[mage lang=”en|es|fr|en” source=”flickr”]telomerase polymerase[/mage] differences and similarities in dna polymerase, primase, and telomerase?
name three differences and three similarities between those three
1. DNA Polymerase III Responsible for DNA Replication
Polymerase III is really an aggregate of ten different polypeptides and is referred to as a holoenzyme. The assembled aggregate has a total molecular weight of 760 kDa. The subunits are present in the holoenzyme in equal ratios. There are only 10-15 holoenzyme molecules per cell, and when the holoenzyme binds to the DNA, it remains firmly attached and proceeds along the growing chain until the strand is complete. The holoenzyme is a dimmer with two active sites, one for synthesizing the leading strand and one for extending the lagging strand. How the holoenzyme by passes the previously synthesized strand to get the next primer on the lagging strand is not exactly understood, but it likely involves folding the DNA at the site of synthesis.
2. The Primosome is Responsible for DNA strand initiation.
The primosome is a huge protein composed of 15 subunits responsible for the initiation of each new Okazaki fragment. As mentioned earlier, DNA polymerase can not initiate new strand synthesis, but RNA polymerases can initiate strand synthesis de novo. The primosome accomplishes initiation of strand synthesis by synthesizing a short segment of RNA for the DNA polymerase to use as a primer. The primase component of the primosome is responsible for synthesis of the short RNA fragment. However, it can not act alone; it must be present in the primosome complex.
It adds specific DNA sequence repeats (“TTAGGG” in all vertebrates) to the 3′ end of DNA strands in the telomere regions, which are found at the ends of eukaryotic chromosomes. This enzyme is active only in germ cells and cancer cells.Embryonic stem cells express telomerase, which allows them to divide repeatedly and form the individual. In adults, telomerase is highly expressed in cells that need to divide regularly (e.g., in the immune system), whereas most somatic cells express it only at very low levels in a cell-cycle dependent manner.
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[mage lang=”en|es|fr|en” source=”flickr”]telomerase information[/mage] When looking at senescence in cells what is the significance of measuring telomerase activity?
ie. what information does it provide (senescence and ageing??.. senescence and cancer??)
Recall that senescence is when cells no longer divide, and typically enter a period of decreased metabolic activity before apoptosis.
One of the proposed mechanisms for cells to determine their own age is the length of telomeres. Telomeres are the ends of chromosomes and, because of the chemistry of DNA replication, become progressively shorter with each subsequent round of replication. Telomerase is a unique enzyme that can repair this shortening by adding extra, non-coding pieces of DNA to the ends of chromosomes, protecting the necessary genetic information from being lost.
Most normal adult cells do not express telomerase. This ensures that when cells become too old to function correctly, they will die (they approach what’s called the Hayflick limit, at which point their chromosomes are no longer long enough for the cell to be functional). If telomerase is mutated and is expressed in adult cells, they lose the ability to stop growing, and they are at a higher risk for becoming cancerous. Indeed, a majority of tumors arise from cells that erroneously express telomerase. I hope this helps…telomerase is emerging as a possible target for chemotherapeutic drugs. Interestingly, despite the heightened level of telomerase, cancer cells tend to have shorter telomeres than other rapidly dividing cells…the hope is that they will be more susceptible to telomere poisoning than other cell populations.