2. Some Informations About DNA:-

2. Some Informations About DNA:-

“ Deoxyribonucleic acid”. The molecules inside cells that carry genetic information and pass it from one generation to the next. See mitosis, chromosomes.

We have heard the term DNA a million times. You know that DNA is something inside cells .We know that each and every one looks different and this is because of they are having different DNA.

Have you ever wondered how the DNA in ONE egg cell and ONE sperm cell can produce a whole human being different from any other? How does DNA direct a cell's activities? Why do mutations in DNA cause such trouble (or have a positive effect)? How does a cell in your kidney "know" that it's a kidney cell as opposed to a brain cell or a skin cell or a cell in your eye? How can all the information needed to regulate the cell's activities be stuffed into a tiny nucleus?

A basic tenet is that all organisms on this planet, however complex they may beperceived to be,are made of the same type of genetic blueprint.The mode by which that blue print is coded is the factor that decides our physical makeup-from color of our eyes to what ever we are human.








To begin to find the answers to all these questions, you need to learn about the biological molecules called nucleic acids.

An organism (be it bacteria, rosebush, ant or human) has some form of nucleic acid Which is the chemical carrier of its genetic information. There are two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) which code for all the information that determines the nature of the organism's cells. As a matter of fact, DNA codes for all the instructions needed for the cell to perform different functions. Did you know that human DNA contains enough information to produce about 100,000 proteins?

Genes are made up of DNA ,which is shaped like a twisted ladder with rungs made up of molecules called nucleotide bases linked together in specific pairs.The arrangement of these bases along the DNA provides the cell with instructions on making proteins. DNA is tightly coiled into rod-shaped structures called chromosomes, which are stored in the nucleus of the cell. There are 22 pairs of chromosomes in each body cell plus two sex chromosomes.

2.1) Structure of DNA:-

This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining ß-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right- handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions.



There is a residue on each every 3.4 A. in the z-direction. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure repeats after 10 residues on each chain, that is, after 34 A. The distance of a phosphorus atom from the fibre axis is 10 A. As the phosphates are on the outside, cations have easy access.

The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact.

The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. The are joined together in pairs, a single base from the other chain, so that the two lie side by side with identical z-co-ordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur.

The hydrogen bonds are made as follows : purine position 1 to pyrimidine position 1 ; purine position 6 to pyrimidine position 6.

If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are : adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine).

In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine ; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined.

It has been found experimentally (3,4) that the ratio of the amounts of adenine to thymine, and the ration of guanine to cytosine, are always bery close to unity for deoxyribose nucleic acid.

It is probably impossible to build this structure with a ribose sugar in place of the deoxyribose, as the extra oxygen atom would make too close a van der Waals contact. The previously published X-ray data (5,6) on deoxyribose nucleic acid are insufficient for a rigorous test of our structure. So far as we can tell, it is roughly compatible with the experimental data, but it must be regarded as unproved until it has been checked against more exact results. Some of these are given in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments.

It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.

2.2) Arrangement of Nucleotieds in DNA :-

• One strands:-

Strands of DNA are long polymers of millions of linked nucleotides. These nucleotides consist of one of four nitrogen bases, a five carbon sugar and a phosphate group. The nucleotides that make up these polymers are named alter,the nitrogen bases that comprise it, namely, Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). These nucleotides only combine in such a way that C always pairs with G, and T always pairs with A. These two strands of a DNA molecule are anti-parallel in that each strand runs in a opposite direction. Here below figure shows two strands of DNA and the bonding principles of the four types of nucleotides.







The linkage of the sugar-phosphate "backbone" of a single DNA strand is such that there is a directionality. That is, the phosphate on the 5' carbon of deoxyribose is linked to the 3' carbon of the next deoxyribose. This lends a directionality to a DNA strand which is said to have a 5' to 3' direction. The two strands of a DNA double helix are arranged in opposite directions and are said to be anti-parallel in that one strand is 5' - 3' and the complementary strand is 3' - 5'.

• Double Helix:-








The particular order of the bases arranged along the suger-phosphate backbone is called the DNA sequnce and the combinations of the four nucleotides in the estimated millions long polymer strands results in a billions of combinations within a single DNA double helix. These massive amounts of combinations allow for the multitude of differences between every living thing on the plane-form the large scale (for example, mammals as opposed to plants)to the small scale (differences in human hair colour). Here the above fig. Shows the double helix shape of the DNA.