Understanding Structure of DNA

 

Deoxyribonucleic Acid

DNA (Deoxyribonucleic Acid) is hereditary material. It packed very tightly and perfectly to form a chromosome. DNA is the most important molecule in organisms

 

 Constituents of DNA

                                        As the DNA is a chemical, DNA is quite simple. It contains three types of chemical components

 1) A deoxyribose Sugar,

2) A nitrogenous base,

3) A phosphate group.

1. Deoxyribose Sugar

The sugar in the DNA is called deoxyribose because it has only H atom at 2’ Carbon, unlike ribose which has (OH) at 2’ Carbon atom.

Ribose is a single-ring pentose [5-Carbon] sugar. The numbering of the carbon atoms runs clockwise, following organic chemistry rules. Note the absence of the hydroxyl (-OH) group on the 2’ carbon in the deoxy-ribose sugar in DNA as compared with the ribose sugar in RNA

 2. Nitrogenous Base

There are four nitrogenous bases present in the DNA, i.e. two purines (Adenine, Guanine), and two Pyrimidine (Cytosine, Thymine). For the convenience the carbon atoms in the Nitrogenous base are numbered as 1, 2, 3, and so on.

And the carbon atoms in the sugar are numbered by the same way but assigned with a prime (2’, 3’, and so on).

a. Purines

                             Purines are double ring structures; there are two purines present in DNA. Adenine and Guanine.

b. Pyrimidines

There is only one ring present in these. These are Cytosine and Thymine in DNA. There is Uracil is also but It is only present in RNA in place Thymine. So Thymine instead of Uracil is present is DNA.

                         

3. Phosphate group

  It’s derived from phosphoric acid (H₃PO₄).

Phosphorus attached to the four Oxides (PO₄)^-3. And - 3 charges is present at this group.

Nucleoside and Nucleotide

When the deoxy ribose sugar and any of the four nitrogenous base joins together through a N-Glycosidic bond it is called as Nucleoside.

Sugar + Nitrogenous base = Nucleoside

When a phosphate group joins the nucleoside through phosphodiester bond, this structure is known as Nucleotide.

Sugar + Nitrogenous base +Phosphate group = Nucleotide

The Nucleotides joins together to form large molecule of DNA. It is convenient to write the first capital letter of the each nucleotide as A, G, C, T.

The nucleotide with adenine base names as “deoxyadenosine 5’-monophosphate”. 5’ is actually the position on sugar where the phosphate group attached.

 

Chargaff’s rules

Chargaff most importantly determining the structure of the DNA. He formulated about the base composition of the DNA.

1. The whole quantity of Pyrimidines is often adequate to the whole quantity of Purines.

2. The quantity of “A” is often adequate to the quantity of “T”.  And quantity of “G” is adequate to the quantity of “C”. However the amount of (A+T) not basically equals to the quantity of (G+C).

3. This quantitative relation varies within the totally different organisms however same within the same organisms

X-Ray Diffraction information

Rosalind Franklin collected this information. X-rays were dismissed on the Deoxy ribonucleic acid, the scattered light-weight is determined through film. It showed spots made by X-rays. The angle of those sports gave info regarding the position of the atoms within the DNA. It absolutely was demonstrated; the DNA is long and has 2 similar components parallel to every alternative. It additionally instructed that the DNA has coiling structure.

The helix model of Structure

          Watson and Crick in 1953 revealed the planned structure of DNA in the journal Nature. The 3 dimensional structure planned by them has 2 strands that runs parallel to every alternative. They’re composed of nucleotides, and twisted along to make a helix structure.

The two ester strands nucleotide stick along by weak interactions of H bonding. The back bone is formed of sugar and phosphates as an alternative, whereas the N- bases protrude outward from the back bone. The every sugar-phosphate back bone is 3’-5’. Each the strands lie opposite one another other i.e. opposing parallel. Every base is connected to the 1’ carbon of the deoxy sugar, and faces inward N-base on the opposite strand. H bonds between the bases of 2 strands hold the 2 strands of polymer. Stacking of N-base pairs happens. This stacking adds the soundness to the DNA molecule. By double coiled structure 2 grooves square measure fashioned one is Major groove the opposite is Minor Groove.

Purine base continuously pair up with pyrimdine base. A pair up with T and C combines with G.

There is three H- bonds gift between G-C bases. And a couple of H- bonds gift between A-T bases.

Hence the deoxyribonucleic acid with a lot of G-C pairs are stable than the one with a lot of A-T pairs.

Forms of DNA

 

B-Form DNA

 

It is the foremost common style of the DNA. It’s the conventional minor and major grooves.

It has dimensions

             •0.34 nm between bp, 3.4nm per flip, regarding ten bp per flip

              •1.9 nm in diameter

             •It is traditional right handed structure

Z-form DNA

 

            •It is left handed double coiling structure

             •The major and minor grooves very little distinction

        A-Form DNA

             •It could be a right handed  structures

•Has shorter and additional compact coiling structure