Describe how DNA damage induced by UV radiation is repaired by prokaryotes

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Describe how DNA damage induced by UV radiation is repaired by prokaryotes

All living organisms have adapted over millions of years to the changing conditions and to colonise new habitats.  However, an individual must be genetically stable in order to survive and reproduce.  According to Alberts et al (1998) the stability is achieved through extremely accurate mechanisms for replicating DNA and also through mechanisms for correcting the rare copying mistakes made by the replication machinery and for repairing the accidental damage.  Errors in DNA sequence can be induced by environmental factors such as radiation, mutagenic chemicals and thermal decomposition of nucleotides (Lodish et al, 1995).  Most of the damage to DNA is temporary and is repaired almost immediately.  If DNA replication and repair processes fail then this will lead to permanent damage (mutations) and will have consequences.

Prokaryotes are the most wide spread organism on the earth and they have a unique structure as they have no defined membrane-bound nucleus.  According to Hames and Hooper (2000) they are spherical (cocci), rodlike (bacilli) or helically coiled (spirilla).  Prokaryotes, like all cells, are bound by a plasma membrane that completely encloses the cytosol and separates the cell from the external environment.  Prokaryotes have a plasma membrane that can be folded to form mesosome which is where DNA replication and enzymatic reactions can occur.

Damage can occur to all cellular molecules.  If RNA or proteins are damaged, they can be degraded and newly synthesised by the mechanisms transcription and translation using DNA as a template, whereas DNA needs to be repaired when damaged.  The agents that damage DNA are certain wavelengths of radiation (gamma, ultraviolet and x-rays), highly reactive oxygen radicals, chemicals in the environment (hydrocarbons in cigarette smoke) and chemicals used during chemotherapy.  These agents can cause the bases in the DNA to be covalently modified at various positions and cause failure in the proofreading during DNA replication producing mismatching of the normal bases.  Also they can cause breaks in the DNA backbone and the formation of covalent linkages in between the bases.  Figure 1 shows DNA damage produced by ultraviolet light.  Alberts et al (1998) explained that DNA damage produced by ultraviolet light can cause the two adjacent thymine bases to become covalently attached to one another, forming a thymine dimer.  Skin cells that are exposed to ultraviolet radiation are especially susceptible to this type of DNA damage.  

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Multiple systems for repairing the damage to the DNA have evolved and this is probably due to the importance of the DNA information content.  Repairing damaged or inappropriate bases can be either done by direct chemical reversal of the damage bases or where the damaged bases are removed and replaced with the correct ones in a localised burst of DNA synthesis (excision repair).  There are three different types of excision repair; base excision repair, nucleotide excision repair and mismatch repair; and they all have different specialised sets of enzymes and proteins.  Table 1 shows the different types of enzymes ...

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