I- Introduction, definition :
originally isolated from the cell nucleus. The nucleic acids are substances, not only in the ring but also in the cytoplasm of cells.
From a chemical standpoint nucleic acids are weak acids.
It exists 2 types & rsquo; nucleic acids: 50 & rsquo; ADN, and 50 & rsquo; ARN
These are long molecules formed by the repetition of & rsquo; subunit called nucleotide.
II- Structure of & rsquo; DNA and L & rsquo; RNA :
L & rsquo; DNA is a molecule & rsquo; great biological importance because it is the support of & rsquo; genetic information. L & rsquo; genetic information that & rsquo; it contains is transmitted to offspring so it is the main vehicle of the phenomenon of & rsquo; heredity.
L & rsquo; DNA is localized mainly in the nucleus, However, there is a DNA into the cytoplasm c & rsquo; is & rsquo; mitochondrial DNA responsible for the & rsquo; mitochondrial inheritance or kindergarten.
L & rsquo; RNA is localized mainly in the cytoplasm, but it also exists in the kernel.
L & rsquo; DNA and & rsquo; RNA are formed by repeating units called nucleotides under.
Each nucleotide consists in turn 3 parties :
- A nitrogenous base
- A sugar or dare to five carbons ribose or deoxyribose
- A phosphate group
The phosphate groups and molecules & rsquo; sugars that make up the nucleic acids are identical and play a structural role by against the bases can be 4 types and contain the & rsquo; genetic information.
Nucleotides have varied and important functions, Those are :
- Compounds “high energy potential” on which the activation of molecules depends,
- Structural coenzyme compounds,
- Of intracellular second messengers to extracellular signals and mediators,
- Regulators d & rsquo; protein activity.
1- Nitrogenous bases :
These are essential components of nucleic acids. These aromatic molecules whose core is either a purine or a pyrimidine.
a- The purine bases :
Derive from the purine nucleus, which is an aromatic ring 9 5C and 4N atoms and result from the substitution of atoms & rsquo; hydrogen of & rsquo; heterocycle by hydroxyl radicals, amines or methyl.
b- The pyrimidine bases :
Derived from the pyrimidine ring which is an aromatic ring 6 4C atoms and 2N and result from the substitution of & rsquo atoms; hydrogen of & rsquo; heterocycle by hydroxyl substituents amines or methyl.
c- The physicochemical properties of purine and pyrimidine bases :
The aromatic character of the purine and pyrimidine bases gives them :
– Resistance to the & rsquo; oxidation
– A characteristic absorption in the & rsquo; UV (Identification and assay)
– The presence of hydroxyl substituents and amines enables purine and pyrimidine bases occur in several tautomeric forms :
- The lactam keto or enol form and shape or lactim. At physiological pH is the keto form predominates.
- The amine form and imine form to the amino group.
Tautomerism is the transformation d & rsquo; a functional group by another group :
– simultaneous displacement of & rsquo; a & rsquo atom; hydrogen and
– d & rsquo; an electron doublet from d & rsquo; an adjacent double bond.
2- the Oses :
These are pentoses under furan form that & rsquo; nucleotides found in nucleic acids. The beta-D (-) ribose and beta D(-) 2 deoxyribose.
The numbers of the pentose carbon atoms are assigned the prime sign to differentiate atoms bases.
3- L & rsquo; phosphoric acid :
4- Les nucleosides :
A nucleoside results from the covalent combination of & rsquo; a dare and & rsquo; a sugar by binding N beta glycoside. This connection unit 1 glycosidic carbon′ s & rsquo; & rsquo dare with; nitrogen number 9 purine bases and & rsquo; nitrogen number 1 pyrimidine bases.
– Les ribonucléosides
– deoxyribonucleosides :
5- nucleotides :
These are phosphate esters of nucleosides, the phosphoryl group can be attached to a free OH of the monosaccharide.
Ribonucleosides can be phosphorylated by 2 ',3'And 5' .de there may exist more cyclic nucleotide. When the molecule of phosphoric acid esterified 2 hydroxyl functional groups of the monosaccharide it give cyclic esters in position 2 ',3'Or 3',5’.
Deoxyribonucleosides may be phosphorylated in the 3 'and 5' since the carbon atom 2 'shall not OH and there can be only one cyclic ester 3',5’.
6- Structure & rsquo; polynucleotide :
A polynucleotide consists of more nucleotides or deoxyribonucleotides linked by phosphate groups specifically, therefore c & rsquo; is a nucleotide polymer. The 3 'hydroxyl of the saccharide portion of the first nucleotide is joined to the 5' hydroxyl of the monosaccharide of the adjacent nucleotide by a phosphodiester bridge.
