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Deoxyribonucleic Acid

DNA or deoxyribonucleic acid is the hereditary material in humans and in almost all other organisms. DNA encodes genetic information which is used in the development of almost all living organisms including viruses. DNA is nucleic acid and is one of the major macromolecules vital to all forms.  

DNA are mostly are made of two strands, coiled to form a double helix. The strands of DNA are made of sequences of nucleotides. Nucleotides are composed of nitrogen bases, monosaccharide sugar (deoxyribose in DNA) and a phosphate group. The nucleotides are linked to each other by covalent bonds between the sugar and the phosphate groups, which results in an alternating sugar-phosphate backbone. The nitrogen bases of two separate polynucleotide are bound with hydrogen bonds to make a double-stranded DNA. DNA stores information, both the strands of DNA store the same biological information.  

DNA Double Helix Structure
The strands of DNA are anti-parallel and opposite to each other. The DNA organized into chromosomes within the cells. During the process of cell division, the DNA are replicated in the process of DNA replication, which gives each cell its own set of chromosomes. Eukaryotic organisms store their DNA in the nucleus of the cell and also in other organelles like mitochondria and chloroplasts. In prokaryotes, DNA is diffused in the cytoplasm. 



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Deoxyribonucleic acid or DNA is defined as a molecule that encodes genetic information which is necessary for the development and functioning of all living organisms. DNA is a double stranded molecule which has information for of factors like cell growth, division and function. DNA is is the form of a double stranded helix. DNA is a polymer of nucleotides which codes fro amino acid sequences during the process of protein synthesis. DNA carries genetic information on genes which are required to construct molecules like proteins. 

DNA Structure

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Primary Structure:
  • DNA is a polymer sequence made up of subunits of nucleotides. Nucleotides of the DNA are made of a sugar(deoxyribose), a nitrogenous base and a phosphate group. 
  • Nitrogenous bases of four types are present in a DNA molecule, namely adenine, guanine, cytosine and guanine, a sugar molecule is the 5-carbon sugar deoxyribose and one or more phosphate groups. 
  • Adenine and guanine are purine nitrogen bases, cytosine and thymine are pyrimidines.
  • A phosphodiester bond is formed with the phosphate group of the nitrogen bases with the OH group of the sugar.
  • The nucleic acid sequence on the nucleotides are complementary to another sequence of the DNA strand.
Secondary Structure: 
  • Secondary structure of DNA is the interaction between the bases, of which strands are bound to each other. 
  • In the double helical structure of DNA, the strands are held together by hydrogen bonds, where the nucleotides on one strand pairs with nucleotide on the other strand. 
  • The secondary structure gives shape to the nucleic acids. The purine base pairs with pyrimidine base by hydrogen bonds. 
  • The secondary structure determines the base-pairing of the strands to form a double helix. 
  • A major groove and a minor groove is formed in the double helix. The strands of DNA are not symmetrical with one another, hence the grooves are unequal. The major groove is 22 Å wide and minor groove is 12 Å wide. 

Tertiary Structure: 
  • Tertiary structure of DNA is it location in three-dimensional space. There are 4 different structural variations in DNA forms: 
  • Right or left handed
  • Length of the helix turn
  • Number of base pairs per turn
  • Difference in size between the major and minor grooves. 

DNA's tertiary arrangement in space includes B-DNA, A-DNA and Z-DNA. The possible confirmations in which the DNA exists include A-DNA, B-DNA and Z-DNA. B-DNA and Z-DNA are the only functional forms that have been observed in organisms. The DNA adopts conformation depending upon the hydration level, DNA sequence and the amount and direction of supercoiling, chemical modification of bases, types and concentration of metal ions and also polyamines in solution. 

Types of DNA


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  • The B-form of DNA is the most common form of DNA. 
  • The B-DNA when compared to the A-DNA is more narrower and elongated. 
  • The major groove is wider and is more accessible to proteins. 
  • The minor groove is narrow. 
  • B-form of DNA is favored at high water concentrations. Base pairs are almost perpendicular to the helix axis. 
  • Helix diameter  23.7 Å
  • Rise per Base pair  3.4 Å 
  • Base-pair per helical turn ∼10 
  • Helical pitch  34 Å 
  • Tilt of the bases -1°


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  • Z-form of DNA has left handed sense. 
  • This form can be formed in-vivo, if is given proper sequence and superhelical tension but function remains hidden. 
  • This structure of DNA is more narrower and elongated than A or B helix. Major groove is not prominent.
  • The minor groove is narrow. 
  • The Z-form of conformation is favored by high salt concentrations, some base substitutions, it requires alternating purine and pyrimidine sequence. 
  • The base pairs are nearly perpendicular to helix axis. 
  • Helix Diameter 18.4 Å
  • Rise per base pair 3.8 Å 
  • Base pair per helical turn ∼ 12
  • Helix pitch 47 Å
  • Tilt of the bases -9°


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  • A-form of DNA are shorter and wider than the B-DNA.
  • Most of the RNA and RNA-DNA duplex are found in this form. 
  • The major groove is deep and narrow and is not easily accessible to proteins. 
  • The minor groove is wide, shallow is accessible to proteins but information content is lower than the major groove.
  • This conformation is favored at lower water concentrations. 
  • The base pairs are tilted to the helical axis. 
  • Helix diameter 25.Å
  • Rise per base pair 2.3 Å
  • Base pair per helical turn ∼ 11
  • Helical pitch 25 Å
  • Tilt of the bases 20°

