Molecular Basis of inheritance /Structure of DNA

                             Molecular basis of inheritance

The DNA:-

DNA is a long polymer structure of deoxyribonucleotide.The length of the DNA is defined by Number of nucleotide.    Bacteriophage lambda has 48502 base pairs,Escherichia coli.has 4.6*106 bp,and haploid content of human DNA is 3.3*109 bp.

Structure of Polynucleotide Chain:-

A Nucleotide has three Component – Nirogenous Base ,Phosphate group and pentose sugar (Ribose in case of RNA And deoxyribose in case of DNA.)

There is two type of nitrogenous bases-Purine (Adenine and Guanine )and pyrimidine(Thymine and Cytosin) ,thymine is replace by Uracil in case of RNA.

Nitogen bases is Attached to Pentose sugar by N-glycosidic linkage and form Nucleoside E.g. Guanosine ,Deoxyguanosine,Deoxythymidine,Cytidine and Deoxycytidine etc.

Mainyl one end of the pentose sugar is attached to 5’ end by phosphate group and 3’ end is attached by hydroxyl group ,

Discoveries:-

DNA IS A ACIDIC SUBSTANCE PRESENT IN NUCLEUS IS DISCOVERED BY FRIEDRICH IN 1869.

He named it as Nuclein.

Watson and crick in 1953 discover DNA double helix model by  x-ray differaction method produce by Maurice Wilkines and Rosalind Franklin.

Chargaff’s Rule :-

Erwin Chargaff’s Proposed that the ration of adenine and thymine and guanine and cytosine is equal to one.

Note that Adenine combine with thymine by 2-H-bonding

And cytosine and guanine is combined by 3-h-bonding.

DNA Double Helix

Characterstic of DNA :-

(1)       It is made up of two polynucleotide chain ,where the backbone is the sugar-phosphate group and the bases project inside.

(2)Two strands of DNA is antiparllel to each other means one have 5’-----3’ stand then other have 3’-----5’ strand.

(3)     The length of the  each turn is approx.     3.4 nm because the number of base pair in each turn may be different.

(4)The stability of the helix structure is due to presence of H-bonding.

Central dogma:-

Frincs crick proposed that the genetic information flows from:-

 DNA-----RNA---PROTEIN.

The length DNA helix in a  typical mammalian cell is approx.. 2.2 metres.

In prokaryotes ,e.g. E.coli. they donot have a defined nucleus.

In eukaryotes, this organisation is much more complex. There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in the basic amino acid residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called as histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome  . A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread-like stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope (EM).

Transformation PRINCIPLE:-

Transformation principle is discovered by Frederick Griffth in 1928. His experiment is based on the poisonous nature of streptococcus pneumonia (pathogen for pneumonia)

S-strain -------- Inject into mice -------Mice die

R-strain --------Inject into Mice-------mice live

Griffth was able to kill the bacteria by heat.

S-strain--------Inject into mice ------Mice live

(heat-killed)

S-strain

(heat-killed)

      +           --------inject into mice ------ mice die R-strain

(live)

He conclude that R-strain bacteria is transformed by heat-killed S-strain bacteria.

The Genetic Material is DNA :-

The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages. The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell. The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures more virus particles. Hershey and Chase worked to discover whether it was protein or DNA from the viruses that entered the bacteria. They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that contained radioactive sulfur. Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not. Similarly, viruses grown on radioactive sulfur contained radioactive protein but not radioactive DNA because DNA does not contain sulphur. Radioactive phages were allowed to attach to E. coli bacteria. Then, as the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge. Bacteria which was infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria. Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicates that proteins did not enter the bacteria from the viruses. DNA is therefore the genetic material that is passed from virus to bacteria.

RNA WORLD :-

From foregoing discussion, an immediate question becomes evident – which is the first genetic material? It shall be discussed in detail in the chapter on chemical evolution, but briefly, we shall highlight some of the facts and points. RNA was the first genetic material. There is now enough evidence to suggest that essential life processes (such as metabolism, translation, splicing, etc.), evolved around RNA. RNA used to act as a genetic material as well as a catalyst (there are some important biochemical reactions in living systems that are catalysed by RNA catalysts and not by protein enzymes). But, RNA being a catalyst was reactive and hence unstable. Therefore, DNA has evolved from RNA with chemical modifications that make it more stable. DNA being double stranded and having complementary strand further resists changes by evolving a process of repair.

Transfer of characters from one parent to offspring by the help of Molecules is called  Molecular basis of inheritance