This structure can be schematically
pApGpCpApT = AGCAT
Read Convention d`un polynucleotide :
By convention we always reads a polynucleotide in the direction towards 5`p 3` free OH.
7- Structure of & rsquo; DNA :
a- The characteristics of these 2 chains :
They are called
– Helicoils :
The 2 DNA chains present in & rsquo; space a helical .They s & rsquo configuration; wrap around & rsquo; an imaginary central axis, forming a double helix 2 nm diameter .The not the & rsquo; propeller is 3.4nm and contains 10 pairs nucledtides.la distance 2 nucleotides of 0.34 nm.
– Antiparallèles :
Means that the two strands are parallel but in opposite directions .For strand is 5'-direction>3 from top to bottom, for the second strand is the direction 5 '-' 3 'from the bottom up.
– Supplement :
The complementary rule is :
Opposite A has a T and G cm lace has a C.
This complementarity rule obeys steric reason or room (in front of & rsquo; a purine base cm has a pyrimidine base, and due to link d & rsquo; hydrogen : l & rsquo; adenine is joined to thymine by two hydrogen bonds and cytosine guanine has united by three hydrogen bonds.
– Denaturing DNA :
If cm heats the DNA there is a rupture of the hydrogen bonds between the bases of the double helix discards 2 strands separate year said that the DNA is denatured. This denaturation is reversible L & rsquo; AD N may also be denatured in a slightly alkaline medium.
■ Règfe de Chargaff
A = T and G = C, which is of great importance in the secondary structure of DNA A + G = C + T
A+T ≠ G+C
9- Structure of & rsquo; RNA :
- or : Ribose
- Base : A U.G. C
- A single chain
- The deciphering of the code gpnétique contained in a molecule of & rsquo; DNA through the formation of & rsquo; an RNA molecule.
L & rsquo; RNA is synthesized during & rsquo; a complex process that & rsquo; cm-called transcription L & rsquo; ribanucléique acid is a polymer of ribonucleotides practices or pyrimidine joined together by bridges 3 ',5’ phosphodiesters, analogpes to those DNA. L & rsquo; native RNA exists as a single-stranded molecule, the single strand of RNA is able to fold on itself in a hairpin.
■ There are several types (TARN:
→ The coding RNAs:
- L & rsquo; mRNA :
– Its sequence is complementary to the & rsquo; one of 2 strands of the molecule & rsquo; DNA
– it contains the & rsquo; genetic information for the biosynthesis of & rsquo; a protein.
– has d & rsquo sequences; translation initiation AUG or GUG and sequences announcing the end of the translation :Drop,UAG,FROM.
→ noncoding RNA:
- ARN ribosomal ,transfer RNA, SnRNA.
- micro RNA, and RNA (regulatory genes).
The RNA nbosomiaux represent over 80% total cellular RNA s & rsquo; associate with proteins to form the ribosome which is the support of protein synthesis. Ribosomes are a combination of 2 subunits : 50S and 30S in prokaryotes and 60S and 40S in eucaiyotes.
The sedimentation coefficient S (Svedberg ) is & rsquo; unit of measure of the sedimentation velocity.
sedimentation coefficient d & rsquo; a particle depends not only on its mass but also its shape and rigidity.
By definition, S sedimentation constant, is the sedimentation rate per unit dAccélération (force g). As this constant is low, we use, as a unit, le Svedberg (un Svedberg = 10′13 second).
Allows the transport of amino acids to the ribosome instead of protein biosynthesis :
a single chain 73 at 93 ribonucleotides its 3 'end by a term or CSF binds
THE A and its initial end is always a 5 pG, The anticodon loop comprises bases Following :pyr-pyr-X-Y-Z-pur-base variable.
- While in eukaryotes, most of P DNA is present in the core, a portion of this DNA is located in the mitochondria.
L’ADN mitochondrial :
- The mtDNA can be detected by fluorescence microscopy, it is located in the mitochondrial matrix.
- The mitochondrion contains 4 at 6 double-stranded DNA molecules.
- L’ADN mt humain, a circular molecule that has been completely sequenced,
- It contains 16 569 pb
- Possesses 13 sequences beginning with a coding ATG (methionine) ends with a stop encoding and are long enough to encode a polypeptide of more than 50 AA
- Unlike nuclear DNA P, it is free of introns and does not contain long coding sequences
- Molecular biology Christian Moussard
- molecular biology Simon Beaumont
- Human biochemistry
- Pierre Louisot
Course of Dr Sifi Karima – Faculty of Constantine