Chromosomes telomers

Figure 2d), including parallel interstrand (most stable) and antiparallel intrastrand (fold-back) structures in DNA and RNA. These types of multistranded DNA structures have been proposed to be important in chromosomal telomer formation, immunoglobulin gene switching, and genetic recombination. The large hydrophilic channel within the qnadruplex structure is stabilized by monovalent metal ions such as K+ > Rb Na+ > Li (in order of their effectiveness) " (also see Section 6). 

Other DNA polymerases. Reverse transcriptases synthesize DNA using an RNA template strand. They are best known for their function in retroviruses (Chapter 28). The HIV reverse transcriptase is a heterodimer of 51- and 66-kDa subunits. The larger subunit contains a ribonuclease H domain.288-2893 The enzyme is a prime target for drugs such as AZT and others.290 291 A different reverse transcriptase is found in all eukaryotic cells in telom-erase, an enzyme essential for replication of chromosome ends. Reverse transcriptases have also been found in rare LI sequences that are functioning ret-rotransposons (Section D).292... 

The replication of a linear DNA molecule in a eukaryotic chromosome creates a problem that does not exist for the replication of bacterial circular DNA molecules. The normal mechanism of DNA synthesis (see above) means that the 3 end of the lagging strand is not replicated. This creates a gap at the end of the chromosome and therefore a shortening of the double-stranded replicated portion. The effect is that the chromosomal DNA would become shorter and shorter after each replication. Various mechanisms have evolved to solve this problem. In many organisms the solution is to use an enzyme called telom-erase to replicate the chromosome ends (telomeres). 

Telomers seal the end of a chromosome. The telomers consist of simple, tandemly-repeated sequences. Typically one strand is G-rich and the other C-rich. A 15-residue long G-rich strand is added to the 3 end of the chromosomal DNA by the telomerase. This allows a primer to bind and initiate synthesis on the other strand.The telomerase maintains the lenght of the chromosome and prevents the loss of coding sequences. 

Telomeres. The DNA sequences at the chromosome ends have a TG-rich strand, such as the (TTGGGG)5o 7o Tetrahymemy and the (TTAGGG) of both human and trypanosome chromosomes. The complementary DNA strand is CA-rich. The S. cereaisiae telomers have 350 base pairs containing the sequences (TGj 3 / C3 3A) as well as one or more copies of a 6.7-kb nonrepetitive sequence and other elements. In many species the repetitive telo-meric sequences have 3 poly(G) tails at the ends of the DNA molecules. These tails are able to form G quartet structures (Fig. 5-26 and Chapter 5, Section C,4). A variety of telomere-binding proteins have been isolated. " Some of these bind to G quartet struc-... 

Telomeres are specialised short DNA sequences that form the ends of the linear DNA of eukayo-tic chromosomes. They contain many copies of the same short sequence (5 -TTAGGG-3 in humans) and their function is to protect the ends of the chromosome from degradation. The enzyme telom-erase is responsible for their synthesis which is independent of the normal replication process. 

Eukaryotic DNA is equipped with special ends called telomers. Telomers are made up of hexanucleotide sequences that repeat at the ends of the DNA. For example, human DNA features repeating AGGGTT sequences. Functionally, telomers protect the ends of chromosomes from being treated as a broken piece of DNA in need of repair. Interestingly, telomers are cut off each time the DNA is replicated, indicating a possible cellular clock that allows only a certain number of cellular replications. Telomerase is the enzyme that catalyzes the synthesis of telomers. Telomerase is present in limited quantities within certain cells such as fetal tissue, adult male germ cells, and stem cells. It is also found in over 85% of tumor cells. Researchers speculate that the telomerase activity may be linked to cancer. Propose an explanation for why telomerase activity could be associated with cancer and speculate on ways in which cancer treatments in the future may capitalize on research on this enzyme. 

Tdomerase is an enzyme that helps to maintain chromosome length during DNA replication in eukaryotes by producing the telomer TTAGGG. This enzyme has a key role in the development of cancers. 

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