![]() ![]() G binds C (with 3 hydrogen bounds: more stable link: 5.5 kcal vs 3.5 kcal). AsĪ corollary, the number of purines residues equals the number of pyrimidine Hold the two strands together (dashed lines in the drawing).Ī purine on one strand shall link to a pyrimidine on the other strand. Hydrogen bounds between the bases of one strand and that of the other strand Planes are perpendicular to the axis of the double helix. The (hydrophobic) bases are stacked on the inside, there DNA stucture 2.2.1 Hydrogen bounds: bases pairing One of 34 A°, corresponding to one helix turn (and also to 10 bases pairs).įigure 6. Of 3.4 A°, corresponding to the distance between 2 bases, and another The general appearance of the polymere shows a periodicity Two strands are antiparallel (id est: their 5->3 orientations are in 2.2 DNA moleculeĭNA is made of two ("duplex DNA") dextrogyre (likeĪ screw right-handed) helical chains or strands ("the double helix"),Ĭoiled around an axis to form a double helix of 20A° of diameter. N) - phosphate-sugar (nucleotide n+1), and so on, covalently linked, the basesįigure 5. Theīackbone of the molecule is made of a succession of phosphate-sugar (nucleotide Successive addition of monomeres in a general 5 -> 3 configuration. 5 sugar, which 3 is free for next step. Then, we start with a phosphate,Ī 5 sugar (+base) and the 3 of this sugar, linked to a second phosphate The phosphate of a mononucleotide (in C5 of its sugar) being linked to theĬ3 of the sugar of the previous mononucleotide. Secondary and tertiary structures of the molecule: Three-dimentional conformation of DNA 2.1 Dinucleotidesĭinucleotides form from a phosphodiester link between 2 mononucleotides. Nucleotide names: deoxyribonucleotides in DNA: deoxyadenylic acid, deoxyguanylicĪcid, deoxycytidylic acid, deoxythymidylic acid (ribonucleotides in RNA: adenylicĪcid, guanylic acid, cytidylic acid, uridylic acid). Nucleoside names: deoxyribonucleosides in DNA: deoxyadenosine, deoxyguanosine,ĭeoxycytidine, deoxythymidine in DNA (ribonucleosides in RNA: adenosine, guanosine, Note: other nitrogenous bases exist, in particular methylated bases derived from the above mentioned methylation of the bases has a functional Pyrimidines: cytosine (C) and thymine (T) ( Note: thymine is replaced by The nucleotide is therefore: phosphate - C5 sugar C1 - base.Īromatic heterocycles there are purines and pyrimidines. ![]() Link), and the phosphate links to C5 (ester link) to make the nucleotide. A nitrogen atom from the nitrogenous base links to C1 (glycosidic Figure 1.Phosphate group 1.2.Sugarĭeoxyribose, which is a cyclic pentose (5-carbon sugar). In DNA,the nucleotide is a deoxyribonucleotide (in RNA, the nucleotide is a ribonucleotide). A nucleotide is made of a phosphate + a sugar + a nitrogenous base. Primary structure of the molecule:covalent backbone and bases asideĪ nucleoside is made of a sugar + a nitrogenous base. These are the most common base pairing patterns but alternative patterns also are possible.I. As shown in figure 3, adenine forms a base pair with thymine, and guanine forms a base pair with cytosine. The interaction between two bases on opposite strands via hydrogen bonds is called base pairing. It is the sequence of these four bases that encode genetic information. ![]() The bases interact via hydrogen bonds with complementary bases on the other DNA strand in the helix. The four bases are adenine (A), cytosine (C), guanine (G) and thymine (T). The sugar and phosphate create a backbone down either side of the double helix. Nucleotides have three components: a base, a sugar (deoxyribose) and a phosphate residue. The monomers of DNA are called nucleotides. Some DNA sequences do not code for genes and have structural roles (for example, in the structure of chromosomes), or are involved in regulating the use of the genetic information for example, repressor sites are DNA sequences that allow binding of a repressor, which stops the process of gene expression.ĭNA consists of two long polymers (called strands) that run in opposite directions and form the regular geometry of the double helix. Genes are the DNA segments that carry genetic information (1). Therefore, DNA is an essential component of independently living organisms. Genetic information is encoded in deoxyribonucleic acid (DNA) molecules. ![]()
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