The genetic code

The genetic code is the system of correspondence between the nucleotide sequences of DNA and amino acid sequences of proteins.

The sequence of four nucleotides A, C, T, G, in a gene sequence must encode the sequence of 20 amino acids in the protein. If a database encodes a single amino acid, only 4 amino acids could be coded unambiguously. Encoding an amino acid therefore requires at least a sequence of 3 bases (64 possible arrangement, or codons). It is therefore possible to encode 61 different amino acids and 3 stop codons for translation: UAA UAG and UGA. The code is called degenerate (known redundancy of the genetic code), an amino acid can be encoded by several codons for that reason.

Possible variations of the spatial structure of DNA

* Curved DNA: DNA that moves a dynamic structures and makes movements.

DNA structures seen in vivo also have a functional role: genetic recombination and mutation.

* Z DNA: a left-handed double helix whose backbone conformation of this structure zigzag May smoother than DNA B.

there is one path that resembles the major groove of DNA B. base pairs that form in the B DNA the major groove near the axis are rejected outside at the DNA level Z. phosphors are closer to each other. Z DNA can not form the figure is formed nucléosome.une GC base supports Z conformation and the methylation of cytosine.

* DNA fusiform and DNA hairpin

Holiday junctions formed during recombination are cruciforms inverted repeats mirroring polypurines DNA segment, is also produced polypyrimidiques structures cruciform or hairpin apparently by intrabrin.

* H DNA or DNA triplexes: inverted repeats (polychrome) DNA segments polyurine, polypyrimidine can form triplex structures. we obtain three-stranded DNA + a single strand.

DNA H could have a role in réguation functional gene expression, as well as RNA. eg repression of transcription.

* G DNA: DNA quadruplex, fold double-stranded sequences rich in G and C on itself forming paired bases Hoogsteen type between guanine 4 and structure is particularly stable and often near the promoters of genes at telomeres.

Different forms of DNA

As explained above, two DNA molecules are paired through hydrogen bonds between their nitrogenous bases to form the double helix of DNA (DNA as double-stranded). This is the form stable DNA is present in living organisms. Yet this double-helix may be opened to allow the execution of basic biological processes (such as replication or transcription) generating DNA single-stranded form. Depending on environmental conditions, these two forms of DNA (single and double stranded) can see their structure vary. These structures are generally rare, and their biological functions (if any) unknown.

Several types of DNA double-strand

Depending on the composition of the external environment, particularly the percentage of water bound to hydrophilic phosphates, the double helix of DNA can adopt three structures:

* 95% water B
* 70% water type A
* 50% water type Z

These structures also exist in vivo:

* DNA-B form of DNA the most common. It pitch propeller clockwise, trays base perpendicular to the helix axis from the center of matching them. It has 10.5 bp per turn (or 21 nucleotides) a rotation of 36 ° between each sugar (or 34A). The sugars are in anti position (core bases outside of sugars), endo and radial with respect to bases. The vertical space between each base pair is 0.34 nm.

* DNA-A: a series of specific DNA transcription. Indeed, RNA is type A, during transcription, RNA stimulates transfer of DNA from B to A. At the end of the transcript, when RNA was liberated, the DNA returns to its conformation B.

Type A is characterized by elevations of very inclined base, a tangential position of the sugars (as well as anti-and endo), an axis passing through the major groove rather than through the middle of base pairing, and 11 base pairs per turn is 32.7 ° between each sugar.

* Z-DNA: its role is to promote interaction bases with the regulatory proteins. Is pitch propeller levorotatory. It is characterized by plateaus slightly sloping (9 ° or less), and an alternative position of sugars into radial and tangential (and an alternate 3'endo syn / anti 5'endo). The axis passes through the minor groove and has 12 base pairs per turn is 30 ° between each sugar. The portion of B-DNA Z-DNA is favored by the presence of multiple cytosines within promoters.

Other structures of DNA

* The triplex: structure formed when a molecule of DNA single strand pair is in the major groove of a DNA double helix

* The G-quadruplex: secondary structure formed by single-stranded DNA when it is rich in guanine, forming a stack of trays ( "Quartet") each composed of 4 guanines.

* The hairpins

Mechanical properties

The mechanical properties of DNA can be studied by numerical simulations of molecular dynamics and by manipulation experiments of single molecules (eg, using optical tweezers or magnetic). Like all polymers, the DNA molecule is elastic. Under weak constraints, a double strand can be described by standard models of polymer physics (model worm, etc.).. However, applying a force 65pN the ends of a double strand, is transiting it to a new form, about 1.7 times longer, called DNA-S (stretched). This can be interpreted by a rotation base pairs: the double helix into a "scale" or "fiber." It seems that this transition plays a role in biological processes such as DNA repair proteins by

Different types of enzymes related to DNA

* DNA helicase: an enzyme that catalyzes the unfolding of the complementary strands of a double helix of DNA.
* DNA ligase: enzyme catalyzing the connection between two separate molecules of DNA, forming phosphodiester bonds between the 3'-hydroxyl of one and the 5'-phosphate of the other. Its natural role is the repair and replication of DNA. Is an essential tool in the technology of recombinant DNA because it allows the incorporation of foreign DNA into vectors.
* DNA polymerase: an enzyme catalyzing the polymerization (5 'to 3') of monodésoxynucléotides triphosphates that make up the DNA. In the absence of monodésoxynucléotides triphosphates, it has a role of exonuclease, removing nucleotides non-instrumented strands "sticky" (sense 3 'to 5')
* DNA primase: an enzyme that catalyzes the synthesis of short RNA primers from which begins the synthesis of DNA strands.
* Topoisomerase or topoisomerase (eg, DNA gyrase): enzyme that catalyzes the introduction or removal of supercoiling in DNA.


Helical structure has inspired a number of artists. Remains the most famous surrealist painter Salvador Dali inspired by it in nine paintings between 1956 and 1976 including The Great Masturbator in a Surrealist Landscape with DNA and Galacidalacidesoxyribonucleicacid

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