The essence and functions of the telomere. Why everyone is obsessed with telomeres and what you need to know about them DNA telomeres

04.03.2024

On the topic: “Telomeres and telomerase.”

Performed:

Zhumakhanova Adina

Faculty: Public Health

Group:

Course:1

Almaty 2012

Introduction………………………………………………………………………………...3

1. Definition of telomere and telomerase…………………………………………..…4-9

1.1.Functions of telomeres………………………………………………………………...5

1.2. The problem of terminal underreplication of DNA………………….…6
2. Telomerase activity in mammals: mechanisms of regulation…………..9-10
3. Telomerase, cancer and aging…………………………………………………………….……11-13
Conclusion…………………………………………………………………………………...…..14
Literature………………………………………………………………………………..…………15

Applications…………………………………………………………………………………..16-17

Introduction.

The work is devoted to the study of the structure and functions of telomeres and telomerase, the study of their influence on cellular structure, the expression of telomerase in normal human cells, as well as the study of telomerase activity and telomere length in tumor cells.

The relevance of the work lies in studying the influence of the telomerase enzyme on the development of tumor cells, studying the possibilities of the process of continuous division due to the activity of telomerase.

Also, the relevance of the work lies in the study of the aging processes of both the organism as a whole and the cell. The work makes it possible to understand how underreplication of the terminal sections of DNA occurs, what processes occur in the cell for its division, what enzymes and proteins are involved in these processes.

The purpose of the work is to study the mechanisms accompanying cell division, study the influence of telomerase on intracellular processes and the connection between telomerase, cancer cells and cell aging.

Telomeres and telomerase

Telomeres(from ancient Greek τέλος - end and μέρος - part) - the terminal sections of chromosomes. Telomeric regions of chromosomes are characterized by a lack of ability to connect with other chromosomes or their fragments and perform a protective function. In most organisms, telomeric DNA is represented by numerous short repeats. Their synthesis is carried out by an unusual RNA-containing enzyme, telomerase.

The existence of special structures at the ends of chromosomes was postulated in 1938 by classic geneticists, Nobel Prize winners Barbara McClintock and Hermann Möller. Independently of each other, they discovered that chromosome fragmentation (under the influence of X-ray irradiation) and the appearance of additional ends lead to chromosomal rearrangements and chromosome degradation. Only the regions of chromosomes adjacent to their natural ends remained intact. Deprived of terminal telomeres, chromosomes begin to fuse with high frequency, which leads to severe genetic abnormalities. Therefore, they concluded, the natural ends of linear chromosomes are protected by special structures. G. Möller proposed calling them telomeres.

In most eukaryotes, telomeres consist of specialized linear chromosomal DNA composed of short tandem repeats. In the telomeric regions of chromosomes, DNA, together with proteins that specifically bind to telomeric DNA repeats, forms a nucleoprotein complex - constitutive (structural) telomeric heterochromatin. Telomeric repeats are very conservative sequences, for example, the repeats of all vertebrates consist of six nucleotides TTAGGG, the repeats of all insects - TTAGG, the repeats of most plants - TTTAGGG.

In subsequent years, it became clear that telomeres not only prevent the degradation and fusion of chromosomes (and thereby maintain the integrity of the genome of the host cell), but are also apparently responsible for attaching chromosomes to a special intranuclear structure (a kind of skeleton of the cell nucleus), called the nuclear matrix . Thus, telomeres play an important role in creating the specific architecture and internal order of the cell nucleus.

1.1.Functions of telomeres:

1. Participate in the fixation of chromosomes to the nuclear matrix, ensuring the correct orientation of chromosomes in the nucleus.

2. The ends of sister chromatids formed in the chromosome after the S-phase are connected to each other. The structure of telomeres, however, allows for chromatid separation in anaphase. Mutation of the telomerase RNA gene with a change in the nucleotide sequence of telomeres leads to chromatid nondisjunction.

3. Protect genetically significant sections of DNA from underreplication in the absence of telomerase.

4. The ends of broken chromosomes are stabilized in the presence of telomerase by adding functional telomeres to them. An example is the restoration of α-thalassemia gene function by adding telomeres to breakpoints in the long arm of chromosome 16.

5. Affect gene activity. Genes located near telomeres are functionally less active (repressed). This effect is called transcriptional silence or silencing. Telomere shortening leads to the abolition of the gene position effect with activation of peritelomeric genes. Silencing may be based on the action of proteins (Rap1, TRF1) that interact with telomeres.

6. Act as a regulator of the number of cell divisions. Each cell division is accompanied by a shortening of the telomere by 50-65 nucleotide pairs. In the absence of telomerase activity, the number of cell divisions will be determined by the length of the remaining telomeres.

