Definition of DNA:
DNA is a complex large organic molecule composed of deoxyribose sugar, phosphoric acid, and nitrogen-containing bases that determines various characteristics as the genetic material in most organisms and transmits them to offspring and controls the formation of polypeptides in living cells.
Discovery and Naming:
Mendel’s work on heredity was published in 1865. 4 years later a German chemist Friedrich Miescher (1869) was able to isolate a white substance containing nitrogen and phosphorus from the cellular nuclei of human pus and from the nuclei of fish sperm. It is different from other elements of the kosher. He thought it was a new type of organic matter. Since that substance is related to the nucleus of the cell, he named it Nuclein. His discovered ‘nuclein’ is slightly acidic, so it was later called nucleic acid. After that there was no work done for almost 50 years. Biologists didn’t know what it did in cells. In 1920, a biochemist named P A Levene discovered the chemical structure. He mentions that there are two types of nucleic acids-
1. Ribonucleic acid or RNA, which has a hydroxyl group (-OH) attached to a specific C-atom (the second carbon atom from the N₂-containing base).
2. De-oxyribonucleic acid or DNA, which has a hydrogen atom (-H) attached to that carbon atom. It was formerly called thymonucleic acid.
Location of DNA:
In eukaryotic cells:
DNA is abundant in eukaryotic chromosomes. This is called chromosomal DNA. Chromosomes are also called nuclear DNA as they are located in the nucleus. In addition to eukaryotic chromosomal DNA, the DNA present in the cytoplasm is called cytoplasmic DNA. This type of DNA is usually found in the cytoplasm of mitochondria and stroma of chloroplasts. They are called circular DNA because of their shape.
In prokaryotic cells:
Circular nucleoid DNA and plasmid DNA are present floating in the cytoplasm of the cell bodies of bacteria and blue-green algae or cyanobacteria. In this case no histone proteins are associated with DNA.
In viruses:
Some viruses have DNA present on the inside of the capsid.
1.Cyclic double-stranded DNA is present in the head of bacteriophage or phage virus.
2. Among the plant viruses, only Cauliflower mosaic virus or CaMV contains DNA.
3. Some animal viruses, such as small pox virus, chicken pox virus and herpes virus contain DNA.
Length of DNA molecule:
Variation in the length of DNA molecules can be observed in different species of organisms.
1. The circular DNA molecule present in the mitochondrial membrane is about 5 µm in length.
2.Nucleoid DNA is about 1.4 mm in length located in the cytoplasm of the bacterial cell body.
3. The length of 1 picogram (pp) extracted DNA molecule is about 31 cm. Based on this, DuPraw and Bahr (1968) determined the length of a 5.6 pg DNA molecule to be 173.6 cm in a human somatic cell [having a diploid number of chromosomes (46 or 23 separate pairs). (1pg 10-12 gl)
Test for the presence of DNA in the cell:
The presence of DNA in the cell can be determined in three ways-
1.With the help of Feulgen reaction:
Scientist Feulgen (1912) discovered that when DNA is hydrolyzed with warm Schiff’s reagent, it turns reddish purple. In 1924, Feulgen pioneered the basic fuchsin staining method, confirming the existence of DNA within chromosomes.
2. With the help of de-oxyribonuclease:
DNA is hydrolyzed under the influence of an enzyme called deoxyribonuclease (deoxyribonuclease or DNase). It does not respond to the ‘Feulgen reaction’ when applied to the nucleus or chromosomes.
3. With UV light:
The wavelength of the visible light spectrum is 6500 Å to 4500 Å. However, light with wavelengths shorter than 4500 Å are UV rays. DNA absorbs UV light. Hence its presence in chromosomes can be proved with the help of UV light without staining the chromosomes.
