Notes on DNA Structure, Chargaff’s rule, Bonds in DNA and Different Forms of DNA
Whatever you see as life in this world, from microscopic bacteria to gigantic blue whale, from beautiful rose to sky loving redwood tree, the magician behind this wonderful phenomenon 'life' is simply the DNA, the key of life.The 4 letters of life (A,G,C,T) from microscopic bacteria moving through humans and reaching blue whale. The 4 letters responsible for the amazing diversity, mind boggling manifestations that probably passed on from the very beginning of life.The DNA in bacteria, in plants, in humans, in elephants, in blue whales, the molecule is the same but all are different and unique in its own. How? This question persists over time. But think of a molecule that connects blue whale to humans and humans to bacteria, the commonality that connects entire organisms, behind the curtain, the master is the same, manifestations may be different and that is the wonder molecule of life "The DNA".
Discovery of DNA
In1869 F.Miecher isolated unidentified substance from pus cells. He gave the name nuclein, later renamed as nucleic acid. All living organisms contain two types of nucleic acids namely DNA (Deoxyribo Nucleic Acid) & RNA (Ribose Nucleic Acid). James Watson & Francis Crick (1953) proposed the model of DNA. The model of DNA they proposed was based mainly on X-ray diffraction pictures taken by Maurice Wilkins & Roslind Franklin. Biochemical evidences from the work of Chargaff also helped in proposing the three dimensional model of DNA. James Watson & Francis Crick shared Nobel Prize with Maurice Wilkins in 1962.
Behind the DNA structure
DNA (Deoxyribo Nucleic Acid)
Almost all the DNA of eukaryotic cells is found in the linear chromosomes of the nucleus. Small amounts of circular DNA are found in cytoplasmic organelles such as mitochondria & chloroplast.
- The bases in DNA have carbon-nitrogen ring structures (nitrogenous bases)
- Adenine &Guanine (double ring nitrogenous bases) are purines.
- Thymine and Cytosine (single ring nitrogenous bases) are pyramidines
Adenine (A)-6-amino purine, Cytosine (C)-2 -oxy-4-aminopyrimidine,
Guanine (G)-2-amino-6-oxopurine, Thymine (T)-2, 4-dioxy-5-methylpyrimidine
Nucleoside vs Nucleotide
Sugar + base=Nucleoside (The combination of a base with the sugar constitutes a nucleoside)
In DNA, the sugar is a form of ribose called deoxyribose (it is missing the oxygen in the 2’ carbon atom)
Sugar + base (Nucleoside) + phosphate=Nucleotide
He found that in DNA the concentration of adenine always equalled the concentration of thymine and the concentration of guanine always equalled the concentration of cytosine ie. the amount of purine=the amount of pyramidine in a given DNA molecule.
A/T or G/C=1, A=T, G=C
A+T=C+G (No. of AT pairs need not equal to the number of GC pairs
Bonds in DNA
Adenine is paired to Thymine by 2 hydrogen bonds
Cytosine is paired to Guanine by 3 hydrogen bonds
DNA contains many mono deoxyribo nucleotides covalently linked by 3’5’-phosphodiester bonds. The resulting long, unbranched chain has polarity, with both a 5’end and a 3’end that are not attached to other nucleotides.
The sequence of nucleotides is read as 5’-3’. DNA exists as a double stranded molecule (A few virusesØX174 contain single stranded DNA as genetic material), in which the two strands wind around each other forming a double helix. The two chains are paired in an anti-parallel manner that is the 5’ end of one strand is paired with 3’ end of the other strand.
Bonds in DNA Structure
Forms of DNA
- 6 different morphological forms of DNA : A,B,C,D,E & Z
- A,B,C & D are right handed helices.
- B & Z occur in rigidly controlled experimental conditions.
- A-DNA is the dehydrated form which occurs in an environment richer in Na+ & less of water.
- B-DNA occurs under salt concentration& high degree of hydration.
- C-DNA & E-DNA are seen under special environmental condition &have slightly different conformation so do not occurin vivo.
- Z-DNA is left handed helices. Z-DNA, is the skinniest DNA, with only one groove and is stabilized by high salt concentration.
DNA is negatively charged
Most DNA molecules are right handed double helices. The stacking of the paired bases by their flat surfaces in the center of the molecules to forms a hydrophobic core. Together with the hydrogen bonds between the base pairs, these hydrophobic interactions contribute to the stability of the molecule.
DNA is negatively charged, why?
At neutral pH,there is no charge on any of the bases, but the phosphates on the surface of the DNA molecules are ionized, giving the molecule a net negative charge.
A complete turn of DNA (pitch) has a length of about 34A0
The spacing between adjacent base pairs is 10 A0 and the helix is 20 Ao in diameter
The ds DNA has 2 different grooves: a major groove and a minor groove, where interactions between specific nucleotide sequence and specific DNA binding enzymes (like DNA polymerase) may occur inorder to activate or repress a gene action.
More about DNA
- DNA Learning Center
DNA, genetics, and biotechnology online resources and hands-on educational programs for students, educators, and the public.
- DNA from the Beginning - An animated primer of 75 experiments that made modern genetics.
Discover the concepts and experiments that define the fields of genetics and molecular biology.
- DNA Interactive: Discovering the DNA Structure and beyond
DNA Interactive is an educational web site resource that celebrates the 50th anniversary of the discovery of the DNA double helix structure.
- The DNA Files
- Difference Between DNA and RNA