DNA REPLICATION AND REPAIR

The process of DNA duplication is usually called Replication.

The replication is termed “Semiconservative” since each new cell contains one strand of original DNA and one newly synthesized strand of DNA. The original polynucleotide strand of DNA serves as a template to guide the synthesis of the new complementary polynucleotide of DNA.


In a cell, DNA replication begins at specific locations in the genome, called "origins of replication.”



STEPS OF DNA REPLICATION




TELOMERES
  • Repetitive sequences at the ends of the linear DNA molecule in eukaryotic chromosomes.
  • With each round of replication, these are shortened because DNA polymerase can’t complete synthesis of 5’ end of the each strand.
  • This contributes to aging of the cells.


TELOMERASE
      
      
  • An enzyme in eukaryotes used to maintain the telomeres.

      
  • Has telomerase reverse transcriptase activity.

      
  • Present only in embryonic cells, germ cells (reproductive) and stem cells.


SIGNIFICANCE OF TELOMERASE

Cancer cells have high levels of telomerase which prevents telomeres from being short and contribute to immortality of malignant cells.

REVERSE TRANSCRIPTASE

RNA dependant DNA polymerase
Requires RNA template to direct the synthesis of new DNA.
Retroviruses (HIV) have this activity.



COMPARISION OF DNA & RNA SYNTHESIS




DNA REPAIR

Can be damaged by chemicals or radiation
Incorrect bases can also be incorporated during replication.
Multiple repair systems have evolved, allowing cells to maintain the sequence stability of their genomes.
     
1. Excision Endonuclease
     
2. Uracil glycosylase
     
3. AP endonuclease


REPAIR OF THYMINE DIMER




REPAIR OF DEAMINATED & MISSING BASES

Cytosine can become deaminated spontaneously by reaction with nitrous acid to form Uracil


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