Deoxyribonucleic acid is abbreviated as DNA. The smallest part of the chromosome that is self-reproducing, regulates biological functions and transmits heritable characteristics and is capable of causing variation, mutation and evolution of organisms is called DNA. This is called the molecular basis of life. About 90% of the chromosomes of organisms contain DNA. There are two types of nucleotides in the DNA molecule. Exon and Intron Nucleotides. Meaningful parts are called exons and nonsense parts are called introns. The molecular weight of DNA is 106-109 daltons.
The discovery of DNA
In 1869, Swiss physician and chemist Friedrich Miescher isolated DNA from the nuclei of sperm, sperm cells, and red blood cells of birds. He named it Nuclein. In 1874 he isolated nucleic from salmon sperm and named it protamine. In 1880 scientist Fisher discovered purine and pyrimidine. In 1889, Altmann named nucleic acid as nucleic acid. Albrecht Kossel discovered adenine, guanine, cytosine and thymine in 1910. In 1910 scientist Leven discovered deoxyribose sugar. In 1914, German chemist Robert Feulgen invented the Feulgen staining method for DNA. In 1928, British scientist Frederick Griffith realized that DNA contains all genetic information. In 1950 Erwin Chargaff mentioned Chargaff’s rule of DNA. At the same time, Maurice Wilkins and Rosalind Franklin presented the physical structure of the DNA molecule using X-ray crystallography. In 1952, Hershey and Chase proved that DNA is the genetic material of organisms. In 1953, J. D. Watson and F. H. Crick proposed the generally accepted model of DNA.
Volume of DNA
The amount of DNA in the cells of each species of organism is specific. If the DNA molecules of the human body are joined together, the chain that will be created is 5-6 times the distance between the Earth and the Sun. The amount of DNA is expressed in picograms. 1 picogram = 10-12 grams. Chicken diploid cells contain 2.5 picograms and sperm contain 1.25 picograms of DNA. Human cells contain 5-6 picograms of DNA. An adult human body contains 100 picograms of DNA.
Location of DNA
All protocellular organisms contain DNA. DNA is always located in chromosomes. Hence it is called chromosomal DNA. Chromosomes are also called nuclear DNA as they are located in the nucleus. Circular DNA is present in the mother body of mitochondria and in the stroma of chloroplasts.
Protocellular organisms Bacteria and cyanobacteria (blue green algae) contain circular nucleoid DNA and plasmid DNA in their cytoplasm. Such DNA molecules do not contain histone proteins.
Viruses such as vaccinia, variola, adeno herpes simplex, T2, TIV, ϕX174, M13, CMV etc. contain DNA. Smallpox, chickenpox and herpes viruses contain DNA.
Types of DNA
There are three types of DNA found in living cells. These are-
1. A-DNA: A-DNA is right-handed and double helix. It consists of 11 base pairs. The distance between their bases is 0.34 nanometers.
2. B-DNA: B-DNA is right-handed and double helix. It consists of 10 base pairs. The distance between their bases is 0.29 nanometers.
3. Z-DNA: Z-DNA is left-handed and double helix. Its chains are decorated in two curved positions.
Chemical Structure of DNA
The DNA molecule is made up of three chemical elements. Nitrogen bases, pentose sugars and phosphoric acid.
1. Nitrogen Alkali: This alkali is formed by carbon, hydrogen, oxygen and nitrogen. Alkaline compounds form rings. Based on the number of rings, nitrogenous bases can be divided into two groups.
(i) Purines: Dicyclic nitrogenous bases are called purines. Its common symbol is C5H4N4. It is composed of adenine and guanine.
(ii) Pyrimidine: A cyclic nitrogenous base is called pyrimidine. Its common symbol is C4H4N2. It is composed of thymine and cytosine.
2. Pentose sugar: Five carbon sugar (sugar) is called pentose sugar. DNA molecules contain deoxyribose sugar. It is a type of monosaccharide. Deoxyribose sugar is so named because carbon number 2 does not contain oxygen.
3. Phosphoric acid: One of the chemical components of DNA molecule is phosphoric acid. Its molecular symbol is H3PO4. It contains one divalent oxygen atom and three monovalent hydroxyl groups. Oxygen and hydroxyl groups combine with a pentavalent phosphorus atom to form phosphoric acid.
Watson and Crick’s DNA model thinking
1. DNA is made up of four types of nucleotides.
2. Know the chemical structure of nucleotides.
3. Since DNA is acidic, the phosphate group will be exposed.
4. Adenine and Thymine are equal and Guanine and Cytosine are equal in Chargaff’s principle.
5. Wilkins and Franklin presented the physical structure of the DNA molecule by X-ray crystallography. Its molecular distance maps are 2 nm, 0.34 nm and 3.4 nm.
6. Two purines cannot reversibly sit within 2.0 nm of each other. The two pyrimidines sit opposite each other at a distance of less than 2.0 nm. So if the purine and pyrimidine sit opposite each other, the distance is 2.0 nm.
7. Adenine and Thymine are joined by two hydrogen bonds and Guanine and Cytosine by three hydrogen bonds.
8. The two sutrakas are different, but complementary.
Physical structure of DNA-Watson Crick model
In 1953 scientists J. D. Watson, Francis Crick and Frederick Wilkins introduced the model of DNA structure. It is called Watson-Clique model. This is called the double helix model.
