Chromosomes

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I- Structure of human chromosomes :

Chromosomes are small organs in the form of rods consist of long molecules of & rsquo; dsDNA associated with two types of proteins, basic type proteins or histone proteins and acid type proteins or non-histone proteins.

This complex set of & rsquo; DNA and proteins may change structure over time .

C & rsquo; is thus that & rsquo; outside the cell division (mitosis or meiosis), structural change chromosomes to form chromatin .

The DNA in chromatin n & rsquo; is not free , but it is integrated in structures made based nucleosomes .

Nucleosomes are balls 100 A diameter surrounded regularly and evenly spaced by a thread 20 A diameter or double-stranded DNA.

The stack adjacent nucleosomes


  • In the human genome, chromosomes come in different sizes. The smallest chromosome is chromosome 21. It has about 50 million base pairs , while the largest chromosome ,is the chromosome 1. It can reach up & rsquo; to 250 million base pairs. Cytogenetic called the study of chromosomes, their structure and their transmission.
  • The establishment of a chromosomal formula of a species allows to detect any structural abnormalities or chromosome number.
  • The normal human karyotype has 46 divided into chromosomes 23 pairs :

– 22 pairs of identical chromosomes in men and women appointed autosomes numbered 1 at 22, according to their decreasing size.
– The last remaining pair is represented by the named sex chromosomes gonosomes : XX in females and XY in humans.

  • The nomenclature of a karyotype is as follows :

We must master the total number of chromosome followed by a comma, sex chromosomes followed by a comma , chromosomal anomaly structure or number when it exists.

  • 46, XY
  • 47,XX ,21
  • 45,X0

II- Chromosomal analysis :

  • The chromosomes of a human cell division are more easily analyzed at metaphase or prometaphase of mitosis of cells in culture .The chromosomes are classified and analyzed after coloring.
  • Each chromosome has a primary constriction or centromere that unites the two sister chromatids and sets the chromosome mitotic spindle .From both sides of the centromere ,one chromatid has a short arm or p arm and a long arm or arms q.
  • Depending on the position of the centromere , we distinguish the chromosomes centromere median or metacentrics ,chromosomes centromere sub median or sub metacentric chromosomes and centromere distal or acrocentric.
  • Telomeres are located at the ends of chromosomes. Telomeres are nucleoprotein structures highly specialized, that protect the chromosome ends. Telomeres are highly repeated regions, GC riches, which makes it very stable double-stranded . There is a shortening of telomeres with each cell division.
  • chromosomes 13,14,15,21 and 22 have small masses of chromatin DNA called satellite connected at their short arm by a narrow stalk = secondary constriction , carrying the genes coding for 18S ribosomal RNA,28 S.


  • The members of a chromosome pair or homologous chromosomes carry genetic information that cad homologous sequence is identical loci on both chromosomes although at each locus ,there may be slightly different or identical forms of the same gene, called alleles.
  • These forms are called alleles, So an allele is an alternative form of a gene occupying a given locus on a chromosome.

One of the features of & rsquo; human species is its diversity. The individuals composing differ from each other.

heterozygous genes (two different alleles) / homozygous genes (two identical alleles)


These sequences variations may occur in genes outside of genes

→ with result
→ with a phenotypic result example : eye colour

One member of each chromosome pair is inherited from the father and one mother. The boy inherits the X chromosome from his mother and transmits it to all his daughters, the Y chromosome is inherited from father to forward to its son.

references
– Jean-Claude Kaplan, Marc Delpech. Molecular Biology and Medicine. 3edition. Paris: Medicine-Sciences Flammarion, 2007.
– Lodish, Berk, Matsudaira, Kaiser, Krieger, Scott, Zipursky, Darnell. Molecular biology of the cell. 5edition. Brussels : the boeck, 2005.
– Genoa
5-William Klug , Michael Cummings , Charlotte Spencer .Génétique 8th editing .2006 Pearson Education France.
– genome 3 Brown 2007 Editor : Garland publishing

Dr K Sifi Karima's course – Faculty of Constantine