We all know our chronological age - the one that our passport says. But at the same time, we notice that not everyone looks their age - some are younger, and some are older. Here it comes down to biological age, which certainly cannot be determined by date of birth. He speaks not only about external freshness, but also about the “youth” of our organs, vital systems, and cells. Biological age is evidence of our rational (or vice versa) use of the genetic program. But it is “recorded” in the telomeres of the cell, about which we have a lot of interesting things to learn today.

What is a telomere?

The word comes from a combination of other Greek. τέλος - “end” and μέρος - “part”. A telomere is the end region of DNA that can be visually compared to the plastic tip of a shoelace. The main distinguishing features of this region are the performance of a protective function and the lack of the ability to connect with other chromosomes or their fragments.

As we know, the DNA of a human cell has 23 chromosomes. The ends of each of them are necessarily protected by such “tips”. Chromosome telomeres protect the genetic program and are responsible for the integrity of DNA.

A little theory

Telomere is a term coined in 1932 by G. Miller. In humans, as in the vast majority of eukaryotic organisms(consisting of cells with nuclei), this is a special linear chromosomal DNA that consists of a number of tandem short repeats.

Thus, in the telomeric regions there is nucleoprotein complex - telomeric structural heterochromatin. It is formed by DNA and proteins that specifically bind to telomeric DNA repeats. The latter are conservative sequences. In us, as in all vertebrates, such a DNA repeat is a strictly defined nucleotide sequence TTAGGG. For most of the plant world it is TTTAGGG. In insects - TTAGG.

Researchers from Cardiff University have found that the maximum length of a human telomere at which chromosomes are connected to each other is 12.8 telomeric repeats.

Story

A revolutionary approach to the telomere effect was outlined not so long ago:

In 1971, Russian scientist A. M. Olovnikov first hypothesized that telomeres shorten with each cell division. The shorter this “tail”, the less the cell’s resource for further division.
Experimental confirmation of the Russian’s hypothesis was announced 15 years later English explorer Howard Cook. But here the first inconsistencies of the theory have already been identified: muscle and nerve cells do not divide, and accordingly, the number of telomeres in them cannot decrease. But at the same time, these tissues age along with all others. This question is still open and causes scientific controversy.

In the early 70s, the same A.M. Olovnikov predicted the existence of the enzyme telomerase, which is capable of “completing” ending telomeres.
In 1985, telomerase was discovered in ciliates, then in yeast, and animals. As for humans, such a valuable enzyme was found only in the reproductive system - in sperm and eggs, and also, oddly enough, in cancer cells. Telomerase makes these particles virtually immortal, as it forms an endless chain of telomeres, which allows the cell to divide forever. But the rest of our somatic tissues do not contain such an enzyme in their components, which is why their cells age and die over time.

The importance of telomeres

Speaking about the fact that these are telomeres, let us highlight their distinctive characteristics:

They do not contain genetic information.
There are exactly 92 telomeres in each cell of our body.
Responsible for genome stability.
They protect chromosomes during replication from accidental fusions and degradation.
Protect cells from aging, mutations and death.
Responsible for the structural integrity of chromosomal endings.

Telomeres and cell life

As you know, new cells are produced by dividing the mother cell in two. Accordingly, chromosomes are divided along with it, as well as telomeres. With each division the number of these protective "tips" decreases. As soon as their number becomes so small that the next division becomes impossible, the cell dies.

This is how the telomere effect affects the biological age of a person and any other living creature. As the number of these protective elements decreases, we age, since there are fewer and fewer cells left in our body that can divide and renew the tissues that make up all the vital organs.

Based on more than 8 thousand studies, the following can be stated:

Long telomeres ensure longevity.
Short telomeres are associated with the steady aging of the body, regardless of the person’s chronological age.

But is it possible to “lengthen” telomeres?

Stop aging

Having established that the telomere is one of the effective properties of controlling a person’s biological age, scientists conducted a number of experimental studies. A panacea was found in the form of telomerase - an amazing enzyme that has the ability to complete almost spent telomeres. As soon as the protective “tip” of DNA is restored, the cell has a resource for further division, and therefore for renewing the tissues of our organs, which slows down the aging of the body as a whole.

How to “lengthen” telomeres? With the help of an enzyme that synthesizes the nucleotide sequence TTAGGG at the terminal regions of DNA (telomeres) - telomerase. It has a number of worthy properties:

Creation of a matrix on which critically short telomeres are completed.
Extending cell life.
Protecting the entire body from premature aging.
Preventing telomere length shortening.
Enables an “old” cell to regain its youth - to function and divide like a young one.

Telomerase - the elixir of youth?

In 1997, scientists from the University of Colorado (USA) obtained the miracle enzyme gene. Already in 1998 their colleagues from the University of Texas Southwestern Medical Center(Dallas) introduced the telomerase gene into human somatic cells - skin, vascular, and visual epithelium. That is, those that by their nature do not contain this enzyme.

So what's the end result? The genetically modified cell remained quite viable while telomerase did its job! The enzyme, as in germ and cancer cells, “sewed” telomeric nucleotide sequences, which is why divisions of this particle of the body did not in any way affect the length of the telomere. So far, in this way it has been possible to extend the life of the cell by 1.5 times.

