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  Structure of DNA

Structure of DNA:

In the nucleus DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are known. DNA and RNA are complex macro molecules and built up of millions of smaller units that are known as nucleotides. Therefore, DNA (Deoxyribonucleic acid) is a macromolecular substance along with double stranded polynucleotide.

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Diagram: Watson and Crick model of DNA


Every nucleotide is formed of pentose sugar, a phosphate group and a nitrogenous base. Ribose in RNA is the constituent sugar and in DNA (Deoxyribonucleic acid) is the Deoxyribose.
The nitrogenous bases are of 2 types:

1. purines and
2. pyrimidines.

Guanine and Adenine are the purines and cytosine and thymine are pyrimidines. The nitrogenous bases available in DNA (Deoxyribonucleic acid) are guanine, adenine, cytosine and thymine, while in RNA thymine is changed by uracil.

The sub-unit comprising just only sugar and nitrogenous base is termed as nucleoside. A nucleoside combines along with phosphate to create a nucleotide.

So, four types of nucleotides are observed in DNA molecule. They are as follow:

1. Adenine nucleotide,
2. Guanine nucleotide,
3. Thymine nucleotide and
4. Cytosine nucleotide.

Therefore, nucleotides are building blocks of DNA.

In the year 1953, James Watson and Francis Crick planned double helix DNA (Deoxyribonucleic acid) model on the base of x-ray diffraction studies along with photographs of DNA (Deoxyribonucleic acid)  taken through Wilkins and Franklin. DNA is a double stranded structure where the two strands are coiled around each other creating a double helix. The DNA (Deoxyribonucleic acid) duplex is “coil of life”. There are 2 grooves that found in DNA molecule they are major and minor grooves. The backbone of the helix is created of sugar and phosphate molecule. The bases of nitrogenous are connected to sugar molecules. The 2 nucleotide strands are held together through unstable hydrogen bonds. Erwin Chargaff in the year 1949 showed that:

(i) The bases pair in particular way. Adenine all the time pairs with thymine and guanine pairs with cytosine.
(ii) Total amount of purine nucleotides is all the time equivalent to the total amount of pyrimidine nucleotides that is

[A] + [G] = [T] + [C].

(iii) The proportion of adenine is equivalent to thymine and thus also of guanine is equal to cytosine. But the [A] + [T] need not essentially be equal to [G] + [C].

These empirical rules concerning to the composition of bases in DNA is collectively termed as law of Chargaff or Base pair rules. There are 2 hydrogen bonds among the adenine and thymine (A= T) and there are 3 hydrogen bonds among the guanine and cytosine (G≡ C) pairing. The two strand run antiparallel in reverse directions that is they run in reverse direction 5’ to 3’ end and 3’ to 5’ end. The two strands are interwined in the direction of clock. The width of DNA (Deoxyribonucleic acid) molecule is 20 Å. The strand finished a turn every 34 Å along the length of it. There are ten nucleotides in every turn. The distance of internucleotide is 3.4 Å. Watson and Crick model of DNA (Deoxyribonucleic acid) is known as B-form DNA. The chains in B-form DNA are in right handed orientation.

 

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