Types of DNA:
Based on the nature and number of stands, DNA can be divided into the following categories-
1. Circular DNA:
In some viruses and bacteria, the DNA is circular and the ends of the DNA are joined by covalent bonds. So it is called CCCDNA or covalently closed circular DNA. Circular DNA can again be divided into two types-[i] Single stranded DNA (ss circular DNA)- DNA in some viruses is single stranded or ss and circular. DNA of viruses such as coliphage X174 and bacteriophage M13. [ii] Double stranded circular DNA (ds circular DNA) – Some viruses and bacteria contain double stranded or ds circular DNA. For example, nucleoid DNA and plasmid DNA present in the body cells of Escherichia coli (E. coli) and other bacteria and DNA in some viruses such as simian virus-40 is double-stranded and circular.
2. Linear DNA (Linear DNA):
In most cases, DNA is linear and has two free ends. Linear DNA can again be divided into two types – [i] Linear DNA (ss linear DNA) – Some viruses have both linear and linear (ss) DNA. As in Colifaz fd. [ii] Double-stranded DNA (ds linear DNA)- DNA is linear and double-stranded (ds) in most cases, ie (a) DNA present in the chromosomes located in the nucleus of eukaryotic cells is linear and double-stranded. (b) DNA is linear and double stranded in some viruses, eg T2, T₁ etc. in phages.
Chemical constituents of DNA:
Chemically, the DNA molecule is made up of three types of components. The components are discussed in detail below-
1. Pentose sugar:
The sugar obtained when an oxygen (0) atom is removed from the -OH group of the second carbon molecule of ribose sugar is called deoxyribose sugar. Its religions are-① The deoxyribose sugar present in DNA is a type of five-carbon sugar, hence it is called a pentose sugar. ② A deoxyribose sugar molecule has four carbon atoms (C) present, which are identified as 1′ [1 prime], 2′,3′, 4′, 5′. ③ 1’C and 4’C are joined by molecular oxygen (0) to form a pentamerous ring. ④ Deoxyribose sugar molecule has a hydrogen atom (H) on one side of 1’C and 3’C and a hydroxyl group (-OH) on the other side. But 2’C has a hydrogen atom (H) on one side and a hydrogen atom (H) on the other. That is, the oxygen (0) of hydroxyl (-OH) is removed, hence the term ‘deoxy’ is used. The rest of the structure is like ribose sugar, so it is called de-oxyribose sugar. Since oxygen is removed from 2’C, it is sometimes called 2-D-oxyribose. ⑤ A hydrogen atom (H) is attached to the 4’C side of the deoxyribose sugar. On the other hand, it is attached to 5’C, which has two hydrogen atoms (H) and a hydroxyl group (-OH).
2.Nitrogenous base:
A nitrogenous base refers to an organic compound with one or two rings consisting of carbon and nitrogen. DNA contains four nitrogenous bases or bases, which can be mainly divided into two groups –
A. Purine bases:
Purine bases contain a total of 5 nitrogen (N) atoms. This type of base consists of two rings- [i] The larger ring is hexagonal and its The O position contains nitrogen atom (N) and the 2, 4, 5 and 6 positions contain carbon atom (C). [ii] The smaller ring is pentagonal and contains nitrogen atoms (N) in positions 7 and 9 and carbon atoms (C) in position 4.
There are two types of purine bases in DNA, namely [i] Adenine (Adenine or A)- with an amino group (-NH₂) attached to C atom number 6 of the adenine base. Hence its chemical name is 6-amino purine. [ii] Guanine (Guanine or G)-An amino group (-NH₂) is attached to the 2 C atom of guanine and an oxygen atom (0) is attached to the 6 5 atom. Hence its chemical name is 2-amino 6-oxy purine.
B. Pyrimidine bases:
The pyrimidine base consists of a hexagonal ring and contains 2 nitrogen (N) atoms. The 2, 4, 5 and 6 positions of this ring contain one carbon atom (C) and one nitrogen atom (N) in the 1O and 1O positions.