For this discovery in 2009, scientists D. Szostak, K. Greider, E. Blackburn were awarded the Nobel Prize, which is why there is no doubt about the seriousness of the research.

How to lengthen telomeres yourself?

In many countries, you can get a telomere test, a test that reveals the length of telomeres in your cells. But what to do if such a resource runs out?

Scientists have discovered that we ourselves can lengthen our telomeres. Moreover, it’s nearby completely uncomplicated solutions:

Healthy eating.
Avoiding prolonged stressful situations.
Confidence in the support of family and friends.

And this is not another call for a healthy lifestyle, but data from the results of research by scientists at the University of California (San Francisco). The experiment lasted 5 years. The “test subject” was a group of men who were diagnosed with prostate cancer at the first stage. 10 participants changed their lives in accordance with the above, and 25 did nothing.

As a result, those who switched to a healthy life increased their telomere length by 10%. And this is an average result: the more positive changes a person made, the more actively his telomere chain lengthened. But for the other part of the observed group, its length decreased by an average of 3%.

Cancer and immortality

Purely hypothetically, telomerase, introduced into the cells of the human body, makes them eternal, and himself immortal. But we should not forget that this is a cancer enzyme, the main cause of the malignant degeneration of matter. Oncology is difficult to defeat because cancer cells are immortal. And it is telomerase that makes them like this.

Hence the question: “Won’t cells modified by this enzyme degenerate into cancer cells?” Nature cannot be deceived by humans: theoretically, we can make the body immortal, but it will inevitably die from cancer.

Thus, the effect of telomere length on our biological age is recognized by the scientific community, although the theory is highly controversial. And also simple rules of life have been identified that will help us enjoy our youth longer without any elixirs of immortality.

Colored human chromosomes and their telomeres

The location of telomeres on a chromosome

Telomeres are the ends of chromosomes. Telomeric regions of chromosomes are characterized by a lack of ability to connect with other chromosomes or their fragments and perform a protective function.

The term “telomere” was proposed by G. Möller in 1932.

In most eukaryotes, telomeres consist of specialized linear chromosomal DNA composed of short tandem repeats. In the telomeric regions of chromosomes, DNA, together with proteins that specifically bind to telomeric DNA repeats, forms a nucleoprotein complex - constitutive telomeric heterochromatin. Telomeric repeats are very conservative sequences, for example, the repeats of all vertebrates consist of six nucleotides TTAGGG, the repeats of all insects TTAGG, the repeats of most plants TTTAGGG.

Scientists from Cardiff University have found that the critical length of the human telomere, at which chromosomes begin to connect with each other, is 12.8 telomeric repeats.

With each division cycle, the cell's telomeres shorten due to the inability of DNA polymerase to synthesize a copy of DNA from the very end. It can only add nucleotides to an already existing 3'-hydroxyl group. For this reason, DNA polymerase needs a primer to which it can add the first nucleotide. This phenomenon is called terminal underreplication and is one of the most important factors of biological aging. However, as a result of this phenomenon, telomeres should shorten very slowly - several nucleotides per cell cycle, i.e. for the number of divisions corresponding to the Hayflick limit, they will be shortened by only 150-300 nucleotides. Currently, an epigenetic theory of aging has been proposed, which suggests that telomere erosion is accelerated tens and hundreds of times due to recombinations in their DNA caused by the functioning of cellular DNA repair systems. The activity of these systems is initiated by DNA damage, primarily caused by mobile elements of the genome that are derepressed with age, which predetermines aging as a biological phenomenon.

There is a special enzyme called telomerase, which, using its own RNA template, completes telomeric repeats and lengthens telomeres. Telomerase is blocked in most differentiated cells, but is active in stem and germ cells.

For the discovery of the protective mechanisms of chromosomes against terminal underreplication using telomeres and telomerase, the Nobel Prize in Physiology or Medicine was awarded in 2009 to an Australian working in the United States, Elizabeth Blackburn, an American, Carol Greider, and her compatriot Jack Szostak.

Have you heard anything about telomeres? We won't believe it at all. But if you really haven't, then it's obviously time for you to catch up, because this is the number one topic in the field of anti-aging today. And this is not another project focused on facial and age-related wrinkles, but a large-scale campaign to study the microscopic elements in our body that determine the features and speed.

It is believed that telomeres - the ends of chromosomes - can be changed to literally turn back time. Recently, telomeres have received more and more attention, and this is the merit of the American cytogeneticist Elizabeth Blackburn, who in 2009, together with colleagues Carol Greider and Jack Szostak, received the Nobel Prize in Medicine “for the discovery of the mechanisms protection of chromosomes by telomeres and the enzyme telomerase.” Simply put, everything here is very, very serious.

