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DNA -BASIC STRUCTURE

 DNA-BASIC STRUCTURE  DEOXY RIBO NUCLEIC ACID DNA is a polymer of deoxy ribonucleotides It is composed of monomeric units namely deoxy adenylate(dAMP), deoxy guanylate(dGMP), deoxy cytidylate(dCMP) and deoxy thymidylate(dTMP). The monomeric deoxy nucleotides in DNA are held together by 3’,5’ –phosphodiester linkage ERWIN CHARGOFF RULE/CHARGOFF’S EQUIVALENT RULES 1. The sum of purines is equal to the sum of pyrimidines Pu/Py=1 A+G=T+C 2.The ratio of adenine to thymine is one A/T=1 3. The guanine and cytosine is also one G/C=1 4.Bases with 6 amino group is equal to bases with 6 keto group. A+C=G+T The ratio of A+T/G+C  is known as dissymmetric ratio . It varies greatly from one species of DNA to other When dissymmetry of ratio exceeds 1 such DNA is called A-T type. The value of dissymmetry of human being is 1.4. STRUCTURE OF DNA[WATSON –CRICK MODEL] 1.DNA molecule consist of 2 polynucleotide chains, coiled around each other on the common in the form of righthanded double he...
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DNA -BASIC STRUCTURE

 DNA-BASIC STRUCTURE 



DEOXY RIBO NUCLEIC ACID

  • DNA is a polymer of deoxy ribonucleotides
  • It is composed of monomeric units namely deoxy adenylate(dAMP), deoxy guanylate(dGMP), deoxy cytidylate(dCMP) and deoxy thymidylate(dTMP).
  • The monomeric deoxy nucleotides in DNA are held together by 3’,5’ –phosphodiester linkage

ERWIN CHARGOFF RULE/CHARGOFF’S EQUIVALENT RULES

  • 1. The sum of purines is equal to the sum of pyrimidines
  • Pu/Py=1
  • A+G=T+C
  • 2.The ratio of adenine to thymine is one
  • A/T=1
  • 3. The guanine and cytosine is also one
  • G/C=1
  • 4.Bases with 6 amino group is equal to bases with 6 keto group.

  • A+C=G+T
  • The ratio of A+T/G+C  is known as dissymmetric ratio . It varies greatly from one species of DNA to other When dissymmetry of ratio exceeds 1 such DNA is called A-T type. The value of dissymmetry of human being is 1.4.


STRUCTURE OF DNA[WATSON –CRICK MODEL]

  • 1.DNA molecule consist of 2 polynucleotide chains, coiled around each other on the common in the form of righthanded double helix. The double helix are called plektonic coils
  • 2.Two inter chain spacing or grooves has width of 12𝐴 ̂,depth -8.5 𝐴 ̂   and a minor groove width 6 𝐴 ̂  ,depth 7.5 𝐴 ̂

  • 3. The two strands are antiparallel that is one stran runs in 5’ to 3’ direction while the other strand runs in 3’ to 5’ direction.
  • 4. The diameter of helix is 20 𝐴 ̂. The bases of 3.4 𝐴 ̂  
  • apart along the helix axis. Each turn of the helix contains 10 nucleotide residues. The pitch of helix is 3.4 𝐴 ̂ (3.4nm)
  • 5. Each strand of DNA has a hydrophilic deoxy ribose phosphate back bone on the outside of molecules where hydrophobic base are stacked inside

  • 6. The two polynucleotide chains are not identical but complimentary to each other.
  • 7. The two strands are held together by hydrogen bonds formed by complementary base pairs. The A-T pair has a hydrogen bond while G-C has 3 hydrogen bonds. The GC is stronger by 50% thant AT 
  • 8.The genetic strand in one of the two strands are called as template strand or sense strand . The opposite strand is antisense strand 


STRUCTURE OF DNA
Primary structure

  • It is a linear sequence of nucleotide link  by phosphodiester bond.
Secondary structure

  • It is interaction between bases strands are bound to each other. 

  • In DNA double helix two strands bound by hydrogen bond
  • The secondary structure has helix bulger loop junction.
  • The secondary structure has single poly nucleotide.
Teritiary structure
  • Teritiary structure in DNA take geometric steric constrains which has large scale folding of linear polymer. 

  • They are 3 areas of structural form differs.
  • 1.Handedness-Right or left
  • 2.Length of helix turns.
  • 3.Nitrogen of base pair per turn
  • 4.Differences in size between major /minor grooves.

Quartinary structure
  • The quartinary structure of DNA is similar to protein has higher level of organisation of nucleic acid as chromatin. 
PAIRING

  • Pairing always occurs between adenine and thymidine  and between guanine and cytosine.
  • This base is complementary is due to steric factor and hydrogen bonding factor




1.STERIC FACTOR

  • The glycosidic bonds that are attached to base pair are always 10-80 𝐴 ̂ apart . 
  • A purine and pyrimidines base pairs fit perfectly in space.
  • In contrary there is insufficient space for two purines where as there is more than enough space for two pyrimidines so that they are far apart to form hydrogen bonds.
  • Cytosine because these would be two hydrogens near one of bonding position and one as O .
  • Similarly guanine cannot pair with thymidine.
References
1.Principles of Biochemistry-Lehinger 
2.Biochemistry-U. Sathyanarayana & U.Chakrapani






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