What are telomeres? |

Eukaryotic Chromosomes and Telomeres

The DNA of bacteria and other related simple organisms (prokaryotes) consists of one double-stranded DNA molecule. Structurally and functionally, the prokaryotic chromosome contains one copy of most genes as well as DNA regions that control expression of these genes. Prokaryotic gene expression depends primarily upon a cell’s moment-to-moment needs. An entire prokaryotic chromosome, its genome, usually encodes about one thousand genes.

The genomes of eukaryotes
are much more complex and may include 100,000 or more genes. The number of chromosomes in different types of eukaryotes can range from just a few to several hundred. Each of these huge DNA molecules is linear rather than the circular molecule seen in prokaryotes. In addition, many individual segments of eukaryotic DNA exist in multiple copies. For example, about 10 percent of the DNA of a eukaryote consists of “very highly repetitive segments” (VRS’s), units that are less than ten deoxyribonucleotides long that are repeated up to several million times per cell. DNA segments that are several hundred deoxyribonucleotide units long represent about 20 to 25 percent of the DNA. They are repeated one thousand times or more per cell. The rest of the eukaryote DNA (from 65 to 70 percent of the total) consists of larger segments repeated once or a few times, the genes, and the DNA regions that control the expression of the genes.

Much of the repetitive DNA, called satellite DNA,
does not seem to be involved in coding for proteins or RNAs involved in making proteins. Telomeres are part of this DNA and consist of pieces of DNA that are several thousand deoxyribonucleotide units long, found at both chromosome ends. They are believed to act to stabilize the ends of chromosomes and protect them from exonuclease enzymes that degrade DNA from the ends. Researchers have concluded this for two reasons. First, the enzymes that make two chromosomes every time a cell reproduces are unable to operate at the chromosome ends. Hence, the repeated reproduction of a eukaryote cell and its DNA will lead to the creation of shorter and shorter chromosomes, a process that can cause cell death when essential genes are lost. Second, as organisms age, the telomeres of their cells become shorter
and shorter.

Telomerase Enzymes

When chromosomes are replicated in preparation for cell division, the internal segments are replicated by a complex process involving the enzymes primase and DNA polymerase. Primase lays down a small segment of RNA on the template strand of DNA, and DNA polymerase uses the primer to start replication. Making the end of a linear chromosome is a problem, however, because primers cannot consistently be produced at the very ends of the chromosomes. Consequently, with each cell division a small portion of the ends of newly replicated chromosomes is single-stranded and is trimmed off by exonucleases. This problem is solved by enzymes known as telomerases, which add telomeres to eukaryote chromosomes. Each telomerase contains a nucleic acid subunit, known as human telomerase RNA component (hTERC), which is about 150 ribonucleotides long. This length is equivalent to 1.5 copies of the appropriate repeat in the DNA telomere to be made.

The catalytic subunit of telomerase is called the human telomerase reverse transcriptase
enzyme (hTERT). hTERT uses this piece of RNA as a template to make the desired DNA strand of the telomere. At the telomere, these two telomerase subunits are present along with other telomere-associated proteins and telomere-binding proteins, which affect the localization and activity of telomerase. The exact mechanism by which the DNA strand is made is not yet confirmed; however, it is thought that telomerase uses its RNA sequence (AACCCC) to bind to the target complementary DNA sequence (TTGGGG) at the end of the parent DNA strand. The polymerase activity and RNA template allows for the addition of nucleotides to the telomere. After six new nucleotides have been added (TTGGGG), the telomerase unit moves down the DNA parent strand and continues adding nucleotides. How the telomerase in any given
species identifies the correct length of telomere repeat for a specific chromosome is not clearly understood, although it may be regulated by various telomere-associated proteins. After the addition of telomeric DNA sequences, the parent DNA strand will be longer than the complementary daughter DNA strand. This so-called “end-replication problem” is hypothesized to be solved by the enzyme primase, which uses the extended telomere to create a primer on the daughter DNA strand that DNA polymerase can then extend to fill in the gap.

Telomerase activity can be lost in certain strains of simple eukaryotes, such as protozoa. When this happens to a given cell line, each cell division leads to the additional shortening of its telomeres. This procedure continues for a fixed number of cell divisions; it then ends with the death of the telomerase-deficient cell line in a process known as replicative senescence.

A related observation has been made in humans. It has been shown that when human fibroblasts are grown in tissue culture, telomere length is longest when cells are obtained from young individuals. They are shorter in cells taken from the middle-aged, and very short in cells taken from the aged. Similar observations have been made with the fibroblasts from other higher eukaryotes as well as with other human cell types. In contrast, the process of telomere shortening does not happen when germ-cell lines—which in the whole organism produce sperm and ova—are grown in tissue culture. This suggests a basis for differences in longevity of the germ cells and the somatic cells that make up other human tissues.

Impact and Applications

The discovery and study of telomeres and telomerases produced new insights into DNA synthesis, the number of times a cell can reproduce, and the aging process. The circular DNA of bacteria (which are prokaryotes) allows them to undergo many more cycles of reproduction than the somatic cells of the eukaryotes. The linear eukaryote chromosome may have evolved because such DNA molecules were too large to survive as circular molecules given their rigidity and fragility. In addition, the observation of telomere shortening in simple and complex eukaryotes raises the fascinating possibility that the life spans of organisms may be related to the conservation of telomeres associated with the replication of these structures by telomerases.

The role of telomere length in longevity is uncertain, but apparently significant. Cells grown in cell culture typically divide only a predictable number of times, and once this limit is reached they can no longer divide. At the same time, telomere length shortens with each division. Sometimes, cells in culture will go through what is called a “crisis,” after which they become “immortalized” and are able to divide an indefinite number of times. Immortal cells also actively express telomerases and maintain constant telomere lengths. Cancer cells typically exhibit these same characteristics. A better understanding of telomeres and telomerase expression might provide insights into aging and cancer, leading to a potential cure for cancer and age-related diseases.

Multiple strategies to inhibit the telomere/telomerase complex are under investigation. For example, antisense oligonucleotides and gene-directed enzyme pro-drug therapy have been tested in vitro and in animals for inhibitory effects on hTERC. The first clinically tested hTERC inhibitor is GRN163L, which is being studied in patients with solid tumors and lymphoproliferative diseases. Small molecule inhibitors of hTERT have also been identified; however, issues with specificity and lengthy time to produce cell death have hampered clinical development. Vaccines are another strategy to target hTERT. In this case, either short protein fragments (known as peptides or epitopes) or whole cells engineered to overexpress hTERT are given to patients with adjuvants (molecules that help stimulate immune responses) so that tumor cells expressing hTERT are killed by cytotoxic T lymphocytes (also known as killer-T cells).

Key terms




eukaryote


:

a unicellular or multicellular organism with cells that contain a membrane-bound nucleus, multiple chromosomes, and membrane-bound organelles





prokaryote


:

a unicellular organism with a single chromosome and lacking a nucleus or any other membrane-bound organelles





Bibliography

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