Structure of DNA :
Biochemical composition of DNA
- Discover – Miescher- Pus-cell -Name Nuclein
- Altman -Nucleic Acid
- Each nucleotide unit consists of three types of compounds which are as follows :
(1) A pentose sugar -the 5 carbon deoxyribose sugar
(2) 2. Phosphoric Acid (H₃PO₄)
(3) Nitrogenous base-a heterocyclic ring containing nitrogen
1. Pentose sugar molecule
- Pentose sugar was identified by Lewine (1909).
- It is 5 Carbon Sugar
- These are Two type –
- Deoxyribose Sugar – In DNA always deoxyribose sugar is found which is lacking in one oxygen atom at 2’ carbon
- Ribose sugar – It is Found in RNA but in this oxygen is present at 2’ carbon
- The deoxyribose sugar has a pentagonal ring with 5 carbon atoms (1″, 2′, 3′, 4′ and 5 carbons).
- Among which 1 carbon is attached to the nitrogenous base and 3′ and 5′ carbons are attached to phosphoric acid.
- At carbon 1′ always the OH group is present which is the point of attachment of the base.
- This is linked to the 1-nitrogen (1-Nitrogenation) atom of the pyrimidine bases and to the 9-nitrogen atom (9-Nitrogenation) of purine bases.
(2) 2. Phosphoric Acid (H₃PO₄)
- It found in the form of H₃PO in DNA
- It joins sugar at 3’ and 5’ carbon by the condensation reaction.
- DNA has an acidic nature due to the presence of phosphoric acid because it have the negatively charged oxygen molecule.
3. Nitrogenous bases
- It has nitrogen in their ring so these are basic nature.
- These are of two types :
(a) Purines and (b) Pyrimidines.
(a) Purines :
- Purines are double ringed nitrogen compounds,
- in which two benzene rings (5 and 6 membered) are fused.
- One ring is called Pyrimidine Ring (2N +4C ) and another is called Imidazole Ring (2N +3C )
- In DNA two types of purines : Adenine (A) and Guanine (G) are found.
(b) Pyrimidines :
- Pyrimidines are 6 membered single benzene ring compounds.
- They are Thymine (T) and Cytosine (C). But in RNA instead of thymine, Uracil (U) is present.
- A molecule of deoxyribose sugar attached with a molecule of nitrogenous base without phosphate group is known as nucleoside.
- In a molecule of DNA four types of nucleosides are found, which are :
(a) Adenosine : Adenine + Deoxyribose
(b) Guanosine : Guanine + Deoxyribose
(c) Cytidine : Cytosine + Deoxyribose
(d) Thymidine : Thymine + Deoxyribose
- A nucleotide consists of one molecule of deoxyribose sugar, one molecule of phosphoric acid and one of the four nitrogenous bases.
- In DNA molecule following
four types of nucleotides are present :
(a) De-oxyadenylic acid : Adenine+Deoxyribose +Phosphoric acid (DAMP)
(b) De-oxyguanylic acid : Guanine+Deoxyribose +Phosphoric acid (dGMP)
(c) De-oxycytidylic acid : Cytɔsine+Deoxyribose +Phosphoric acid (DCMP)
(d) De-oxythymidylic acid : Thymine+Deoxyribose +Phosphoric acid (dTMP)
- According to this rule the amount of Purine is equals to that of Pyrimidine in DNA
- According to this law A=T and GC are always equal to one except for o x 174 Coliphage here A:T and G:C ratio is not one.
- Chargaff’s rules are also called as “Base rule” or ‘base ratios’.