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  or Polynucleotide chain 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 or nitrogenous bases.

• Adenine always pairs with thymine by two hydrogen bonds whereas thymine makes a pair with cytosine with the three hydrogen bonds.Adenine and Guanine are purine whereas Thymine and Cytosine are pyrimidine.

• 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.

A/T =G/C =1

Questions : If A double strand DNA has 600 nucleotides. Out of these 120 are adenine. Calculate other nucleotides in DNA.

Solution : According to chargaff , Ratio of adenine and thymine should be equal hence :

Adenine are 120 than thymine are also 120.

Now out of 600 , adenine and thymine are 240

To make ratio one:

A/T =G/C =1

120/120 =180/180 =1

Now , adenine = 120

Thymine = 120

Guanine= 180

Cytosine = 180

Total = 600