MOLECULAR BASIS OF INHERITANCE

 

THE DNA:

  • DNA is a long polymer of deoxyribonucleotides.
  • 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]