1-Introduction
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George Mendel settled since one century that the biological characteristics are inherited and he formulated a set of rules to explain that inheritance.
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It was proposed that genes that are resided on chromosomes made from proteins and they are responsible for genetic materials.
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Later on, techniques for gene mapping were developed and had produced a comprehensive analysis of the relative positions of over
2000 genes on the four chromosomes of the fruit fly, Drosophila melanogaster .
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The discovery that genetic information is coded along the length of a polymeric molecules composed of only four types of monomeric units was one of the major scientific achievements of this century.
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This polymeric molecules, DNA , is the chemical basis of heredity and is organized into genes, the fundamental units of genetic information.
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Genes control the synthesis of various types of RNA , most of which are involved in protein synthesis .
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Genes replication and function are controlled by feedback loops as fig. 1

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DNA is found in close association with proteins forming nucleoproteins .
Avery and his colleagues 1944
………
DNA is the genetic information's.
Genetic information of rough capsule could be transferred from pneumococcus to smooth one by transfer of the DNA.
Hershey and Chase used radioactive bacteriophage by labeling the outer protein with 35 S and the inner DNA with 32 P .
When the bacteriophage infect the bacterial cells only the DNA will inter the cell and replicate inside the cell and the new phages arise contain labeled DNA with 32 P from the parental DNA.
Nucleoproteins is the most important proteins of the cells.
Nucleoproteins under the effect of Pepsin
…….
Protein + insoluble nuclein.
Insoluble nuclein under the effect of Trypsin
…….
Nucleic acid + simple protein
The simple protein principally Histones and protamies.

The basic amino acid are rich in arginine and histidine .
Nucleoproteins is the large part of the nucleus and present in ribosomes.
The chromatin material is mainly Nucleoproteins
Nucleic acid consists of long chain of nucleotides attached together by phosphate diester linkages .
Each Nucleotide consists of three molecules; phosphoric group
(P), pentose sugar (C ) and one Nitrogenous base .
The phosphate diester bond is between the phosphate group of one nucleotide and the pentose sugar of the previous nucleotide.
There are two types of nucleic acids:
1- R ibo n ucleic a cid………
RNA
2- D eoxyribo n ucleic a cid……..
DNA
They differ from each other in the types of pentose sugar and nitrogenous bases.
Pentose sugar ( D-ribose in RNA )…( D-2-Deoxyribose in
DNA).

They are Purine or Pyrimidine bases:
The pyrimidine ring is heterocyclic ring that contains four carbon atoms and two nitrogen atoms .
There are three types: a- cytosine ( 2- oxy-4-aminopyrimidine ).
B- thymine( 2,4-dioxy-5-methyl pyrimidine ). C- uracil ( 2,4dioxypyrimidine ).
2Purine bases:
The purine ring is heterocyclic ring composed of pyrimidine ring fused to imidazole ring .
There are two major purines, adenine ( 6-aminopurine ) and guanine ( 2-amino-6-oxypurine ).
Nucleosides are composed of a purine or a pyrimidine base to which a sugar ( D-ribose D-2-Deoxyribose ) is attached in Blinkage at N9 or N1 , respectively. Figure 3 كلضف نم لودجلا يرظنا

They are Nucleosides +Phosphate groups at one or more hydroxyl groups of the pentose sugar.
The phosphate group is attached to C3 of Ribose in RNA ,
While its attached to C5 in DNA .
DNA is a polymer of thymidylic acid, Deoxy cytidylic acid, deoxy adenylic acid and deoxy guanylic acid . While RNA is a polymer of Uridylate , Cytidilate, adenylate and guanylate .