ABOUT DNA AND ITS STRUCTURE

                                              STRUCTURE OF THE DNA 

DEFINITION 

     DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria and Chloroplast. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people.  The genome of the organism consists of Introns(Non coding genes)  and Exons (coding region). And only 2% of the total genome codes for protein, and others are intron (which is non - coding genes). 

HISTORY

     In the 1950s, Francis Crick and James Watson worked together at the University of Cambridge, England, to determine the structure of DNA.   In Wilkins’ lab, researcher Rosalind Franklin was using X-ray crystallography to understand the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule using Franklin’s data.  Watson and Crick also had key pieces of information available from other researchers such as Chargaff’s rules. Chargaff had shown that of the four kinds of monomers (nucleotides) present in a DNA molecule, two types were always present in equal amounts and the remaining two types were also always present in equal amounts. This meant they were always paired in some way.

STRUCTURE OF THE DNA

  

a)   The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base. 

* Deoxyribose Sugar Structure 

* N2 Bases Structure 

* Bonds: Hydrogen Bond, Phosphodiester bond

b) KEY POINTS: 

⦁ DNA is made of two helical chains that intertwine with each other to form a double helix. The most widely accepted structure of DNA is right-handed helix DNA also known as the B-form of DNA, which is 1.9 nm in diameter.

⦁ These helical chains run anti-parallel to each other, one polynucleotide chain runs from 5’ to 3’ and the other polynucleotide chain runs from 3’ to 5’. These chains are connected to each other via nitrogen bases through hydrogen bonding.

⦁ Hydrogen bonding contributes to the specificity of base pairing. Adenine preferentially pairs with Thymine through 2 hydrogen bonds. Similarly, Cytosine preferentially pairs with Guanine through 3 hydrogen bonds. 

⦁ We can even say, that the base pairing happens when Pyrimidines pair with Purines because Pyrimidines refers to the single ring structure of Thymine and Cytosine and Purines refers to double-ring structures, Adenine and Thymine.

⦁ The base pairs A = T and G ≡ C are known as complementary base pairs. Hence, the amount of Adenine is equal to the amount of Thymine, and the amount of Guanine is equal to the amount of Cytosine.

⦁ The geometry of the DNA is influenced by the distance between the backbones and the angle at which the nitrogenous bases are attached to the backbone.

⦁ The major groove occurs when the backbones are far apart from each other and the minor groove occurs when they are close.

⦁ The regularity of the helical structure forms two repeating and alternating spaces: Major and Minor grooves.

⦁ These groves act on base-pair recognition and binding sites for protein, the major groove contains base pair specific information while the minor groove is largely base-pair nonspecific, caused by protein interactions in the grooves

⦁ The double-helical structure of DNA is highly regular, each turn of the helix measures approximately 10 base pairs. In addition to hydrogen bonding in between the bases, the staging of bases also stabilizes the structure, there are pi-pi interactions between staged aromatic rings of the bases.

⦁ The distance between each turn is 3.4 nm.

⦁ The major groove is 2.2 nm wide and the minor groove is 1.1 nm wide.

⦁ Chargaff's Rule: Chargaff's rules state that DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine.