Structure of DNA - Watson and crick, Chargaff rule

 

DNA( Deoxyribose Nucleic acid)  is a molecule that carries genetic information in all living organisms.  It is found inside the nucleus of a cell of  living organisms.  DNA is the  chief  storage of genetic  information in  most of the  animals and plants, and even some viruses.

The double helix  description of DNA is a  brief elaboration of the molecular shape of a double-stranded DNA molecule. In 1953, Francis Crick and James Watson first described the molecular structure of DNA. Watson and  Crick proposed this double helix model of DNA on the basis of an X-ray diffraction experiment   of Maurice Wilkins and Rosalind Franklin.

Remember Gene is a physical vehicle that carries a particular trait  or code a protein in an organism.

The Strands of DNA 

The DNA molecule is made up of  two strands  and both strands run in such a manner as a double helix.  The backbone of each strand is made up of  deoxyribose sugar  and phosphate. The two strands are held together by  hydrogen bonds that are present between the bases like adenine and  thymine similarly  cytosine with guanine.

Both strands of DNA are coiled in a right handed fashion so it is called B-DNA. one turn of the spiral has about 10 nucleotides on each strand of DNA. It occupies a distance of about 3.4 nm (34Å) so that adjacent nucleotides or their bases are separated by a space of about 0.34 nm (3.4 Å)

The double helix  model describes that DNA Is made up of two linear strands that run opposite to each Other or anti-parallel. Each DNA strand is made up of smaller units called nucleotides that form a chain. The chemical backbones of the double helix are made up of sugar and phosphate molecules that are connected by chemical bonds  known as phospho disaster bonds. Both strands of DNA have opposite polarity. One strand has polarity from 5 prime to 3 prime and the other strand has polarity from 3 prime to 5 prime.

Nitrogenous base or Nucleotides 

The bases in DNA molecules are  adenine, cytosine, thymine, and guanine and represented  by A, C, T, and G. DNA is also a polymer of these  bases.

DNA is organized structurally into chromosomes . DNA is also regarded as genes or we can say that a piece of gene is called DNA .

The two  strands are joined to each other through the nucleotides, also called base pairs of nitrogenous bases. Adenine always pairs with thymine by two hydrogen bonds whereas thymine makes a pair with cytosine with the three hydrogen bonds.Out of four nitrogenous bases , adenine and guanine are purine whereas thymine and cytosine are pyrimidine.

According to Erwin Chargaff, In DNA the ratio between adenine and thymine , and cytosine and guanine is always equal to one or constant.