What is a chromosome?

The chromosomes are:
  – small linear fragments,
  -the condensed form of chromatin during mitosis.
  – carriers of genetic information, made up of long DNA molecules.

Chromosomes exist in the cells of all living things in variable numbers specific to each species.
In humans, there are 46 (diploid 2n) having in 22 pairs of identical chromosomes (homologous), the autosomes and a pair of sex chromosomes or gonosomes, except in reproductive cells or gametes where they are 23 chromosomes (haploid n).

Chromosome morphology

The appearance of chromosomes varies during mitosis.

  • Metaphasic chromosome: it has a length between 3 and 10µm.
    Each chromosome is made up of two chromatids, each chromatid representing one of two identical DNA molecules resulting from replication. These two chromatids are linked by a primary centromere or constriction, which represents a reference which makes it possible to separate the chromosome into two main regions: a short arm (by convention located above the centromere) and a long arm (below the centromere).

Chromosome identification criteria:

  • Centromeric index: which determines the type of chromosome, ratio between the size of the short arm on the short arm + the long arm.
  • Secondary constrictions
  • Satellites and telomeres: telomeres represent the ends of the chromosomal arms, having a particular architecture on the molecular level.

The different types of chromosome:
  – Metacentric chromosome: the arm p (short arm) is equal to the arm q (long arm) .Ex: chromosomes1, 2, 3, 16, 19.20.
  -Submetacentric chromosome: the arm p is less than the arm q.
  -Acrocentric chromosome: the arm p is clearly less than q.

-Teleocentric chromosome.

Chromosome ultrastructure

Two main proteins make up the axial skeleton of the chromosome:

  • Topoisomerase II is a protein which has the essential function of allowing a correct separation of two chromatids during the stage of condensation of the chromatids.
  • Condensins I and II are multi-protein complexes with the ability to bind to DNA and generate super turns and loops, resulting in condensation of the chromatin.

Cohesins: are proteins that are not part of the axial skeleton of the chromosome, but play a fundamental role in the structure and physiology of the metaphasic chromosome.

Chromosome and diseases

the karyotype allows us to detect anomalies relating to:

**The number of chromosomes: Aneuploidies:

-Monosomies (one of the chromosomes of the haploid batch is present in 1 copy). Ex: Turner’s syndrome 45 X0.

-Trisomies (one of the chromosomes of the haploid lot is present in 3 copies), ex: trisomy 21,

**The structure of the chromosome: Deletions, duplications, inversions, Translocations….

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