There are two types of pyrimidine bases in DNA, namely- [i] Cytosine (or C)- Cytosine has an oxygen atom (0) attached to C-atom number 2 and an amino group (-NH2) to atom number 4. remains Hence its chemical name is 2-oxy 4-amino pyrimidine. [ii] Thymine (Thymine or T)- One oxygen atom (0) is attached to number 2 and number 4 C atoms of thymine. A methyl (-CH₁₂) group is attached to the 5th C atom. So its chemical name is 2,4-dioxy 5-methyl pyrimidine.
3. Phosphoric acid or phosphate group:
One of the structural elements of DNA is Phosphoric acid (H₃PO₄), which is attached to the 5’C of the pentose sugar to form a sugar-phosphate structure. Note that phosphoric acid has three hydroxyl groups (-OH), one of which is linked to the 5’C of the deoxyribose sugar by an ester or phosphoester bond. Phosphoric acid exists as a negatively charged phosphate group in DNA molecules. This results in the negative charge of the DNA stem.
Structural unit of DNA:
Deoxyribonucleotide units constitute the building blocks of the DNA molecule. These units are formed by the addition of a phosphate unit to a deoxyribonucleoside. Below is a brief discussion of D-oxyribonucleoside and D-oxyribonucleotide-
1. Deoxyribonucleoside:
In DNA, a molecule of de-oxyribose sugar, a pentose sugar, and an N₂-containing base (any of A, T, C, G) are chemically joined by a glycosidic bond to form an organization called a D-oxyribonucleoside. Nucleosides are named based on the N₂-containing base, and thus four types of deoxyribonucleosides are found in DNA.
Different Types Of D-Oxyribonucleosides:
| N₂ Base | Nucleoside | Common Name | Symbol |
| Adenine (A) | Adenine D-oxyribonucleoside | D-oxyadenosine | AdR or D-A |
| Guanine (G) | Guanine D-oxyribonucleoside | D-oxyguanosine | GdR or D-G |
| Cytosine(C) | Cytosine D-oxyribonucleoside | D-oxycytidine | CdR or D-C |
| Thymine (T) | Thymine D-oxyribonucleoside | D-oxythymidine | TdR or D-T |
D-Oxyribonucleotide:
In DNA, a molecule of de-oxyribose sugar, an N₂-containing base, and phosphate or phosphoric acid are chemically joined to form a D-oxyribonucleotide. That is, D-oxyribonucleotide is the combined form of D-oxyribonucleoside and phosphoric acid. There are four types of deoxyribonucleotides found in DNA based on the position of the N₂-containing base and phosphate group.
Different Types Of D-Oxyribonucleotides:
| N₂-containing bases | Nucleotide | Common Name | Symbol |
| Adenine (A) | D-oxyadenosine- 3′-monophosphate D-oxyade6nosine- 5′-monophosphate | D-oxyadenylic acid | 3′-dAMP 5′-dAMP |
| Guanine (G) | D-oxyguanosine- 5′-monophosphate | D-oxyguanylic acid | 5′-dGMP |
| Cytosine (C) | D-oxycytidine- 5′-monophosphate | D-oxycytidylic acid | 5′-dCMP |
| Thiamine (T) | D-oxythymidine- 5′-monophosphate | D-oxythymidylic acid | 5′-dTMP |
Highlight:
| Polydeoxyribonucleotide |
| Except for some viruses (eg TMV), DNA is the genetic material in most prokaryotic and eukaryotic cells. In most cases, DNA is double stranded and forms a double helix. Each strand of DNA consists of a number of deoxyribonucleotides. That is, each strand is a polymer of deoxyribonucleotides, i.e., it is composed of polydeoxyribo-nucleotides. The 5’C of the deoxyribose sugar of each deoxyribonucleotide is linked to the 3’C of the previous deoxyribonucleotide through a 3’5′ phosphodiester bond to form a strand of DNA called a polydeoxyribonucleotide. In this case the -OH group of the phosphoric acid attached to the 5’C reacts with the -OH group present at the 3’C to liberate one molecule of water and two de-oxyribonucleotides are added through a P-O bond. |
Chargaff’s Alkali Formula or Base Equivalence Formula:
Scientist Erwin Chargaff and his colleagues mentioned an important formula called base equivalence formula in the structure of DNA in 1950 AD. They collect DNA from different organisms. They applied chromatographic method (chromatographic method) to these DNAs and determined their quantity by separating the four N₂-containing bases A, T, C, G. Based on that, Chargaff mentions this formula. According to this formula, the total amount of purine bases and the total amount of pyrimidine bases in an organism’s cell or DNA are equal. That is, the total amount of adenine and guanine (A + G) and the total amount of cytosine and thymine (C + T) are equal. Their ratio is 1:1.
