I- Definition :

The gene is the unit of genetic information, it consists of a nucleic acid sequence coding for the synthesis of a specific polypeptide or RNA.

II- General characteristics of genes :

Genes are stable entities. They are transmitted from parents to their offspring without undergoing changes in their sequences, but they may occasionally undergo changes in their sequences, such a change is called mutation.

– An organism carrying the normal gene is said wildtype.

– A body carrying the altered gene is a mutant.

– For each gene we inherit two alleles, each is carried by one of two chromosomes received from our parents ie by the pair of homologous chromosomes.

The genes can then exist in alternative forms that determine the expression of a particular feature. These forms are called alleles, So an allele is an alternative form of a gene occupying a given locus on a chromosome.

for example, the gene that codes for the color of our eyes exist in duplicate in our genome ,1A carried by the homologous paternal chromosome and the second by the maternal chromosome.

But as, there are several eye colors so this gene exists in different alleles : the determining allele obtaining blue eyes, the determining allele obtaining brown eyes, that permitting obtaining green eyes etc.…

III- Classification of genes :

There are at least 3 classes of genes :

A) The class I genes or genes transcribed by RNA polymerase I :

These are the ribosomal genes coding for the synthesis of 3 RNA of the ribosome ie : L’ARN 28S, 18 S and 5,8S. These genes are not scattered throughout the genome, but gathered in groups. They can exceed 200 copies.

The class I genes belong to the DNA coding category moderately repetitive and have the characteristic of beings repeated thousands of times.

B) Class II gene or genes transcribed by RNA polymerase II :

Class II genes encode proteins. They are usually single or in small numbers except for the genes coding for histone. They are classified according to the number of copies , on distingue :

– Unique or almost unique genes :

The vast majority of genes belonging to this class.

– The gene families :

These are genes that encode proteins roughly similar. The expression of each copy depends on the type or cell state.

Example :

  • The gene family of beta globin
  • The gene family of actin
  • The family of myosin genes

– The housekeeping genes (house keeping gene) :

These are genes that are expressed only in certain tissues .11 encode proteins domestic, such as enzymes of glycolysis , breathing and indispensable intermediary metabolism to cell survival .These genes have low transcription rates and continuous.

– pseudogenes :

These are non-functional nucleotide sequences, because they are not transcribed or translated. Their functionality can not result from either lack sufficient reading frame (excess STOP codons), or of lack initiator methionine codons or promoter region, by the appearance of mutations. c) Class III genes or genes transcribed by RNA polymerase III :

These genes encode small RNAs (ARNt, ARNr 5S, ARNsn). These genes are repeated in tandem (1200 in humans). They are classified in the category moderately repetitive coding DNA.

III- Anatomy of a gene encoding a protein :

– The genes encoding a protein or class II genes do not have a defined absolute structure.

– They are among the most common genes.

– The structure of these genes is not limited to their transcribed part and even less to their coding part.

– They usually have their genetic information is almost always fragmented.

These genes start 5 'non transcribed sequence whose presence is required for the transcription is carried out quantitatively and qualitatively in a normal manner. These sequences can very distant beings (qques kb upstream) and are difficult to define precisely, is the regulatory region of a gene.

  • Towards -100 bp relative to the initiation site of transcription initiates said promoter region.
  • Towards -70 at -80 bp is often a sequence CAAT, or attach one or more protein transcription factors.
  • Towards – 25 at -30 bp found except in rare cases (housekeeping genes) TATA sequence called TATA box (Goldberg -Hogness box), this is the level at which the RNA polymerase binds IL If this sequence is deleted artificially, the transcription rate is decreased and loyalty exact point of initiation is lost .Celle one will be qques bases before or after the usual site .However transcription is never abolished in its entirety.

Then comes the transcription initiation site, the basis for that site is usually a purine, follows a noncoding portion of variable length until the sequence ATG, methionine codon that signals the location of translation initiation. then after alternating sequences conserved exons or introns discontinued in the final version of mRNAs cytosolic.

– Called exon : all transcribed sequences found in the messenger cytosolic (This does not mean that they match the coding parts of the gene), indeed more or less long non-coding exon sequences may exist downstream of the first codon STOP.

– Called intron : any sequence transcribed removed by splicing during the processing of the primary transcript, So not found in the messenger cytosolic Wall.

  • The signal for the translational stop codon is given by a STOP : Drop, UAG, FROM.
  • Finally 10 at 20 bases before the end of the last exon is found the sequence AATAAA polyadenylation sequence improperly called, recognition sequence for the cleavage of the primary transcript ,binding site of the poly A polymerase.
  • The gene ends 3 'of a region rich in G and C whose nature is not known.

The gene boundaries are imprecise, their sizes vary widely up to 2 million base pairs. There is no direct relationship between the size of the protein and the gene length.

IV- The gene family of beta globin :

The family of beta globin genes located on the short arm of chromosome 11 and 11p. It contains in the order genes Epsilon, G Gamma, A Gamma, Delta, beta and a pseudogene Psi betal. The most gene 5 'is separated epsilon gene by 15 at 18 kb group A Gamma – G Gamma. These 2 genes are separated by 5 at 6 kb. Delta and Beta genes are located 15 brood 8 kb en aval.

These genes have 2 introns, Tintron IVSII a caractéristique d'être plus long (850 to 900b).

Coding sequences for A and G Gamma Gamma are almost identical, indicating that their duplication is a recent phenomenon.

2 types of repeat sequences are found in the inter gene DNA . The Alu sequence and LINE sequences I.

During the first weeks of pregnancy Hb is mainly embryonic-like (Xetá 2 Epsilon 2) ; then dice the first month, Alpha channels are synthesized, which is reflected by the appearance of fetal Hb (Alpha 2 Epsilon 2). Finally immediately after birth, there is an abrupt decrease in the synthesis of gamma chains and an increase in the synthesis of chaînesBeta (Alpha 2 Beta2), inducing the replacement of fetal THb by THBA, Ci is the Hb very large majority until the end of life.

This transition HbF->HbA door Switch the phenomenon name.

To Fetus, the fact of owning a different Hb her mother is an advantage, indeed Hb F having a higher affinity for oxygen than Hb A.

Fig. 1 : Anatomy of a class II gene
Fig. 2 : G.nes family of the β globin in localis.e 11p

references :
– genes VI. Author : Benjamin Lewin
– Molecular Biology and Medicine (2° Ed.) Coll. From biology to clinic. authors : KAPLAN Jean Claude, Marc Delpech.
– Molecular biology of the cell. authors : Mr Harvey Lodish, Mr Arnold Berk, Paul Matsudaira, James Darnell.
-Principles of nomenclature for human genes. Volume 60. No. 3. May – June 2002. F. Wojcik

Dr K Sifi Karima's course – Faculty of Constantine