DNA | Definition, Discovery, Functions, Bases, Facts & Structure - Chemosmart

           DNA As a Genetic Material
 
                The living organisms are characterized by their remarkable ability to reproduce and transmit characteristics, generations after generations and maintain continuity of inherited traits. Many experiments were carried out to identify the genetic material. After using bacteria and viruses in the experiment then discovered that, DNA is a genetic material. So, in this article let's know about- What is DNA? DNA structure, Functions of DNA etc.

 
                The living organisms are characterized by their remarkable ability to reproduce and transmit characteristics, generations after generations and maintain continuity of inherited traits. Many experiments were carried out to identify the genetic material. After using bacteria and viruses in the experiment then discovered that, DNA is a genetic material. So, in this article let's know about- What is DNA? DNA structure, Functions of DNA etc.

 

Hershey-Chase Experiment:

             Two Scientists Hershey and Chase worked together with viruses which are infect bacteria (which is called as Bacteriophages). Bacteriophages are contains DNA and Protein. When viruses infect bacteria, their genetic material is injectedin the bacterial cell. 

             Hershey and Chase used radioactive phosphorus (32P) in the medium for some viruses and radioactive sulphur (35S) for some others. DNA contains phosphorus and not sulphur while proteins contains sulphur and not phosphorus. Therefore some viruses were with radioactive DNA and some with radioactive protein coat. Both the types of radioactive viruses were allowed to infect E. Coli bacteria. As the infection peoceeded, the viral coats were removed with the help of centrifuge. 

Experiment by Hershey & Chase

             In this experiment, the bacteria which were infected by viruses with radioactive DNA were radioactive while the bacteria which were infected by viruses with radioactive protein were not radioactive. This shows that DNA enters the bacterial cells and not proteins. 

              From above experiment, Hershey and Chase prove that DNA is a Genetic material. 
    

Definition of gene:

             Genes are units of inheritance. A gene is a segment of DNA that provides instructions for synthesis of a specific protein or a particular type of RNA. i.e. gene is responsible for inheritance and expression of a particular character. 

What is DNA ( DNA full form):

            Long of DNA is  Deoxyribo Nucleic Acid. DNA is a genetic material of living organisms. It should be able to express accurately and have capacity to replicate with minimum errors. i.e. in simple word, DNA is the hereditary material in living organisms except some viruses.

 DNA structure:

       In 1953 James Waston and Francis Crick proposed DNA structure. The following points are shown in DNA molecule. 

Structure of DNA

1. DNA molecule as a double helix:

            DNA molecule consists of two long strands coiled around a common, imaginary, central axis to form a double helix. 

2. Structure of each strand in DNA:

            In DNA, each strand consists of number of nucleotides. Each nucleotide is made up of deoxyribose sugar, phosphate group and a nitrogen base. 

Nucleotides:  They contains structural units of nucleic acid. Each nucleotide has three components. 

Structure of Nucleotide

1) Sugar- It is a pentose sugar called as deoxyribose or ribose sugar. It contains five carbon and has pentagonal ring structure. 

2) Phosphate group- It is Phosphoric acid i.e. H3PO4. It contains three hydroxyl groups in which two hydroxyl groups are involved in strand formation. 

3) Nitrogenous bases- These are cyclic compounds contains Carbon, Hydrogen, Oxygen and Nitrogen atoms. There are five nitrogenous bases: a) Adenine (A) , b) Thymine (T), c) Cytosine (C), d) Guanine (G), and Uracil (U). These are classified as :

Purine : Adenine and Guanine which are double ring compounds. 

Pyrimidine:  Thymine, Cytosine and Uracil which are single ring compounds.

Purines & Pyrimidines

      A base-sugar unit is called nucleoside and a base-sugar phosphoric acid unit is called nucleotide.

Nucleoside & Nucleotide

            Nucleotides are joined together through phosphate ester linkage.  The proportion of heterocyclic bases and the sequence in which they follow each other along the polynucleotide chain differ from one kind of nucleic acid to another. This is called it's primary structure. In 1980 Sanger worked on nucleic acid and found loops of DNA molecule to form a gaint ring containig 5386 nucleotides.
     
Uracil is absent in DNA and present in RNA. 
   Now, 
Pentose sugar + Nitrogen base =                                                                 Nucleoside
                                  
Pentose sugar + Nitrogen + phosphate
                         = Nucleotide    
i.e. Nucleoside + phosphate =                                                                        Nucleotide 

Nucleoside:  It s a combination of a pentose sugar and a purine or pyrimidine base. 

Nucleotide: It is a combination of nucleoside ( pentose sugar+ Nitrogen base) and phosphate. 

                   The successive nucleotides of the same strand are linked by 3'-5' phosphodiester linkages. i.e. phosphate attached to 5th carbon of sugar of nucleotide is joined to 3rd carbon of another. 

3. Complementary base pairing:

            In DNA, the nitrogen bases present on one strand, pair with the nitrogen bases of opposite strand. Purine base Adenine is always pair with the pyrimidine base Thymine by two hydrogen bonds and the purine base Guanine is always pair with the pyrimidine base Cytosine with three hydrogen bonds. 

               As the base sequence present on one strand of DNA shows that base sequence of the other strand, the strand is called as Complementary strands. 

4. Purine : Pyrimidine ratio:

                  Due to complementary strands base pairing total number of purine bases is always equal to the total number of pyrimidines bases (1:1 ratio). This is called as Chargaff's rule. 
i.e. A+G = T+C= 1

5. Polarity of strands:

               There are two strands, one end is considered as 5' end and other end is 3' end. Due to this, each strand in DNA has a specific polarity. At 5' end there is a free phosphate group while at the 3' end there is a free -OH group.

            One of the strands runs in 5'- 3' direction while the other in 3'-5' direction. This arrangement of two strands of DNA is considered antiparallel due to which 5' end of one strand lies close to the 3' end of the other strand and vice versa. 

6. Dimensions of DNA molecule:

            The diameter of DNA molecule is 2 nm. The spiral ladder like arrangement of DNA molecule is due to deflection angle of 36° between two successive steps. 

              DNA molecule makes one complete 360° turn after covering a distance of 3.4 nm. There are 10 base pairs in one complete spiral. 

DNA molecule

Packaging of DNA:

       In eukaryotes, the packaging or organization of DNA is rather complex. 

                          (A) Nucleosome

             Histones are required for the packaging of DNA. Histones are proteins that are rich in the basic amino acid residues lysines and arginines which carry positive charge in their side chains. Eight molecules of Histones (two each of H2A, H2B, H3 and H4) get organized to form histone octamer. DNA is negativity charged and is wrapped around the positively charged hisotne octamer to form nucleosome. 

                        (B) Beads on string

              Under the electron microscope, nucleus shows chromatin network. The nucleosomes in chromatin are seen as 'beeds-on-string'. Around the octamer DNA molecule is wrapped as 1 and 3/4th turn. This DNA is called core DNA and it consists of about 146 base pairs. 

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