The length of the DNA depends on, number of nucleotide pair present in it.
Characteristics of the organism depend on the length of the DNA.
Bacteriophage ø174 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.
Structure of polynucleotide chain:
A nucleotide has three component:-
A nitrogen base
A pentose sugar ( ribose in RNA and deoxyribose in DNA)
A phosphoric acid.
There are two types of nitrogen bases:
Purines ( Adenine and Guanine)
Pyrimidines ( Cytosine, Uracil and Thymine)
Adenine, Guanine and Cytosine is common in 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.
More than two nucleotides joined to form polynucleotide chain.
Polynucleotide chain has a free phosphate moiety at 5’ end of sugar, is referred to as 5’ end
In the other end of the polymer with 3’-OH group called 3’ end.
The backbone of the polynucleotide chain is sugar and phosphate.
Nitrogen bases linked to the sugar moiety project from the backbone.
In RNA every nucleotide has an additional –OH group at 2’ of ribose.
In RNA Uracil is found in place of thymine.
5-methyl uracil is the other name of thymine.
History of DNA:
DNA is an acidic substance in the nucleus was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein”
1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray defraction data produced Maurice Wilkins and Rosalind Franklin.
Hallmark 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:
Made of two polynucleotide chains.
Sugar and phosphate forms the backbone and bases projected to inside.
Two chains have anti-parallel polarity.
Two strands are held together by hydrogen bond present in between bases.
Adenine of one strand pairs with Thymine of another strand by two hydrogen bonds and vice versa.
Guanine of one strand pairs with Cytosine of another strand by three hydrogen bonds and vice versa.
A purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.
The two chains are coiled in a right – handed fashion.
The pitch of the helix is 3.4 nm or 34 A0
There are roughly 10 bp in turn.
The distance between the bp in a helix is 0.34nm or 3.4 A0.
The plane of one base pair stacks over the other in double helix.
H-bond confers stability of the helical structure of the DNA.
Central dogma of flow of genetic information: DNA→ RNA→ Protein.
Packaging of DNA Helix:
Distance between two conjugative base pairs is 0.34nm, the length of the DNA in a typical mammalian cell will be 6.6 X109 bp X 0.34 X10-9 /bp, it comes about 2.2 meters.
The length of DNA is more than the dimension of a typical nucleus (10-6m), how is such a long polymer packaged in a cell?
Packaging in prokaryotes:
They do not have definite nucleus.
The DNA is not scattered throughout the cell.
DNA is held together with some proteins in a region is called ‘nucleoid’.
The DNA in nucleoid is organized in large loops held be proteins.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]
