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DNA --

posted Jan 27, 2015, 12:40 AM by Ranmini Perera   [ updated Dec 1, 2015, 2:22 PM by Upali Salpadoru ]

DNA -- 
The Living Molecule

Fig.1. Double Helix nature of DNA.

No life has been discovered free from DNA (or RNA in a few viruses).  These are the only known polymers that can make chemical copies of themselves. Deoxyribonucleic acid is formed by the condensing (joining) of monomers called ‘nucleotides’. The credit for the discovery of the double helix structure of the DNA goes to Francis Creek (UK) and Jimmy Watson (US). Many eminent physicists, chemists and biologists and a few crystallographers like Rosalind Franklin (UK) and Maurice Wilkins (NZ) have pav

 Fig.2  They discovered the structure.

If the DNA of one human cell is stretched out, it would be almost2 m. long and contain over three billion base pairs.

How does all this fit into the nucleus of one cell? Human female egg cell is only 1/10th  of a cm.


A nucleotide consists of a phosphate, (shown in brown) a 5 Carbon sugar (blue) and a heterocyclic compound, (red) which forms the base. While the phosphate group and the pentose sugar gets repeated the bases vary. There are two types of bases;

Fig.3.  Linking of base pairs to join the 2 helical strands of the DNA molecule.

There are two types of bases; Purines which are double ringed and single ringed Pyrimidines,. Adenine (A) and Guanine(G) are purines while Cytosine(C) and Thymine(T)  are pyrimidines A always link with T while C pairs with G.

The DNA molecule is unique in many respects. This is the only known molecule which can replicate itself provided the conditions are right. It is well known fact that all living things except the viruses (which seem to be on the fence between the living and nonliving) are made up of microscopic units called the cells. An average cell whether it is from the gigantic blue whale or a tiny baby would not be bigger than 1/5000th of a cm. An adult human being has over 70 trillion cells. In each of these cells, embedded in the nucleus there will be a DNA molecule which would extend to a meter in length when stretched. This would contain, in coded form the characteristics and features which would be called the genome of the organism.

Fig 4.  A nucleotide of the DNA polymer

So it is a mystery why our cells in a finger cannot think or why the brain cells are incapable of secreting gastric juice.

The hereditary information is recorded in small units called genes. This is performed by the arrangement of the 4 base units, A, T, G and C. Only 20 amino acids are involved in the forming of these base units. Microbiologists can separate the genes in a sample of DNA and show the sequence of the bases. In the studies of the human genome it has been found that, whether it comes from a European, African or an Asian, genes are 99.9% identical. (Ref. The Cannon by Natalie Angier) According to the same source the mouse DNA is 70% agreeable with ours while that of fruit fly 47%.

Although we cannot say how the DNA molecule was born, the biologists are certain that all living forms originated from a single cell; monophylactic origin. Almost every living specimen today, (except in the case of planting a branch as in vegetative propagation) whether it is an oak tree or a yeast; a bacteria or a buffalo starts life as a single cell.

What is this cell called?

It is the egg cell. Some students will wonder how a buffalo comes from an egg. It has to be reminded that even the females of the mammals produce microscopic eggs. These eggs get fertilized by the sperms from the male and develop inside the uterus. A fertilized egg, still one cell, is called a zygote. This zygote undergoes division, mitosis, producing identical cells up to a stage called blastocyst, carrying the individuals’ genome.