The bottom line is that short telomeres are associated with a shorter human lifespan, while the opposite is true for long telomeres. What else you need to know about these amazing sections of DNA that protect chromosomes from damage, The Guardian asked Elizabeth Blackburn herself. And we summarized the most interesting of the interviews in this material.

What are telomeres?

“If you think of the chromosomes, which contain genetic material, as shoelaces, the telomeres are the little protective tips on them. They are made up of repeated short sequences of DNA and tend to wear out with age. When telomeres cannot protect chromosomes properly, cells begin to perform less well. This provokes physiological changes in the body that increase the risks of conditions and diseases associated with aging: diabetes, cancer and many others.

But in a sense, this process can be influenced, because it occurs in the body of each of us at a certain speed, which can change. An enzyme called telomerase can add DNA to the ends of chromosomes to slow, prevent and partially reverse their shortening.”

Can telomeres prevent aging?

“Every person has an important indicator of health - the number of years during which he remains active and pain-free. Aging and the body’s transition from the sphere of health to the sphere of disease is facilitated by the shortening of telomeres. But work on telomeres is not being done with the goal of increasing telomeres, although all this, of course, is connected, but with the goal of preventing some age-related diseases.”

How quickly does telomere length change?

“Sometimes you can see the effect in a few weeks. But most often these are long-term changes that occur over a period of one to ten years.”

How to preserve telomeres?

“The good news is that you don't have to go to the gym for three hours a day or once a week. People who exercise moderately—about three times a week for 45 minutes—have the same telomeres as professional athletes. Mixing different types of activities is also a good idea. One study found that people who don't focus on just one sport have longer telomeres.

At the same time, many studies in which people suffering from stress took part show that the size of their telomeres depends on how severely this stress is experienced by the person himself. But taking care of telomeres is especially important for people who lead. Even 10-15 minutes of light exercise will benefit telomeres if you sit most of the time.”

What diet is good for telomeres?

“Having adequate amounts of telomere in the menu appears to be associated with improved telomere health, and is the easiest way to maintain normal telomeres. There is some data on the benefits, but, in my opinion, they are not entirely complete. One thing is clear: the telomeres of a person who eats healthy will be longer than the telomeres of someone who consumes large amounts of processed meat, sugary drinks and white bread.”

How does marriage affect telomere length?

“There is a general trend for longer telomeres among people with regular partners. But we also looked at women who were previously in marriages in which they experienced regular domestic violence, and their telomeres were of course shorter, and this correlated with the number of years (the longer the marriage, the shorter the telomeres). This is probably due to the fact that the women were in a stressful situation for a long period. In children, one study suggested that children may help telomere health, but this has not been confirmed by independent research, so it is too early to talk about a trend.”

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Telomere functions

1. Some functions can be conditionally designated as mechanical.

a) Telomeres are involved in the fixation of chromosomes to the nuclear matrix. This is important for the correct orientation of chromosomes in the nucleus, and this circumstance is especially evident in meiosis.

At the zygotene stage of meiosis prophase, directed movements of the ends of chromosomes occur on the surface of the nuclear membrane - so that the ends of homologous chromosomes close and pairing (conjugation) of these chromosomes in strictly homogeneous areas begins from them.

b) In addition, telomeres link the ends of sister chromatids (formed in the chromosome after the S phase) to each other. Perhaps this linkage occurs due to hybridization of sister DNA telomeres.

At the same time, the structure of telomeres is such that it allows chromatids to separate in anaphase. However, a mutation is possible (at the level of the telomerase RNA gene; see below), which changes the nucleotide sequence of telomeres; then chromatid segregation is blocked.

2. The functions of the second group are stabilizing.

a) The most important of them is already familiar to us: if there is no telomerase (or ALT) in the cell, then the presence of telomeres protects genetically significant sections of DNA from underreplication.

b) If there is telomerase activity in the cell, then another possibility arises - stabilization of the ends of broken chromosomes.

Thus, when a chromosome is accidentally broken, fragments are formed, at one or both ends of which there are no telomeric repeats. In the absence of telomerase, these fragments undergo fusion and degradation, which blocks the cell cycle and leads the cell to death.

In the presence of telomerase, telomeric DNA is attached to the break sites. This stabilizes the chromosomal fragments and allows them to function.

In particular, this phenomenon was found in patients with a-ta-lassemia: breaks in chromosome 16q occur in the a-globin genes, and telomeric repeats are added to the damaged end.

3. Effect on gene expression.

Another interesting property of telomeres is designated as ff t position: the activity of genes located next to telomeres is reduced (repressed). This effect is often referred to as transcriptional silencing or silencing.

With significant shortening of telomeres, the position effect disappears and peritelomeric genes are activated.

a) Silencing may result from the action of proteins (such as Rapl or TFR1) that interact with telomeres. Moreover, as already noted, these proteins reduce the accessibility of telomeric DNA for a number of enzymes.

b) On the other hand, the position effect may be due to proximity to the nuclear envelope. Thus, according to the hypothesis of A.M. Olovnikov, Ca2" channels may be located in this shell, and the flow of Ca" ions affects the interaction of proteins with nearby genes.

