dna strand


DNA Structure

DNA stands for Deoxyribonucleic acid. It is a chromosomal component that carries hereditary information. It contains a blueprint of instructions for the development and functioning of the living organisms. All the living organisms contain the DNA genomes.

James Watson, the 23-year old biologist from the US worked with Francis Crick, an English physicist at the University of Cambridge. They used the X-ray crystallography data to decipher DNA's structure. Their research added to the existing information about the structure of the DNA molecule.

They published their hypothesis, entitled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" in the British journal Nature (April 25, 1953. volume 171:737-738.) It is interesting to note that this paper has been cited over 800 times since its first appearance!

Some basic facts about the DNA structure are:

DNA is made up of subunits called nucleotides. Each nucleotide is made up of a sugar, a phosphate and a base.

The four different bases in a DNA molecule are:

  1. adenine (a purine) ,
  2. cytosine (a pyrimidine) ,
  3. guanine (a purine)  and
  4. thymine (a pyrimidine).

The number of purine bases equals the number of pyrimidine bases
The number of adenine bases equals the number of thymine bases
The number of guanine bases equals the number of cytosine bases

The basic structure of the DNA molecule is helical, with the bases being stacked on top of each other.

According to James Watson and Francis Crick:

"...This (DNA) structure has two helical chains each coiled round the same axis...Both chains follow right handed helices...the two chains run in opposite directions. ..The bases are on the inside of the helix and the phosphates on the outside..."

"The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases... The (bases) are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side...One of the pair must be a purine and the other a pyrimidine for bonding to occur. ...Only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine)."

"...in other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined."

"...It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

The DNA does not usually exist as a single molecule in the living organisms. It exists in form of a tightly associated pair of molecules. These two long stands of molecules intertwine each other like vines and form the shape of a double helix. The nucleotide repeats consist of both the backbone of the molecule that keeps the chain together, and a base, that interacts with the other DNA strand in the helix. Generally speaking a nucleoside is the base linked to a sugar and a nucleotide is a base that is linked to sugar and one or more phosphate groups.

The sugar in DNA is the pentose (five carbon) sugar 2-deoxyribose. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand


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