Monday, July 27, 2020

Molecular Basis of Inheritance - Notes | Class 12 | Part 1: The DNA

6. MOLECULAR BASIS OF INHERITANCE


·     Nucleic acids (DNA & RNA) are the building blocks of genetic material.

·     DNA is the genetic material in most of the organisms.

·     RNA is the genetic material in some viruses. RNA mostly functions as messengers.

THE DNA


STRUCTURE OF POLYNUCLEOTIDE CHAIN

Polynucleotides are the polymer of nucleotides. DNA & RNA are polynucleotides. A nucleotide has 3 components:

1.    A nitrogenous base.

2.    A pentose sugar (ribose in RNA & deoxyribose in DNA).

3.    A phosphate group.

Nitrogen bases are 2 types:

} Purines: It includes Adenine (A) and Guanine (G).

} Pyrimidines: It includes Cytosine (C), Thymine (T) & Uracil (U). Thymine (5-methyl Uracil) present only in DNA and Uracil only in RNA.

A nitrogenous base is linked to the OH of 1' C pentose sugar through an N-glycosidic linkage to form nucleoside.

Nucleosides in RNA

Nucleosides in DNA

Adenosine

Deoxyadenosine

Guanosine

Deoxyguanosine

Cytidine

Deoxycytidine

Uridine

Deoxythymidine

A phosphate group is linked to OH of 5' C of a nucleoside through phosphoester linkage to form nucleotide (or deoxynucleotide).

In RNA, each nucleotide has an additional –OH group at 2' C of the ribose (2’- OH).

2 nucleotides are linked through 3’-5’ phosphodiester bond to form dinucleotide.

When more nucleotides are linked, it forms polynucleotide.


STRUCTURE OF THE DNA


} Friedrich Meischer (1869): Identified DNA and named it as ‘Nuclein’.

} James Watson & Francis Crick (1953) proposed double helix model of DNA. It was based on X-ray diffraction data produced by Maurice Wilkins & Rosalind Franklin.


} DNA is made of 2 polynucleotide chains coiled in a right-handed fashion. Its backbone is formed of sugar & phosphates. The bases project inside.

} The 2 chains have anti-parallel polarity, i.e. one chain has the polarity 5’→3’ and the other has 3’→5’.

} The bases in 2 strands are paired through H-bonds forming base pairs (bp).

 A=T (2 hydrogen bonds)      C≡G (3 hydrogen bonds)

} Purine comes opposite to a pyrimidine. This generates uniform distance between the 2 strands.

} Erwin Chargaff’s rule: In DNA, the proportion of A is equal to T and the proportion of G is equal to C.

   [A] + [G] = [T] + [C]     or     [A] + [G] / [T] + [C] =1


v Ф 174 (a bacteriophage) has 5386 nucleotides.

v Bacteriophage lambda has 48502 base pairs (bp).

v E. coli has 4.6x106 bp.

v Haploid content of human DNA is 3.3x109 bp.


Length of DNA = number of base pairs X distance between two adjacent base pairs.

Number of base pairs in human    = 6.6 x 109

Hence, the length of DNA            = 6.6 x109 x 0.34x 10-9

                                                    = 2.2 m

In E. coli, length of DNA          =1.36 mm (1.36 x 10-3 m)

Therefore the number of base pairs 


= 4 x 106 bp

PACKAGING OF DNA HELIX

§ In prokaryotes (E.g. E. coli), the DNA is not scattered throughout the cell. DNA is negatively charged. So it is held with some positively charged proteins to form nucleoid.

§ In eukaryotes, there is a set of positively charged, basic proteins called histones.

§ Histones are rich in positively charged basic amino acid residues lysines and arginines.

§ 8 histones form histone octamer.

§ Negatively charged DNA is wrapped around histone octamer to give nucleosome.

§ A typical nucleosome contains 200 bp.

Therefore, total number of nucleosomes in human =

§ Nucleosomes constitute the repeating unit to form chromatin. Chromatin is the thread-like stained bodies.

§ Nucleosomes in chromatin = ‘beads-on-string’.

§ Chromatin is packaged → chromatin fibres → coiled and condensed at metaphase stage → chromosomes.

§ Higher level packaging of chromatin requires non-histone chromosomal (NHC) proteins.

§ Chromatin has 2 forms:

·   Euchromatin: Loosely packed and transcriptionally active region of chromatin. It stains light.

·   Heterochromatin: Densely packed and inactive region of chromatin. It stains dark.


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