1. Double Helix: There are two strands in the DNA molecule. It is called double strand or double helix. The arrangement of the two sutras is like a stairwell. Each helix is a polynucleotide.
2. Reverse Strands: The two strands of a DNA molecule lie side by side in opposite directions. One is 5-3 carbon oriented and the other is 3-5 carbon oriented. The positions of the two sutras are called antiparallel. These two formulas are positioned around an imaginary central axis.
3. Railings: Deoxyribose sugars and phosphates form the railings on both sides of the ladder. Deoxyribose sugars and phosphates are sequentially linked to form two railings.
4. Sopan: There are many steps in the middle of the stairs. These steps are called steps. Different types of nitrogenous alkalis are added to form this terrace.
5. Hydrogen Bonding: Nitrogenous bases are bonded to each other by hydrogen bonding. According to the bonding principle, Adenine and Thymine are bonded by two hydrogen bonds (A=T) and Cytosine and Guanine are bonded by three hydrogen bonds (G≡C). There are 25 hydrogen bonds per patch of DNA molecule.
6. Carbon attachment: Nitrogenous bases are always attached to the 1st carbon of the pentose sugar. A phosphate is attached to carbon 5 of one nucleotide while a phosphate is attached to carbon 3 of another nucleotide.
7. Alkalinity: DNA molecules contain equal amounts of adenine and thymine and guanine and cytosine.
8. Diameter: The diameter of the double helix of each DNA molecule is 20 Å. But this diameter may vary according to the length.
9. Patch Length: Each patch of double helix consists of 10 pairs of mononucleotides. A pair of mononucleotides is 3.4 Å in length. Thus the length of each patch is 10×3.4= 34 Å or 3.4 nm.
10. Grooves: There are two grooves in each turn of the double helix. One is the major (deep) groove 22 nm and the other is the minor (shallow) groove 12 nm.
11. Molecular Weight: Molecular weight of DNA is 106-109 daltons.
Among all contributions to biology, the Watson-Clique model of the structure of the DNA molecule is a unique contribution. This contribution has brought medical science to the golden pinnacle of development today. So the people of the world have conquered death and attained superiority from world to world.
Chargaff’s rule
In 1950, scientist Erwin Chargaff mentioned the principles about the structure of DNA through research called Chargaff’s rule. Scientist Chargaff found that the amount of Adenine and Thymine in the DNA molecule is equal. Again, the amounts of Guanine and Cytosine are also equal. This is called Chargaff principle. Half of the nitrogenous bases in the DNA molecule will be purines (A+G) and half will be pyrimidines (T+ C). Hence A pairs with T and G pairs with C.
Function of DNA
1. DNA acts as the building block of chromosomes.
2. DNA acts as the molecular or physical basis of heredity.
3. It carries and controls all the characteristics of the organism.
4. It transfers the characteristics of organisms from generation to generation.
5. It manifests all the characteristics of the organism.
6. DNA causes variation, mutation and evolution of organisms.
7. DNA controls the biological and physiological functions of organisms.
8. It plays a role in protein synthesis.
9. DNA has a role in identifying each species.
10. DNA controls the metabolic functions of organisms.
11. It maintains the species of organisms through replication.
Contribution of DNA as hereditary material
The components of a cell that undergo replication and mutation and transmit hereditary characteristics are called hereditary material or genetic material. DNA acts as the container and carrier of hereditary characteristics. DNA is the only unit of heredity in living organisms. It transfers the characteristics of the parent to the offspring. Hence DNA is called the chemical basis of heredity.
Why is DNA called the container and carrier of heredity?
The reasons why DNA is called the container and carrier of heredity are:
1. DNA acts as a transmitter of biological signals in organisms.
2. DNA causes accurate replication during cell division.
3. DNA transfers the characteristics of parents to offspring.
4. It synthesizes proteins and RNA specific to the cell.
5. It changes the structure of organisms through mutation.
6. DNA is the master molecule of organisms.
7. During meiosis cell division, parental DNA moves directly into the germ cell.
8. It causes variation, mutation and evolution.
9. DNA is species specific and immutable. DNA molecules are changed only through mutation.
10. A change in the DNA molecule through mutation is passed on to the next generation.
From the above significant discussion, it can be said that DNA is the container and carrier of the heredity of living organisms.
Role of DNA in Controlling Traits
1. DNA performs biological and physiological functions in organisms. It regulates the characteristic features of organisms through biological and physiological functions.
2. DNA acts as a transmitter of biological signals in organisms. It provides the genetic code of the organism. Hereditary signals act as indicators of the characteristic traits of organisms.
3. DNA makes accurate copies. Therefore, the replication process creates identical DNA. New DNA molecules carry genetic information.
4. DNA serves as the molecular or physical basis of heredity in organisms. It is the holder and carrier of the heredity of living beings. Bears and controls all the characteristics of living beings. So only DNA controls the characteristics of organisms.
5. DNA gives expression to all the characteristics of an organism. It causes variation, mutation and evolution of organisms. As a result, the properties change.
6. Human DNA is damaged 1000 to 1000000 times every day. Fortunately, these damages can be repaired. Cancer occurs when the repair process is disrupted.