UKPCS Science Life Sciences_animal_bio: CHROMOSOMES # 21

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CHROMOSOMES are thread like structure found in the nucleus with  a part or all the genitic material of an organism chromosomes are normally visible under a light microscope only when it is undergoing the METAPHSE of the cell  division .

Before this happens every chromosome is copied once (S PHASE) and the copy is joined to the original by CENTROMERE resulting in an X shaped structure .The original and the copy chromosomes are now called as SISTER CHROMATIDS

Chromosomes vary widely between different organisms:

The typical chromosomes formation is absent.In eukaryotes the genetic material is organized as distinct structural entities called chromosomes.
Only a single chromosome per cellAlways two to many chromosomes per cell.
The prokaryotic chromosome is comparatively shorter.The eukaryotic chromosomes are larger than that of prokaryotes.
Prokaryotic chromosome contain a covalently closed circular DNAEach eukaryotic chromosome contains a linear DNA with two ends.
Prokaryotic chromosomes code for few proteins.Codes a larger no. of proteins.
Chromosome occupies freely in the center of the cell and not covered by the nucleus.Chromosomes are always enclosed in the  nucleus.
Due to the absence of the nucleus ,the prokaryotic chromosomes stay in direct contact with the cytoplasm.


Eukaryotic chromosomes are separated from the cytoplasm by the nuclear membrane.
THE DNA is associated with histone proteins in prokaryotic chromosomes .DNA is associated with histone protein in a eukaryotes.
Nucleosomes are not formed  in prokaryotes .In eukaryotes, the association of DNA with the histone produces distinct structural repeats called nucleosomes.
Prokaryotic chromosome contains only a single origin of replication and can occur at any stage of life cycle.Eukaryotic chromosomecontain many origin of replications and the genetic material will only replicate at the S phase of cell cycle.
Telomere is absent since the chromosome is circularTelomere is present in the tip of eukaryotic chromosome.
Structures like centromere, kinetochore, secondary constriction and chromosomal arms are not formed in prokaryotic chromosomes.Eukaryotic chromosome contains centromere kinetochore and chromosomal arms.


HUMAN CHROMOSOMES can be divided into two:

  1. autosomal chromosomes
  2. sex chromosomes

Human cells have 23 pairs of chromosomes(22 pairs of autosomes and 1 pair of sex chromosomes)giving total of 46 per cell.

Chromosomal aberration: disruption in the normal chromosomal content of a cell and cause major changes in the genetic condition in human ,such as: down syndrome , klinefelter syndrome etc.

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