The DNA

It is an acidic long polymer of deoxyribonucleotide that forms two complementary strands that run antiparallel to each other and are held together by hydrogen bonds between their opposite nitrogenous bases. It is made up of nucleotides which make the length of the DNA. Some of the examples are; 

• Human = 6.6×10⁹ BP, 
• E.Coli =4.6×10⁶ BP, 
• Lambda phage =48502 BP, Ø X 174 phage =5386 nucleotides (Single-stranded DNA).

Structure of the polynucleotide chain

A polynucleotide is a polymer of nucleotides. Both DNA & RNA are polynucleotides. It is made up of three components; a nitrogenous base, a pentose sugar, and a phosphate group. 

The nitrogenous bases are of two types; purines [Adenine (A) and Guanine (G)], and pyrimidines [Cytosine (C), Thymine (T) & Uracil (U)]. Note that thymine is only present in DNA and in the case of RNA Uracil is present instead of thymine. Pentose sugar is ribose in RNA and deoxyribose in DNA. And the phosphate group makes the nucleotide acidic. The nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form nucleosides. The phosphate links two nucleotides to form a dinucleotide with the help of phosphodiester bonds. 

• Nitrogenous base + sugar = Nucleoside
• Nucleoside + phosphate group = Nucleotide

Each polynucleotide chain has 2 free ends- a 3 prime (3’) end and the opposite 5 prime (5’) end. In the 3’ end, the 3rd C-atom of the sugar is free, i.e., it is not linked to any nucleotide. Similarly, in the 5’ end, the 5th C of sugar is free.

Structure of DNA

Based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, James Watson, and Francis Crick proposed a very simple but famous double helix model of DNA in 1953 which was seconded by the finding of Erwin Chargaff for a double-stranded DNA. The findings were;

1. The purines and pyrimidines are always equal in amount i.e. A+G = T+C or A+G/T+C=1.
2. The amount of adenine will always be equal to thymine and the amount of guanine will always be equal to cytosine i.e. A=T and G=C.
3. Adenine is joined to thymine with 2 H-bonds while guanine is joined to cytosine by 3 H-bond.

Salient features of the Double Helical Structure of the DNA are;

1. DNA is made of two polynucleotide chains, where the backbone is made of sugar-phosphate, and the bases are projected towards the inside.
2. The two strands have anti-parallel polarity. It means if one strand has the polarity 5′ –> 3′, then the other has 3′ –> 5′.
3. The bases in two strands are paired through hydrogen bonds (H-bonds) forming base pairs (bp). Purine always comes opposite to pyrimidine. This generates approximately uniform distance between the two strands of the helix (20Å).
4. The double-stranded structure is coiled in a right-handed fashion. The pitch of the helix is 3.4 nm, and there is roughly 10 bp in each turn which makes the distance between a bp in a helix approximately equal to 0.34 nm.
5. The plane of one base pair stacks over the other in a double helix pattern along with the H-bonds conferring stability of the helical structure.

Packaging of the DNA helix

The length of DNA is far greater than the dimension of a typical nucleus i.e. around 2.2 meters. This long-sized DNA is compacted and packed in the relatively smaller region of the nucleus.

In Prokaryotes the DNA is not packed inside a nucleus but it is compacted to form a structure called the nucleoid. In the nucleoid region, the negatively charged DNA is held by some positively charged non-histone proteins forming a looped structure. 

In Eukaryotes the DNA is packed inside a membranous structure called the nucleus. Inside the nucleus, the DNA is wrapped around a unit of 8 molecules of positively charged protein histone in the form of histone octamer to form a structure called nucleosome that contains around 200 bp of DNA helix. The nucleosomes in turn form repeating units like “beads-on-string” structures in the nucleus called chromatin.  Chromatin is packaged to form chromatin fibers which are further coiled and condensed at the metaphase stage of cell division to form chromosomes.

CBSE Class 12 Molecular Basis of Inheritance: Inheritance is transmitted by certain molecules that Mendel termed as ‘factors’, but their nature was discovered later with the development of various scientific techniques. The molecules which govern the inheritance are called genes and it is of two types; Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). In all organisms except viruses, DNA acts as the genetic material whereas RNA acts as the messenger molecule, however in some viruses RNA functions as the genetic material. It is because DNA is a more stable molecule as compared to RNA and hence, it evolved as the genetic material of almost all the organisms.