The position effect can also affect internal genes if one of these genes becomes a transposon (a gene capable of moving to another piece of DNA) and is inserted into the telomeric region. Or if a chromosome breaks and telomeric repeats form at the ends of the break: with the help of the latter, it is possible that telomeric proteins bind and attach to the nuclear membrane.

4. “Counting” function.

Finally, the telomeric sections of DNA act as a clock device (the so-called replicameter), which counts the number of cell divisions after the disappearance of telomerase activity. Indeed, as already noted, each division leads to a shortening of the telomere by 50-65 nt. P.

Moreover, what is much more important for the cell is not how many divisions have already passed, but how much is left before the critical shortening of the telomere. Therefore, we can say that telomeres are a device that determines the number of divisions that a normal cell can make in the absence of telomerase.

Having reached a critically short length, telomeres lose the ability to perform all or many of the above functions. The cell cycle is disrupted and the cell eventually dies.

Telomerase activity is the activity of telomerase, an enzyme that, using a special mechanism, synthesizes telomeric DNA, and thereby affects cell growth. High telomerase activity is characteristic of germ cells and stem cells. As stem cells begin to differentiate, telomerase activity decreases and their telomeres begin to shorten.

Telomeres (from the ancient Greek fElpt - end and mespt - part) are the terminal sections of chromosomes. Telomeric regions of chromosomes are characterized by a lack of ability to connect with other chromosomes or their fragments and perform a protective function.

The term “telomere” was proposed by G. Möller in 1932.

Scientists from Cardiff University have found that the critical length of the human telomere, at which chromosomes begin to connect with each other, is 12.8 telomeric repeats.

With each division cycle, the cell's telomeres shorten due to the inability of DNA polymerase to synthesize a copy of DNA from the very end. It is only able to add nucleotides to the already existing 3"-hydroxyl group. For this reason, DNA polymerase needs a primer to which it could add the first nucleotide. This phenomenon is called terminal underreplication and is one of the most important factors of biological aging. Moreover However, as a result of this phenomenon, telomeres should shorten very slowly - several (3-6) nucleotides per cell cycle, i.e., for the number of divisions corresponding to the Hayflick limit, they will shorten by only 150-300 nucleotides.Currently, epigenetic theory of aging, which suggests that telomere erosion accelerates tens and hundreds of times due to recombinations in their DNA caused by the functioning of cellular DNA repair systems. The activity of these systems is initiated by DNA damage, caused primarily by mobile elements of the genome that are derepressed with age, which predetermines aging as a biological phenomenon.

There is a special enzyme - telomerase, which, using its own RNA template, completes telomeric repeats and lengthens telomeres. Telomerase is blocked in most differentiated cells, but is active in stem and germ cells.

For the discovery of the protective mechanisms of chromosomes against terminal underreplication using telomeres and telomerase in 2009, the Nobel Prize in Physiology or Medicine was awarded to Australian Elizabeth Blackburn working in the United States, American Carol Greider and her compatriot Jack Szostak. Szostack).

In many modern textbooks, telomeres are called specialized terminal regions of linear chromosomal DNA, consisting of repeatedly repeated short nucleotide sequences. This definition is incomplete. Telomeres also include many proteins that specifically bind to telomeric DNA repeats. Thus, telomeres (as well as all other regions of the eukaryotic chromosome) are built from deoxynucleoproteins (DNP), that is, complexes of DNA with proteins.

In subsequent years, it became clear that telomeres not only prevent the degradation and fusion of chromosomes (and thereby maintain the integrity of the genome of the host cell), but are also apparently responsible for attaching chromosomes to a special intranuclear structure (a kind of skeleton of the cell nucleus), called the nuclear matrix (Fig. 1). Thus, telomeres play an important role in creating the specific architecture and internal order of the cell nucleus. Moreover, we will show that the presence of special telomeric DNA at the ends of chromosomes allows us to solve the so-called problem of terminal DNA underreplication.

Telomeric DNA came to the attention of molecular biologists relatively recently, when effective methods for determining the nucleotide sequence of nucleic acids were developed. The first objects of research were single-celled protozoa (the ciliated ciliate Tetrahymena, in particular), since due to the structural features of the nuclear and chromosomal apparatus they contain several tens of thousands of very small chromosomes and, therefore, many telomeres in one cell (for comparison: in higher eukaryotes on there are less than one hundred telomeres per cell).

Repeatedly repeating blocks in the telomeric DNA of protozoa consist of only six to eight nucleotide residues. In this case, one DNA chain is highly enriched in guanylic acid residues (G-rich chain; in tetrahymena it is built from TTGGGG blocks), and the complementary DNA chain is correspondingly enriched in cytidylic acid residues (C-rich chain).