A + G = C + T; A+G/C+T= 1; (A + G) : (C + T) = 1:1
Through this formula, Chargaff also states that the amount of adenine base (A) present in DNA is equal to the amount of thymine base (T) and the amount of guanine base (G) is equal to the amount of cytosine base (C). ie-
A = T G = C; A/T=1; G/C=1
This is called Chargaff’s base equivalence formula. Note that this applies to DNA, not RNA.
Double-stranded structure of DNA:
Based on Wilkins and Franklin’s X-ray diffraction and Chargaff’s base equivalence formula, American biochemist JD Watson and British biophysicist F H C Crick (1953) described the dauble helix model of DNA, which is universally accepted. Their model is also known as Watson and Crick’s model. According to this model, the main structural features of double stranded DNA are mentioned below-
1. DNA, consists of two helical and parallel strands. They spiral and parallel each other around a central axis to form a double helix, which looks like a twisted iron ladder. By nature this horoscope is Dakshinavarti.
2. Each stand of DNA consists of numerous de-oxyribonucleotides i.e. each stand is actually a polyde-oxyribo-nucleotide. The main axis of each stand consists of de-oxyribose sugars and phosphates. This part can be compared to an iron stair handle. N₂-containing bases extend inward when linked to sugars, likening them to the rungs of a ladder.
3. In each stand of DNA the 5’C of the deoxyribose sugar molecule of a deoxyribonucleotide is linked by a phosphodiester bond to the 3’C of the deoxyribose sugar molecule of the preceding nucleotide. In this case, the sequence phosphate-sugar-phosphate-sugar (P-S-P-S) is observed in each stand.
4. Any N₂-containing base between adenine (A), thymine (T), cytosine (C) and guanine (G) with the 1’C of the deoxyribose sugar belonging to the deoxyribonucleotide of each stand of DNA. are linked by glycosidic bonds.
5. The polydeoxyribonucleotide stands of double-stranded DNA are arranged antiparallelly. That is, if in one stand the O’ end of the sugar molecule is on one side and the 5′ end is on the other side (3’5′), in the other stand this arrangement will be completely reversed (5’3′). Since the two strands of double-stranded DNA are aligned parallel to each other, the strand is 20 Å or 2 nm wide everywhere. That is, the distance between the two stands is 20A. A complete (360°) turn of double-stranded DNA consists of 10 complementary base pairs (bp) or steps equidistant. The length of a complete puck is 34A or 3.4 nm. That is, the combined length of the 10 steps is 34A. So the distance between two side-by-side steps or base pairs is 3.4A or 0.34 nm.
6. The nitrogenous base of a de-oxyribonucleotide of one stand of double-stranded DNA is cross-linked to the N₁-containing base of an adjacent de-oxyribonucleotide of another stand of that DNA by a hydrogen bond (H bond), comparable to the steps of an iron ladder. goes Notably, the adenine (A) of one strand is always linked by two hydrogen bonds to the thymine (T) of the other strand and the cytosine (C) of one strand to the guanine (G) of the other strand by three hydrogen bonds. ‘A = T’ and ‘C = G’ base pairs are called complementary base pairs and DNA strands are called complementary stand.
7. Two types of grooves are formed in the helix of DNA as it spirals out. These are the wide groove or major groove and the narrow groove or minor groove.
