8. DNA Replication

Telomeres are found at the ends of eukaryotic chromosomes. What is the sequence composition of telomeres?

Lagging strand telomeres are replicated in the same way as the rest of the chromosome

Which cell type contains the most telomerase

Telomeres are composed of what type of DNA molecule?

In the discussion, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:

How do we know that satellite DNA consists of repetitive sequences and has been derived from regions of the centromere?

There is a problem completing the replication of linear chromosomes at their ends. What is the function of telomerase, and how does it operate to synthesize telomeres?

There is a problem completing the replication of linear chromosomes at their ends. Describe the problem and identify why telomeres shorten in each replication cycle.

In 1994, telomerase activity was discovered in human cancer cell lines. Although telomerase is not active in most human adult cells, all cells do contain the genes for telomerase proteins and telomerase RNA. Since inappropriate activation of telomerase may contribute to cancer, why do you think the genes coding for this enzyme have been maintained in the human genome throughout evolution? Are there any types of human body cells where telomerase activation would be advantageous or even necessary? Explain.

Telomeres are found at the ends of eukaryotic chromosomes. Why is telomerase usually active in germ-line cells but not in somatic cells?

Telomeres are found at the ends of eukaryotic chromosomes. What is the functional role of telomeres?

Telomeres are found at the ends of eukaryotic chromosomes. How does telomerase assemble telomeres?

At the end of the short arm of human chromosome 16 (16p), several genes associated with disease are present, including thalassemia and polycystic kidney disease. When that region of chromosome 16 was sequenced, gene-coding regions were found to be very close to the telomere-associated sequences. Could there be a possible link between the location of these genes and the presence of the telomere-associated sequences? What further information concerning the disease genes would be useful in your analysis?

Go to the OMIM website (https://www.ncbi.nlm.nih.gov/omim) and type 'dyskeratosis congenita autosomal dominant 1' (DKCA1) into the search bar. The result will include a clickable link to the disorder that has an OMIM number of 127550. Review the OMIM information you retrieve and notice that this disorder is caused by a mutation of a telomerase gene that results in abnormally rapid shortening of telomeres and the appearance of disease symptoms at progressively younger ages in successive generations of the affected families. Use this and other information on OMIM to assist with this problem. Go to reference number 15 at the bottom of the OMIM page for a link to a 2004 paper by Tom Vulliamy and colleagues that appeared in the journal Nature Genetics. Click on the 'Full text' option and download a copy of the paper. Look at Table 1 of the paper on page 448. This table lists the lengths of telomeres measured in members of the families in this study. Telomeres shorten with age, and the telomere lengths in Table 1 are age-adjusted. The negative numbers for telomere lengths in the table indicate that telomeres are shorter than average for age, and the more negative the number, the shorter the telomere. Based on Table 1, the discussion in the Vulliamy et al. (2004) paper, and information available on OMIM, answer the following:

How do telomere lengths in children compare with telomere lengths of their parents?