Oracle IAS, the best coaching institute for UPSC/IAS/PCS preparation in Dehradun brings to you UKPCS Science Life Sciences (paper 6)
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 cell||Always 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 DNA||Each 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 circular||Telomere 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:
- autosomal chromosomes
- 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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