SOS response in E. coli: 

The LexA protein is the repressor in this system, which has an operator site near each gene. Because the RecA gene is not entirely repressed by the LexA repressor, the normal cell contains about 1,000 RecA monomers. 

TLS Online TPP Program

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#Unit 3. Fundamental Processes

Short repeating sequence from the human telomere forms three stacked G quartets.

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#Unit 3. Fundamental Processes

Telomerase, a reverse transcriptase that contains an RNA template, repeatedly adds TTAGGG repeats

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#Unit 3. Fundamental Processes

Embryonic cells, germ-line cells, and stem cells produce telomerase, but most human somatic cells produce only a small amount of telomerase as they enter S phase.

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#Unit 3. Fundamental Processes

As a result of their modest telomerase activity, their telomeres shorten with each cell cycle.

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#Unit 3. Fundamental Processes

Extensive shortening of telomeres is recognized by the cell as a double-strand break and consequently triggers cell cycle arrest and apoptosis.

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#Unit 3. Fundamental Processes

In mammals, six telomeric proteins (TRF1, TRF2, Rap1, TIN2, TPP1, and POT1) comprise a complex called shelterin