In yeast, the repeating blocks in telomeric DNA are noticeably longer than in protozoa, and often less regular. Imagine the surprise of scientists when it turned out that human telomeric DNA is built from TTAGGG blocks, that is, it differs from the simplest by only one letter in the repeat. Moreover, telomeric DNA (or rather, their G-rich chains) of all mammals, reptiles, amphibians, birds and fish are built from TTAGGG blocks. The telomeric DNA repeat in plants is equally universal: not only in all land plants, but even in their very distant relatives - seaweeds - it is represented by the sequence TTTAGGG. However, there is nothing particularly surprising here, since telomeric DNA does not encode any proteins (it does not contain genes), and in all organisms telomeres perform universal functions, which were discussed above. True, as often happens in living nature, there are rare but important exceptions to this general rule. The best known of these is the telomeric DNA of the fruit fly Drosophila. It is represented not by short repeats, but by retrotransposons - mobile genetic elements (for more information about mobile genetic elements and the role of retrotransposons in the formation of telomeres, see the articles by V.M. Glaser “Homologous genetic recombination” and “Genetic recombination without homology: processes leading to rearrangements in the genome" and V.A. Gvozdev "Mobile DNA of eukaryotes. Parts 1-2" in the "Soros Educational Journal" (1998. No. 7, 8).

A very important characteristic of telomeric DNA is its length. In humans, it ranges from 2 to 20 thousand base pairs (kb), and in some species of mice it can reach hundreds of kb.

It has been observed that in many species, double-stranded telomeric DNA contains a single-stranded “tail” at the very end. This single-stranded region of telomeric DNA is represented by its G-rich chain and ends with a free 3"-hydroxyl group. Accordingly, telomere proteins are usually divided into two groups: proteins that are associated with single-strand telomeric DNA, and proteins associated with double-stranded telomeric DNA. These proteins are studied very intensively, but we still know little about them. There is no doubt that telomere proteins are involved in all functions of telomeres, maintaining their structure and regulating the length of telomeric DNA (as we will see below, telomere length is an extremely important parameter). It has been established that Some of the proteins associated with double-stranded telomeric DNA regulate the activity of certain genes, increasing or suppressing their expression. An example is the yeast protein Rap1p. This DNA-binding protein is undoubtedly involved in the regulation of telomeric DNA length. At the same time , even being part of a telomere, it is involved in the activation and repression of transcription.This means that changes or disturbances in the structure of telomeres can affect not only their own functions, but also the expression of vital genes located in other regions of the chromosomes. In addition, proteins important for maintaining the overall structure of chromosomes are located on DNA immediately adjacent to telomeric DNA (sometimes called subtelomeric DNA).

Telomeres and the problem of “terminal underreplication of DNA”

telomere chromosome telomerase aging

It is known that DNA polymerases, when synthesizing a daughter DNA strand, read the parent strand in the direction from its 3" end to its 5" end. Accordingly, the daughter chain is synthesized in the 5" 3" direction. The enzyme cannot catalyze the synthesis of a DNA chain in the opposite direction (Fig. 2). In addition, DNA polymerase begins synthesis only with a special RNA primer - a short RNA primer, complementary to DNA. After DNA synthesis is complete, the RNA primers are removed, and the gaps in one of the daughter DNA strands are filled in by DNA polymerase. However, at the 3" end of the DNA such a gap cannot be filled, and therefore the 3" terminal sections of DNA remain single-stranded, and their 5" terminal sections remain under-replicated. Hence it is clear that each round of chromosome replication will lead to their shortening. It is clear , that first of all the length of telomeric DNA should be reduced.

A.M. was the first to draw attention to the problem of “terminal underreplication of DNA”. Olovnikov in 1971. He hypothesized that the loss of terminal DNA sequences due to their underreplication leads to cell aging. In other words, it was assumed that the process of telomere shortening is the clockwork mechanism that determines the replicative potential of a “mortal” cell, and when the length of telomeres becomes dangerously short, this mechanism prevents further cell division. A.M. Olovnikov also suggested that in non-aging cells (and these include, in addition to cancer cells, germ cells, stem cells and other generative cells) there should be a specialized enzymatic system that controls and maintains the length of telomeric DNA.

Hypothesis A.M. Olovnikova found convincing confirmation in subsequent years. Firstly, it was found that the telomeres of normal (that is, doomed to aging) cells are indeed shortened by 50-60 nucleotide units with each cell division. Secondly, in 1984, E. Blackburn and E. Grider isolated an enzyme that, using a mechanism different from the mechanism of reactions underlying DNA replication, synthesizes telomeric DNA. This enzyme was named telomerase.

How does telomerase work?

