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DeoxyriboNucleic Acid
The Genome is the complete set of instructions to create and maintain an organism alive. DNA molecules are support for the Genome of all living organisms.
Structure The DNA Molecule consist of two unbranched polynucleotides chains (strands) held together in an antiparallel manner by hydrogen bonds formed between specific pairs of bases [Adenine-Thymine] [Guanosine-Cytosine]. Thus the bases sequence (code) in one strands determines the code of the other strands (complementarity).
Your Browser is not Java-compatible The joined anti-parallel strands are twisted about each other in the shape of a right-handed double helix. Indeed DNA is often depicted as a twisted ladder in which rungs are bases pairing and sides are deoxyribose-phosphate chains.
The double helix structure is mainly stabilized by hydrogen bonds between bases pairs. Since the hydrophobic bases are stacked inside and the hydrophilic ribose-phosphate chains are on the outside, Van der Waals forces and hydrophobic interactions are also deeply involved in the stabilization of the double helix.
The N-glycosic bonds (sugar-base) are not directly opposite one strand another, therefore two grooves of different width appear between ribose-phosphate chains on the surface of the molecule. In this major or minor grooves, the bases are exposed to solvent and to other molecules. By this way, some chemical and biochemical substances may have interactions with specific bases without disrupting the double helix structure.
Polymorphism There are three natural forms of DNA (A, B and Z). The origin of these different forms are related to the conformation of the sugar (C2'-endo/ C3'-endo) and the orientation of the base relative to the sugar (syn/anti).
Thus depending on base composition and physical conditions (Hydration/Salt-Content), DNA can assume several different conformations (A, B, Z).Each conformation possesses specific parameters: diameter of the helix, number of bases per tour and distance between plan of bases.
- The B-form is the common natural form, prevailing under physiological conditions of low ionic strength and high degree of hydration. B-DNA arranges 10 nucleotides per helix tour, all of conformation C2'-endo/anti . The plane of the bases is nearly perpendicular to the helix axis and the helix surface exhibits two prominent grooves (major and minor).
- The Z-form (Zigzag chain) is observed in DNA G-C rich local region. Z-DNA is longer, thinner and possess an unusual left-handed helix (of 12 bases pairs/tour) with a single narrow deep groove. These Zigzag form mainly results from the alternation of purines (C3'-endo/syn) and pyrimidines (C2'-endo/anti).
- The A-form is sometimes found in some parts of natural DNA in presence of high concentration of cations or at a lower degree of hydration (<65%). A-DNA possess 11 nucleotides per tour (all C3'-endo/anti) and two grooves (a narrow deep major and a wide shallow minor).
- The C-form and D-form are unusual subclasses of B-type. C-DNA is sometimes observed under 45% of hydration while D-DNA is only found in artificial DNA.
Packaging In Prokaryotic cells (cells without nucleus), the two ends of the DNA molecule are joined to form a circular DNA. The circular DNA is coiled into a super helix and often organized in a compact structure containing various proteins and RNAs, named Nucleoid,
In Eukariotic cells, the DNA is packaged in Chromatin within the nucleus. The structure of chromatin is determined and stabilized through the interaction of the DNA with specific bindings proteins.
Nucleosomes are the fundamental structural packing units of chromatin.
A nucleosome is a complex of DNA tightly wrapped around basic proteins called Histones. The nucleosome core consist of two tetrameric molecules, each having four histone-subunits (H2A, H2B, H3 and H4). The DNA helix coils twice around the histone octamer. It is bound to the nucleosome core through electrostatic interactions between the negatively charged phosphate groups in nucleotides and the positively charged basic amino acids in histones.
An external ninth histone (H1 linker histone) is added which holds the nucleosome structure together. A nucleosome plus one H1-histone is sometimes termed a chromatosome.
Nucleosomes are separated one another by a linker segments of 20-200 nucleotides pairs. This gives unfolded chromatin a "beads-on-a-string" appearance.
With the aid of histones H1, nucleosomes may be packed together and wound into a regular coil called solenoid. A solenoid contains six to eight nucleosomes per turn and forms the 30nm nucleoprotein fibers.
Packaging DNA in nucleosomes and then in solenoid reduce its length by a factor of about 50.
Between cell divisions (Interphase), chromatin exist as a tangle of fibers of 10-30 nm (10-9m) diameter and 0.25-2 mm length. The unfolded (beads-on-a-string) regions are referred to as euchromatin and the more condensed ones as heterochromatin.
Just before a cell division (Mitosis) the chromatin condense into ****phasis chromosomes. During this condensation the DNA packaging factor increase dramatically from 50 to about 7000. Actually, high order chromatin arrangement is not clearly understood. However, it is generally admitted that nucleoprotein chromatin fibers are folded and organized with specific non-histone proteins, into subdomains of coiled loops. These subdomains are supposed to be wrapped around other specialized proteins to form a chromosome.
A Chromosome is made of two identical, symmetrical DNA molecules called chromatids. The chromatids are joined by a centromere which attach them to the mitotic spindle. Thus, each chromosome contains 2 chromatids, 1 centromere, 4 telomeres (ends of DNA molecules) and many replication origins (ARS :Autonomously Replicating Sequences).
Diploid organisms such as mammals contains two sets of chromosomes. Thus the human cells contains 46 chromosomes (23 pairs), one set inherited from each parent. There are 24 different chromosomes, 22 autosomes and two possible *** chromosomes which define the *** of the holder: X for female (XX), Y for male (XY).
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