Alternative Forms of DNA:
Different forms of DNA have been found by DNA isolation under different conditions. Scientists Watson and Crick described the double stranded DNA (double stranded DNA), in fact, Rosalind Franklin’s X-ray diffraction analysis (X-ray diffraction analysis) obtained B form or B-DNA. This type of DNA is physiologically useful and important in organisms. They also mention the existence of A form DNA or A-DNA while performing the test. Subsequently, more information about the organization of A-DNA became possible. Three more right-handed DNA forms have been discovered in the laboratory. These DNAs are C-DNA, D-DNA and E-DNA. In 1979, Andrew Wang, Alexander Rich and their colleagues discovered Z-DNA, which is left-handed DNA. Recently Jean Francois Alexandra and his co-scientists discovered P-DNA. Some other alternative forms of DNA are discussed below-
A-DNA:
① It is dehydrated form of DNA. ② has 11 base pairs present in each leg. ③ has a helix diameter of 23A. ④ The distance between two consecutive nucleotides in the same stand is 2.3A. ⑤ The base pairs are slightly tilted with respect to the central axis.
C-DNA:
① Contains 9.33 base pairs per base. ② has a helix diameter of 20Å. ③ This type of DNA is narrower than B-DNA and A-DNA. ④ In this case also the base pairs are slightly tilted with respect to the axis as in A-DNA.
D-DNA:
① Only found in T₂ bacteriophage. ② 7-4 base pairs are present in each paka. ③ This type of DNA does not contain guanine, only A-T base pairs.
Z-DNA:
① anti-clockwise helical DNA. ② 12 base pairs present in each paka. ③ The distance between two consecutive nucleotides on the same stand is 3.8Å. ④ The phosphodiester bond in each strand is zig-zag, so it is called Z-DNA.
Some Other Types Of DNA:
| Repetitive DNA | Selfies DNA | Palindromic or P-DNA | Junk or J-DNA |
| Repeats of the same N₂-containing bases are observed in DNA near the centromere and telomere segments of eukaryotic chromosomes, such regions of DNA are called repetitive DNA. | Repetitive regions of DNA are incapable of providing the code for protein synthesis or polypeptide chain formation, but replicate themselves during replication. Hence the DNA in that region is called Selfish DNA. | Thus, in the case of DNA, the arrangement of N₂-containing bases in the two strands is exactly the same, when read from two different directions (5′ 3′ and 3′ 5′). Such arrangement is called palindromic arrangement. | The part of the chromosomal DNA in the cell where the sequence of nucleotides and N₂-containing bases does not carry the signal or code required for protein or polypeptide synthesis is called junk or non-coding DNA (non-coding DNA). For example, in humans about 98% of DNA is junk DNA, while in bacteria about 2% is junk DNA. |
Functions Of DNA:
Mentioned below are the major functions of DNA-
1. Hereditary transmission of traits:
Mainly chromosomal DNA or genes determine most of the characteristics (traits) of organisms and help them to be transmitted through the gamete to offspring. Thus, DNA or genes are the container and carrier of heredity.
2. Protein Synthesis:
DNA or genes control protein synthesis i.e. the formation of polypeptide chains in living cells. Note that when a section of double-stranded DNA is unwound, mRNA is synthesized by the transcription process from one strand, which brings the protein synthesis signal (code) from DNA into the cytoplasm as codon. There, with the help of ribosomes, tRNA, amino B molecules, etc., polypeptide chains are formed in the process of translation, i.e., protein synthesis takes place.
3. Replication:
In living cells, DNA is synthesized from dsDNA in phase 1 of interphase, i.e., DNA is replicated. This phenomenon is called replication. This process is controlled by DNA or genes.
4. Exhalation:
DNA or genes control the reproduction of organisms. Thus, the formation and development of the body takes place under the control of DNA or genes.
5. Genetic differences:
Genetic diversity is maintained in species through DNA recombination.
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