Molecular Basis of Inheritance

Notes on Molecular Basis of Inheritance

DNA
(1) DNA is a long polymer of deoxyribonucleotides.
(2) The length of the DNA depends on the number of nucleotide pairs present in it.
(3) Bacteriophage lambda has 48,502 base pairs.

Central dogma of molecular biology
(1) Crick proposed the Central dogma in molecular biology
(2) It states that the genetic information flows from DNA à RNA à Protein.
(3) In some viruses like retroviruses, the flow of information is in reverse direction, which is from RNA à DNA à mRNA à Protein.

Structure of polynucleotide chain:

(1) A nucleotide has three components-
(a) A nitrogen base
(b) A pentose sugar (ribose in RNA and deoxyribose in DNA)
(c) A phosphoric acid.

(2) There are two types of nitrogen bases:
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine, Uracil and Thymine)

(3) Adenine, Guanine and Cytosine are common in RNA and DNA.
(4) Uracil is present in RNA and in DNA in place of Uracil, Thymine is present.
(5) In RNA, Pentose sugar is ribose and in DNA, it is Deoxyribose.

(6) Based on the nature of pentose sugar, two types of nucleosides are formed – ribonucleoside and deoxyribonucleotides.
(7) Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.
(8) More than two nucleotides join to form polynucleotide chain.
(9) The two strands of DNA (called DNA duplex) are antiparallel and complementary, i.i., one in 5’->3’ direction and the other in 3”->5” direction.

History of DNA

(1) DNA is an acidic substance in the nucleus.
(2) It was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein”
(3) In 1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray diffraction data produced Maurice Wilkins and Rosalind Franklin.

Packaging of DNA Helix

(1) The basic unit into which DNA is packed in the chromatin of eukaryotes.
(2) Nucleosome is the basic repeating structural (and functional) unit of chromatin, which contains nine histone proteins.
(3) Distance between two conjugative base pairs is 0.34nm
(4) 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.
(5) The length of DNA is more than the dimension of a typical nucleus (10-6m)

DNA Replication

(1) DNA is the only molecule capable of self duplication so it is termed as a living molecule.
(2) All living beings have the capacity to reproduce because of DNA.
(3) DNA replication takes place in S-phase of the cell cycle. At the time of cell division, it divides in equal parts in the daughter cells.
(4) Delbruck suggested three methods of DNA replication i.e.
(i) Dispersive
(ii) Conservative
(iii) Semi-conservative
(5) The process of DNA replication takes a few minutes in prokaryotes and a few hours in eukaryotes.

RNA
(1) RNA is the first genetic material.
(2) RNA is a non hereditary nucleic acid except in some viruses (retroviruses).
(3) RNA used to act as a genetic material as well as catalyst.
(4) It is a polymer of ribonucleotide and is made up of pentose ribose sugar, phosphoric acid and nitrogenous base (A,U,G,C).
(5) RNA may be of two types – genetic and non-genetic.

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