DNA Properties

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  • DNA is a polymer made from repeating units of nucleotides. 
  • The double helical structure of DNA was first discovered by Watson and Crick. 
  • The DNA structure is if two helical chains that are coiled round the same axis. 
  • The pitch of the structure being 34 Å and radius being 10 Å. 
  • The DNA chain measured is 22 Å to 26 Å wide and one nulceotide is 3.3 Å. 
  • Individual repeating units of DNA is very small, where DNA polymers can be very large molecules containing millions of nucleotides. 
  • DNA usually exists in the form of pairs, that are held tightly togehter. The two strands of DNA are twisted in the shape of a double helix. 
  • A nucleotide unit consists of the segment of the backbone of the molecule which holds the chain together and also a nucleobase which interacts with the other DNA strand in the helix.
  • A nitrogen nucleobase linked to a sugar molecule is known as nucleoside. 
  • A nitrogen base linked to a sugar and one or more phosphate groups is called a nucleotide. 
  • Monomer units of nuleotides are linked to form a polynucleotide as in DNA.
  • Alternating phsophate and sugar molecules form the backbone of the DNA strand. 
  • The sugar in the DNA molecule is a pentose, deoxyribose sugar. 
  • The sugars are linked together by phosphodiester bonds between the third and fifth carbon atoms of the adjacent sugar rings. 
  • In the double helix structure of DNA, one strand runs opposite to the direction of the other strand and are antiparallel.
  • The ends of the molecule are asymmetric and are known as the 5' and 3' ends, the 5' end has a terminal phosphate group and 3' end had a terminal hydroxyl group. 
  • The difference between the DNA and RNA is the sugar molecule, the RNA has pentose ribose sugar instead of the deoxyribose sugar of the DNA. 
  • The helix of the DNA is held by two forces: hydrogen bonds are present between bonds which are present between nucleotides and base pairing interaction of the nucleobases.
  • The conjugated Pi bonds of the nucleotide bases, in the aqueous environment of the cell, align the nucleotide bases perpendicular to the axis of the DNA molecule, hereby minimizing their interaction with the solvation shell and the Gibbs free energy. 
  • The bases found in the DNA are adenine, cytosine, guanine and thymine. These bases are attached to the sugar and phosphate to form the complete nucleotide. 

DNA and RNA Properties

DNA Base Pairs

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  • In the double helical structure of the DNA, each type of nucleobase on one strand bonds with one type of nucloebase on the other strand, this is known as complementary base pairing. 
  • The purine bases pairs with pyrimidines by hydrogen bonds where adenine base pairs with thymine with 2 hydrogen bonds and cytosine bonds with guanine with 3 hydrogen bonds.
  • This bonding of two nucleotides along the double helix is called a base pair. 
  • The hydrogen bonds are not covalent and they can be pulled apart like a zipper, by mechanical force or high temperature. 
  • As a result of complementary base pairing, the information in the DNA helix is duplicated on each strand
  • The interactions between complementary base pairs is critical for the functioning of the DNA. 
  • The two types of base pairs form different types of hydrogen bonds, the AT base pairs with two hydrogen bonds and the GC base pairs with three hydrogen bonds. 
  • DNA with higher content of GC is more stable and than the DNA with low GC content. 

DNA Base Pairing Structure

DNA Supercoiling

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  • DNA can be present in a twisted rope in a process known as DNA supercoiling. 
  • In its relaxed state, a DNA strand usually circles the axis of the double helix once every 10.4 base pairs. 
  • DNA, that is twisted in the direction of the helix is said to be positive supercoiling and the bases are held more together tightly. 
  • DNA, that is twisted in opposite direction is negative supercoiling, and bases come apart more easily.
  • Most DNA in nature, have a slight negative supercoiling that is brought in the enzyme called topoisomerases. 
  • The topoisomerase enzymes are also needed to relieve the twisting stresses in the DNA strands during the process of transcription and DNA replication. 

DNA Damage

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  • Damage to DNA can be caused by many sorts of mutagens which change the sequence of DNA. 
  • Mutagens includes substances like oxidizing agents, alkalyting agents and also high-energy radiations like uv rays and X-rays. 
  • The type of damage occurred depends on the type of mutagen. 
  • UV light can cause damage by producing thymine dimers. 
  • Oxidants like free radicals or hydrogen peroxide produce damage in may forms like base modifications, specifically guanosine and double strand breaks. 
  • Out of the damage caused due to oxidation, the most dangerous form of mutations are double-strand breaks, as these mutations are difficult of repair and produce point mutations, insertions and deletions from the DNA sequence and also chromosomal translocations. 
  • Sometimes, double-strand mutations can lead to cancer. 
  • Damage to DNA is also caused by various cellular processes that produce reactive oxygen species. 
  • Most of the damages in DNA are repaired but in any cell some DNA damage may remain though repair processes occur. 
  • These DNA damages accumulate with age in mammalian tissues. This accumulation can be an important cause of aging. 

Biological Functions

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  • DNA is the genetic material in the eukaryotic organisms, that is present in the form of linear chromosomes, it is circular in prokaryotes. 
  • The genetic information is carried by DNA on sequences known as genes. 
  • The complete set of information in an organism is known as the genotype. 
  • Gene is defined as the basic unit of heredity. 
  • It is the region of DNA that influences a particular characteristic in an organism. 
  • The gene sequence of DNA influences the phenotype of an organism.
  • The sequence of bases on the DNA strand defines a messenger RNA sequence which then gives one or more protein sequences.
  • The genetic code is a sequence of three nucleotides which determine the amino-acid sequence of the proteins. 
  • Cell division is a process by which organism grows, the DNA are replicated and each daughter cell receives same genome as their parents. 
  • The double strand of the DNA helps DNA replication. 
  • The strands are unwound and for each strand a complementary strand is recreated. 
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