The Chemical Constitution of DNA

Introduction

DNA is the Deoxyribonucleic acid molecule that is composed of various groups and two polynucleotide chains that are arranged in an antiparallel manner and are coiled around each other to form a double helix structure of DNA. DNA is responsible for carrying the genetic information from the parents to the next generation in almost all living organisms including several viruses. DNA is the type of nucleic acid that is essential for living beings along with carbohydrates, proteins, lipids and is the important macromolecule. DNA is composed of several chemicals that include sugar, phosphate, nitrogen bases, etc. DNA helps mainly in the generation of proteins. Nucleic acid was first discovered from the nuclei of the pus cells by Friedrich Miescher. The nucleic acids are of two types: deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). DNA was first isolated by Freidrich Meischer, a Swiss physician in 1869 but its structure was not clear until 1953. Later the proper structure was discovered by James Watson and Francis Crick which was accepted by many biologists and led them to win the Nobel Prize for their work. Their work was however very similar and reliant to the work of Rosalind Franklin. Franklin was working by herself on the isolation of the structure of DNA and it was her X-ray photograph that gave the exact double-helical structure of DNA. She was yet to publish her work but somehow Watson and Crick got access to her work and published it without her knowledge due to which she could not win the Nobel Prize. The DNA in the case of the viruses and the bacteriophages are enclosed in a protein coat while in the case of bacteria and mitochondria, and plastids of the eukaryotic cells the DNA present is circular and is without cover, naked inside the cytoplasm. While in the case of the eukaryotic cells DNA is in the form of long spirally arranged threads called chromosomes. According to various experiments, it is known that DNA is found in the chromosomes while RNA and proteins are found in the cytoplasm of the cell.

Structure of DNA

The double helix structure of DNA constitutes two strands of DNA that are intertwined together and are made up of nucleotides. The nucleotides are the monomeric unit in the nucleic acid that is composed of three main constituents: – a five-carbon sugar, a nitrogenous base, and phosphoric acid. The sugar and phosphate groups are the repeating units that are combined together and form a sugar-phosphate backbone. The nitrogenous bases have a heterocyclic structure that is aromatic molecules. There are two types of nitrogenous bases that are found, purines and pyrimidines. Adenine and guanine are the two types of purines while thymine, cytosine are the types of pyrimidines.

Phosphate group

The phosphate group is composed of a phosphorus atom that consists of four negatively charged oxygen atoms that are attached to it.

5-carbon sugar

It is also known as pentose sugar that includes the deoxyribose in DNA and ribose in RNA. They have five carbon atoms along with hydrogen atoms and hydroxyl groups and one oxygen atom. In the case of the deoxyribose sugar, the hydroxyl groups are attached to the carbon atom of the first and third number while to the second carbon number only a hydrogen atom is attached.

Nitrogen base

In nucleic acid, the nitrogen atom acts as a base as it gives electrons to various molecules that results in the formation of new molecules. It can also create a ring structure to bind with carbon, hydrogen, and oxygen molecules. Ring structures are of two types: purines (double rings), and pyrimidines (single rings). Pyrimidines consist of cytosine, thymine, and uracil while purines consist of adenine and guanine. Purines are larger than pyrimidines, this size difference helps in determining the pairings in the strands of DNA.

Nucleic acid bonds

The glycosidic and the ester bonds are those bonds that help in holding the sugar, phosphorus, and nitrogen molecules together. The bonds between the first carbon atom of the 5-carbon sugar and the nitrogen atom of a ninth number of the nitrogenous base are called glycosidic bonds. The bond between the fifth carbon atom of the 5-carbon sugar and the phosphate group is called ester bonds. These bonds hold single nucleotides and also several nucleotides together that form the polynucleotides and result in the formation of deoxyribonucleic acid (DNA). These polynucleotide chains will be formed by the bonding of the fifth carbon atom in a 5-carbon sugar and the third carbon atom in the next 5- carbon sugar that results in the formation of the sugar-phosphate backbone. For the process of formation of DNA, the primer RNA strand will be bonded to the polynucleotide with the help of a hydrogen bond but will be in an antiparallel manner. The linking of the purines and the pyrimidines are responsible due to the hydrogen bonds. The pairing of adenine to thymine and guanine to cytosine is termed as a complementary base pairing. In humans, it is shown that there are around 150 million base pairs in a single molecule. Our body cells divide, regenerate and die which is only possible when the DNA in the cell replicates themselves continuously. The DNA is usually made of 4 million nucleotides per cell and present in the chromosome and little is found in the mitochondria and chloroplasts. The RNA nucleotides are less in number, up to 12000 per cell, and are distributed throughout the cytoplasm of the cell except for the vacuoles.

Denaturation of DNA

The two DNA strands are denatured or are separated when they are passed through severe conditions like high temperature, pH, etc. This process is called denaturation. The DNA can be regained to its original structure by the removal of denaturants like pH, temperature, etc. This process is called renaturation or annealing.

Structure of double-stranded DNA

DNA is of various types, but the two most common types of DNA are:

B DNA

• The double strand of the long polynucleotide chain is coiled around the axis
• The two strands of the DNA are arranged in an antiparallel manner
• Nitrogenous bases like adenine base pairs with thymine while guanine base pairs with cytosine
• The hydrogen bonds between adenine and thymine formed are two while the hydrogen bonds formed in between guanine and cytosine are three in numbers

Z DNA

• The structure of Z DNA is found to be thinner than B DNA
• The bases of purine and pyrimidine are alternatingly arranged
• To make the structure of DNA stabilized it is constituted of high salt concentration

Conclusion

DNA is the storehouse of biological information that is transferred from one generation to the next. They help in the process of generation of proteins in the body which is very essential for the development of body parts. In the case of human beings, around 98% of the DNA is non-coding- introns (does not participate in protein synthesis so is non expressive) and is similar while only around 1-2% of the DNA is the coding regions- exons (participate actively in protein synthesis so are genetically expressed) which differs in all the individuals making them different from others. Mutation or changes in DNA may lead to various diseases that include sickle cell anemia, cystic fibrosis, etc. Thus, DNA is the building block of all living organisms.