So, the main purpose of telomerase is to synthesize tandemly repeating DNA segments that make up the G-strand of telomeric DNA. Thus, it belongs to the class of DNA polymerases, and it turns out that telomerase is an RNA-dependent DNA polymerase or reverse transcriptase. Enzymes of this class, which synthesize DNA on RNA templates, are very well known to molecular biologists. They are encoded and contained in retroviruses (for example, in the human immunodeficiency virus, which causes AIDS) and serve to synthesize DNA copies of their genomes, which in a retrovirus is represented by RNA. In the cellular genome, reverse transcriptases are encoded in retrotransposons.

RNA, used by telomerase to synthesize telomeric DNA as a template, is part of this enzyme. This is the uniqueness of telomerase: today it is the only known RNA-containing reverse transcriptase. Telomerase RNAs vary greatly in length and structure among different organisms. Protozoan telomerase contains RNA 150-200 nucleotide residues (nt) long, human telomerase RNA is 450 nt long, while yeast telomerase contains an abnormally long RNA (about 1300 nt). Like any other RNA in a cell, telomerase RNA has a specific secondary and tertiary structure. The secondary structure of isolated telomerase RNA has been reliably established only for protozoan telomerase. The spatial structure of telomerase RNA as part of the enzymatic complex is still unknown.

The template region is represented only once in telomerase RNA. Its length does not exceed the length of the two repeats in telomeric DNA that it encodes and to which it is, of course, complementary.

Since telomerase synthesizes DNA segments that are repeated many times using only one segment of its RNA, it must have the ability to periodically (after completion of the synthesis of each repeat) move (translocate) the template region to the 3" end of the synthesized telomeric DNA. The energy source for this The movement is apparently carried out by the synthesis reaction of the telomeric DNA chain itself, since deoxynucleoside triphosphates, the substrates of this reaction, are high-energy substances.

and in the first stage, telomerase finds the 3" end of telomeric DNA, with which part of the template region of telomerase RNA forms a complementary complex. In this case, telomerase uses the 3" end of chromosomal DNA as a primer. Next comes the turn of the RNA-dependent DNA polymerase activity of telomerase. It is provided by a special subunit of telomerase, which in the structure of its catalytic center is in many ways similar to the reverse transcriptases of retroviruses and retrotransposons. When the synthesis of the DNA repeat ends, translocation occurs, that is, the movement of the template and protein subunits of the enzyme to the newly synthesized end of the telomeric DNA, and the whole cycle is repeated again.

Familiarity with even a very schematic description of the mechanism of the telomerase reaction leads to the conclusion that two components - reverse transcriptase and telomerase RNA - cannot be used to carry it out. There is no doubt that it must contain a subunit responsible for searching for and binding the 3" end of the chromosome (and thus performing a kind of anchor function); a subunit responsible for translocation; subunits that bind the reaction product (single-strand DNA). The composition of telomerase usually also contains a protein subunit with nuclease activity, which apparently cleaves off several nucleotides one after another from the 3" end of telomeric DNA until at this end there is a sequence complementary to the desired section of the template segment of telomerase RNA. These telomerase subunits, which perform various functions during the synthesis of the G-strand of telomeric DNA, are depicted in Fig. 4, which shows the hypothetical structure of yeast telomerase. It must be emphasized once again that the complete protein composition of the enzyme is not yet known in any case. Therefore, in Table. Table 1 shows the characteristics of only well-studied protein subunits of several telomerases.

The wide distribution of telomerase among eukaryotes suggests that the mechanism of telomeric DNA synthesis that we observe in modern organisms arose a very long time ago. Moreover, an evolutionary genetic comparative analysis of the nucleotide sequences of the genes for the catalytic subunits of telomerase and other reverse transcriptases shows that this mechanism could have existed even before the appearance of the first eukaryotic cells.

The C-strand of telomeric DNA is synthesized using conventional DNA polymerase. Therefore, the 3"-terminal region of the G-strand, on which, apparently, the RNA primer was originally located, ultimately remains in a single-stranded state (that is, in principle, it is ready for telomerase to grow a new repeat on it).

Telomerase, cancer and aging

Let us consider the data on the length of telomeric DNA and telomerase activity in various human cells given in Table. 2.

High telomerase activity is observed in human germ cells throughout his life. Accordingly, their telomeres consist of the largest number of DNA repeats and contain all the necessary proteins for normal cell proliferation. A similar situation is observed for stem cells. Let us remember that stem cells divide indefinitely. However, a stem cell always has the opportunity to give rise to two daughter cells, one of which will remain a stem cell ("immortal"), and the other will enter the process of differentiation. Thanks to this, stem cells serve as a constant source of a variety of cells in the body. For example, bone marrow stem cells give rise to hematopoiesis, the process of formation of blood cells, and various skin cells originate from the basal cells of the epidermis. As soon as the descendants of germ cells or stem cells begin to differentiate, telomerase activity drops and their telomeres begin to shorten. In cells whose differentiation is completed, telomerase activity drops to zero, and, as we have already noted, with each cell division they inevitably approach the state of senescence (stop dividing). Following this, a crisis occurs and most cells die. This picture is characteristic of the vast majority of known eukaryotic cell cultures. However, there are rare but important exceptions: telomerase activity is found in such “mortal” cells as macrophages and leukocytes.

