Genomic instability

• Describe telomeres and how they change over time

  • Telomeres consist of repetitive regions that cap the ends of chromosomes to protect them from digestion.
  • Telomere length decreases over time with the number of cell divisions
  • Telomere length determines the cells capacity to undergo division
• Describe the structure of telomeres

  Consists of a double stranded region of telomeric DNA followed by a single stranded 3’ overhand of G-rich telomeric DNA, which is unstable due to the action of DNase. The 3’ overhang forms the displacement loop, which binds to the C-rich 5’ end and this forms the T loop. Telomeric protein, TRF1 and TRF2, bind to repetitive regions within the telomere, which allows binding of TIN2, TPP1 and POT1.
• Describe why telomeres form


  Replication occurs from 5’ to 3’ and short RNA primers bind to the DNA in order to allow replication machinery to bind. More distal primers bind to the DNA and Okazaki fragments form. When the primer is removed there is an overhang where no primer can bind, which is where a telomere will form.
• Describe the action of hTERT


  This is the human telomerase holoenzyme, which is composed of two core subunits; the catalytic domain and the hTR RNA subunit. The holoenzyme attaches to the 3’ end of the G-rich strand overhang, via hydrogen bonding with hTR. Reverse transcription of sequences in hTR subunit allows hTERT to extend the G rich strand by six nucleotides.
• Describe the risks of telomere loss to chromosomal stability 


  Telomere erosion means that chromatid ends become unprotected and can fuse together, which prevents separation of chromatids during mitosis and therefore senescence. It can also leads to the formation of an anaphase bridge and breakage of chromosomes.

  
• Describe how cancers mutate telomerase


  Cancere upregulate the action of hTERT in order to maintain the cell within the cell cycle and prevent senescence. Examples of cancers which do this are neuroblastoma and Ewing’s sarcoma. Usually cancers will mutate the promoter sequence in order to alter the binding site of transcription factors and increase the expression of TERT.
• Describe how senescence occurs

  • Senescence occurs when the cell leaves the cell cycle due to natural accumulation of cell cycle inhibitors, which prevents CDK activation of CDKs
  • CDK inhibitors include p27, p21 and p16