogenous bases. A sugar residue and a base make up a nucleoside. Thus, a nucleotide is a nucleoside plus a phosphoric acid molecule. There are four different kinds of nucleotides based on the nature of the four bases. The order and arrangement of bases varies from organism to organism. The molecular ratio of A+T: G+C of any DNA may differ from species to species but remains constant for a particular species. However, the purine and pyrimidine bases occur in equimolecular ratio.The three dimentional molecular model of DNA was proposed by J.D. Watson (USA) and F.H.C. Crick (England) in 1953 for which they shared the Nobel Prize in medicine in 1962 with M.H.F Wilkins (England). Watson and Crick published the concept of the DNA model in the Journal ‘Nature’ in 1953.
Double Helical Structure of DNA (DNA-A double helix) :-
Watson and Crick in 1953 designed the structure of DNA. It is called the watson and crick model of DNA. They are awarded Novel price in 1962 for this work.
1. Watson and Crick suggested that a DNA molecule consists of two polynucleotide chains.
2. The two chains are antiparallel i.e., they lie in a head to tail position. They run in opposite directions (one chain runs in the 5′-3′ direction while the other runs in the 3′-5′ direction.
3. The two chains are coiled around each other, hence the name double helix.
4. Each chain has a sugar-phosphate backbone with nitrogenous bases projecting at right angles to the central axis of the helix.
5. The bases from opposite chains form hydrogen bonds across the double helix.
6. The bases are hydrophobic but the sugar-phosphate backbone is hydrophilic.
7. The pairing of bases is very specific. Adenine always pairs up with thymine (A=T) and guanine pairs up with cytosine (G=C).
8. There are two hydrogen bonds between A=T base pair and three hydrogen bonds between G=C base pair.
9. Each chain forms a right-handed helical spiral around a common axis.
10. The double helix resembles a spiral staircase.
11. A single twist (one complete helix) is 34 Aº long and comprises 10 base pairs. The mean diameter of the helix is about 2000 nm.
12. The adjacent base pairs are 3.4 Aº apart and are inclined at an angle of 36º.
13. Electron micrograph of DNA shows that a DNA molecule is about 20 Aº wide. As each chain is 10Aº in thickness, it follows that a single DNA molecule consists of two polynucleotide chains lying side by side.
14. The sequence of bases in one chain determines the sequence of bases on the other chain. This shows that the two chains are complementary to each other.
15. There are two grooves; one major and the other minor, running along the chain, parallel to the sugar-phosphate backbone. The grooves provide surface area for interaction with proteins.
16.The form of DNA explained by Watson and Crick is B DNA. There are other forms of DNA as shown in the following table.
Single stranded DNA:-
The usual form of DNA is double stranded. However, single stranded DNA is found in bacteriophage. DNA is the ultimate hereditary unit of plants and
animals. Gene is a part of DNA that can produce a trait.In other words, we can say that the genes are functional units of DNA. A single DNA molecule may be the seat for many genes. DNA and proteins (histone) combine to form chromosomes. Chromosomes are thread-like bodies found in the nucleus of a cell. In resting cells, chromosomes are spread out in the form of network called chromatin network or chromatin reticulum. Chemical analysis of chromatin has shown 60 % histone protein, 35 % DNA and 5 % RNA.
DNA is made up of 3 chemical components;namely:-
(2). Phosphoric acid
(3). Nitrogenous bases
The sugar present in the DNA is called deoxyribose. It is a pentose sugar which contain 5 carbon atoms . It contain 1oxygen atom less than the ribose sugar. The 2nd carbon of deoxyribose bond with 2hydrogen atoms, but in ribose sugar, the 2nd C-atom bonds with 1hydrogen atom and 1hydroxyl group (OH).
(2) Phosphoric acid:-
Phosphoric acid links consecutive Nucleotides by joining their pentose sugar with a phosphate diester bonds. This bond links carbon 5′ in one Nucleoside with carbon 3′ in the next Nucleoside.
These are nitrogen containing organic compounds they are 2types namely:-
The purins are double linked compound. A purine molecular consists of a 5-membered Imidazole ring join to a pyrimidine ring at position 4′ and 5′.The most common purine of DNA are Adenine(A).
These are the single ring compounds, with Nitrogen imposition with 1′ and 3′ of a single membered benzene ring. The 2most common pyrimidine of DNA are Cytosine and Thymin.
Biological significance of DNA:-
DNA is the chief hereditary material in most living organisms. All biosynthetic and biological activities of the cell are controlled by DNA. It is the carrier of genetic information not only from parent cell to daughter cells but also from one generation to the next generation. DNA has two important functions : autocatalytic function and heterocatalytic function.
Autocatalytic Function of DNA: When DNA directs its own synthesis, it is called autocatalytic function.
Heterocatalytic Function of DNA: When DNA controls the synthesis of molecules other than DNA such as RNA and proteins, it is called heterocatalytic function.
Function of DNA:-
1.DNA act as the carrier of genetic information from generation to generation.
2.DNA control all developmental process of an organism and all life activities.
3.DNA synthesis RNA.
4.DNA has a genetic information for protein synthesis.