Oracle IAS, the best coaching institute for UPSC/IAS/PCS preparation in Dehradun brings to you UKPCS Science Life Sciences (paper 6) – Chromosomes
CHROMOSOMES
These are thread like structure found in the nucleus with a part or all the genitic material of an organism. They 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
Difference between Prokaryotes and Eukaryotes:
Chromosomes vary widely between different organisms:
| Prokaryotes | Eukaryotes |
| 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.
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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 chromosome contain 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
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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