DNA is a macromolecule formed by the polymerisation of deoxyribonucleotides.
The length of the DNA depends on, number of nucleotide pairs present in it.Foeexample Bacteriophage ΓΈ174 has 5386 nucleotides,Bacteriophage lambda has 48502 base pairs,Escherichia coli have 4.6 X 106 base pairs,Human genome (haploid) is 3.3 X 109 bp.
Polynucleotide chain:
A nucleotide has three component:-
A nitrogen base,a pentose sugar ( ribose in RNA and deoxyribose in DNA) and a phosphoric acid.
There are two types of nitrogen bases: namely Purines ( Adenine and Guanine) and Pyrimidines ( Cytosine, Uracil and Thymine)Adenine, Guanine and Cytosine is present in both RNA and DNA.Uracil is present in RNA and Thymine is present in DNA in place of Uracil.
Pentose sugar is ribose in RNA and Deoxyribose in DNA.
A nitrogen base attached to the pentose sugar at C1 of pentose sugar by N-glycosidic linkage to form a nucleoside.
According to the nature of pentose sugar, two types of nucleosides are formed ribonucleoside and deoxyribonucleotides.
Ribonucleosides are: Adenosine,Guanosine,Cytidine,Uridine
Deoxyribonucleosides are: ,Deoxyadenosine,Deoxyguanosine,Deoxycytidine,Deoxythymidine.
Phosphoric acid attached to the 5’ OH of a nucleoside by Phosphodiester linkage a corresponding nucleotide is formed. (Ribonucleotide or deoxyribonucleotides depending on the sugar unit).
Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.Many nucleotides joined to form polynucleotide chain. Polynucleotide chain has a free phosphate group at 5’ end of sugar, is referred to as 5’ end and in the other end of the polymer with 3’-OH group called 3’ end.
History of DNA:
DNA is an acidic substance present in the nucleus was first identified by Friedrich Meischer in 1869. He named this as ‘Nuclein”
1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray diffraction study conducted Maurice Wilkins and Rosalind Franklin.
Key feature of their proposition was base pairing between two strands of polynucleotide chains. This was based on observation of Erwin Chargaff.
Chargaff’s observation was that for a double stranded DNA, the ratio between Adenine and Thymine, and Guanine and Cytosine are constant and equal one.
Salient features of Double helix structure of DNA:
Each DNA is made of two polynucleotide chains.
Sugar and phosphate forms the backbone and bases projected to inside.
Two chains have anti-parallel in polarity.
Two strands are held together by hydrogen bond present in between nitrogen bases.
Adenine of one strand joins with Thymine of another strand by two hydrogen bonds and vice versa.
Guanine of one strand joins with Cytosine of another strand by three hydrogen bonds and vice versa.
A purine basecomes opposite to a pyrimidine base. This results approximately uniform distance between the two strands of the helix.
The two polynucleotide chains are coiled in a right – handed fashion and the pitch of the helix is 3.4 nm or 34 A0.There are roughly about 10 bp in turn. The distance between the base pair in a helix is 0.34nm or 3.4 A0.
Polynucleotide chain:
A nucleotide has three component:-
A nitrogen base,a pentose sugar ( ribose in RNA and deoxyribose in DNA) and a phosphoric acid.
There are two types of nitrogen bases: namely Purines ( Adenine and Guanine) and Pyrimidines ( Cytosine, Uracil and Thymine)Adenine, Guanine and Cytosine is present in both RNA and DNA.Uracil is present in RNA and Thymine is present in DNA in place of Uracil.
Pentose sugar is ribose in RNA and Deoxyribose in DNA.
A nitrogen base attached to the pentose sugar at C1 of pentose sugar by N-glycosidic linkage to form a nucleoside.
According to the nature of pentose sugar, two types of nucleosides are formed ribonucleoside and deoxyribonucleotides.
Ribonucleosides are: Adenosine,Guanosine,Cytidine,Uridine
Deoxyribonucleosides are: ,Deoxyadenosine,Deoxyguanosine,Deoxycytidine,Deoxythymidine.
Phosphoric acid attached to the 5’ OH of a nucleoside by Phosphodiester linkage a corresponding nucleotide is formed. (Ribonucleotide or deoxyribonucleotides depending on the sugar unit).
Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.Many nucleotides joined to form polynucleotide chain. Polynucleotide chain has a free phosphate group at 5’ end of sugar, is referred to as 5’ end and in the other end of the polymer with 3’-OH group called 3’ end.
History of DNA:
DNA is an acidic substance present in the nucleus was first identified by Friedrich Meischer in 1869. He named this as ‘Nuclein”
1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray diffraction study conducted Maurice Wilkins and Rosalind Franklin.
Key feature of their proposition was base pairing between two strands of polynucleotide chains. This was based on observation of Erwin Chargaff.
Chargaff’s observation was that for a double stranded DNA, the ratio between Adenine and Thymine, and Guanine and Cytosine are constant and equal one.
Salient features of Double helix structure of DNA:
Each DNA is made of two polynucleotide chains.
Sugar and phosphate forms the backbone and bases projected to inside.
Two chains have anti-parallel in polarity.
Two strands are held together by hydrogen bond present in between nitrogen bases.
Adenine of one strand joins with Thymine of another strand by two hydrogen bonds and vice versa.
Guanine of one strand joins with Cytosine of another strand by three hydrogen bonds and vice versa.
A purine basecomes opposite to a pyrimidine base. This results approximately uniform distance between the two strands of the helix.
The two polynucleotide chains are coiled in a right – handed fashion and the pitch of the helix is 3.4 nm or 34 A0.There are roughly about 10 bp in turn. The distance between the base pair in a helix is 0.34nm or 3.4 A0.
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