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Sunday, January 06, 2008

cell types contain the enzyme telomerase which protects the length of telomeres at the end of chromosomes

015. All of the following cell types contain the enzyme telomerase which protects the length of telomeres at the end of chromosomes, except :

1. Germinal

2. Somatic.

3. Haemopoetic.

4. Tumour.

Answer

2. Somatic.

Reference:

Lippincot 3rd edition Page 405

Ganong 22nd Edition Page 20

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Discussion

Telomerase is an enzyme, discovered by Elizabeth Helen Blackburn and Carol Greider, that adds specific DNA sequence repeats, ("TTAGGG" in all vertebrates) to the 3' ("three prime") end of DNA strands, in the telomere regions at the ends of chromosomes. The enzyme is a reverse transcriptase that carries its own RNA template; the RNA is used as a template for DNA synthesis.. Telomerase (TE-LÓM-ER-ACE) is a ribonucleoprotein enzyme complex (a cellular reverse transcriptase) that maintains chromosome ends and has been referred to as a cellular immortalizing enzyme. Telomerase is a ribonucleoprotein reverse transcriptase enzyme (composed of both RNA and proteins) that uses its internal RNA component (complementary to the telomeric single stranded overhang) as a template in order to synthesize telomeric DNA (TTAGGG)n, directly onto the ends of chromosomes. Telomerase is present in most fetal tissues, normal adult male germ cells, inflammatory cells, in proliferative cells of renewal tissues, and in most tumor cells. After adding six bases, the enzyme is thought to pause while it repositions (translocates) the template RNA for the synthesis of the next six base pair repeat. This extension of the 3' DNA template end in turn permits additional replication of the 5' end of the lagging strand, thus compensating for the end-replication problem.

Interpretation

Telomerase is needed for the cell that actively divides. We know that germinal celss, tumour cells and hemopoietic cells are actively dividing cells

Comments

Telomeres are repeated DNA sequences that protect the ends of chromosomes from being treated like a broken piece of DNA needing repair. Without telomeres, the ends of the chromosomes would be "repaired", leading to chromosome fusion and massive genomic instability. Telomeres are also thought to be the "clock" that regulates how many times an individual cell can divide. Telomeric sequences shorten each time the DNA replicates. When at least some of the telomeres reach a critically short length, the cell stops dividing and ages (senesces) which may cause or contribute to some age-related diseases. In cancer, a special cellular reverse transcriptase, telomerase, is reactivated and maintains the length of telomeres, allowing tumor cells to continue to proliferate.

Tips

Telomeres are repeated DNA sequences that protect the ends of chromosomes from being treated like a broken piece of DNA needing repair. Without telomeres, the ends of the chromosomes would be "repaired", leading to chromosome fusion and massive genomic instability. Telomeres are also thought to be the "clock" that regulates how many times an individual cell can divide. Telomeric sequences shorten each time the DNA replicates. When at least some of the telomeres reach a critically short length, the cell stops dividing and ages (senesces) which may cause or contribute to some age-related diseases. In cancer, a special cellular reverse transcriptase, telomerase, is reactivated and maintains the length of telomeres, allowing tumor cells to continue to proliferate.

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