It was recently found that normal somatic cells lack telomerase activity because the expression of the gene for its catalytic subunit (reverse transcriptase) is completely suppressed in them. Other components of telomerase, including telomerase RNA, are formed in these cells, although in smaller quantities than in their “immortal” progenitors, but constantly (or, as they say, constitutively). The discovery of this important fact by J. Shea, W. Wright and their collaborators became the basis for that sensational work to overcome the “Hayflick limit”. Indeed, everything else was a matter of technique (albeit a very difficult one).

Telomerase reverse transcriptase genes were introduced into normal somatic cells using special vectors constructed from viral DNA. The level of gene expression in a eukaryotic cell depends on many factors, including proteins - transcription factors that bind to specialized sections of DNA located in the chromosome adjacent to this gene. The genomes of viruses that need to quickly multiply in the host cell contain DNA sections that can greatly enhance the expression of a particular gene. The researchers made sure that in their designs the human telomerase reverse transcriptase gene was surrounded by precisely such sections of viral DNA. The results of their experiments can be summarized briefly: cells in which telomerase maintained telomere length at a level characteristic of young cells continued to divide even when control cells (without telomerase) became decrepit and died.

In this and similar works, the absence of cancer cells in the cell culture is especially carefully monitored. It is known that the cells of most cancer tumors studied to date are characterized by fairly high telomerase activity, which maintains telomere length at a constant level. This level is noticeably lower than, for example, that of embryonic cells, but it is sufficient to ensure unlimited division of cancer cells in culture. There is a hypothesis, which has many supporters, suggesting that the loss of telomerase activity in somatic cells of modern organisms is a property acquired in the process of evolution that protects them from malignant degeneration.

The relatively short telomere length of most cancer cells suggests that they originate from normal cells that have reached a pre-crisis state. As we have already noted, this condition is characterized by dysregulation of many biochemical reactions. In such cells, numerous chromosomal rearrangements occur, which also lead to malignant transformation (for more details on the origin of malignant tumors, see the article by G.I. Abelev “What is a tumor”: Soros Educational Journal. 1997. No. 10). Most of these cells die, but in some of them, as a result of random mutations, constant expression of telomerase genes can be activated, which will maintain the length of telomeres at a level necessary and sufficient for their functioning.

For some time, it was puzzling that about a fifth of the cancer tumors and cells analyzed did not contain active telomerase at all. It turned out, however, that the length of telomeres in them is maintained at the proper level. Thus, in these cells there is a different (not telomerase, but rather recombination) mechanism for the formation of telomeric DNA (see the article by V.M. Glaser “Homologous genetic recombination”: Soros Educational Journal. 1998. No. 7). In other words, such cells are in the same series of exceptions to the rule as the Drosophila.

Conclusion

What practical conclusions follow from what has been learned so far about the connection between telomerase activity, cancer growth and cell aging. It would seem that they lie on the surface: if you don’t want to grow old, activate telomerase; If you want to kill a cancerous tumor, kill the telomerase in it first.

The lightness of the first conclusion (namely, it was picked up by the media) is obvious: there is a huge distance between a cell culture and a cellular tissue, and even more so an organism. The time has not yet come to seriously discuss the problem of obtaining transgenic human organs for transplantation into sick people (although theoretically this is, of course, possible). And most importantly, the aging process not only of the organism, but also of the cell is an extremely complex set of changes in many biochemical reactions, and it can hardly be reversed by acting only on one of them. At the same time, there are very real plans to activate telomerase in skin cells that are transplanted into patients with severe burns, and thereby activate their growth. Or try to “rejuvenate” retinal cells in the same way by taking them from a patient suffering from retinal clouding (a common disease in older people that leads to blindness), and then returning them back.

As for the development of methods for selective suppression of telomerase activity in cancer tumors, this is now an important direction in the search for new means of combating malignant diseases. So far, most of the work is related to testing inhibitors of reverse transcriptases (catalytic subunits of telomerase). The experience of fighting AIDS, where they are trying to solve a similar problem, suggests that there are certain hopes of finding such a cure. The main difficulty is that the catalytic subunit of telomerase is one of the DNA polymerases, and the desired inhibitor must be aimed specifically at telomerase DNA synthesizing activity. Otherwise it will be toxic to normal cells.

Recent studies that describe selective suppression of telomerase RNA, which causes the death of cancer cells in culture, seem more promising. In normal cells, as we noted above, telomerase RNA is synthesized, but these cells lack telomerase activity and, most likely, they do not need telomerase RNA.

The study of the fine structure of telomeres and the mechanism of action of telomerase is still only in its early stages. However, they attract great interest from researchers working in various fields of biology and medicine, and new interesting discoveries can be expected in the near future.

Literature

1. Alberts B., Bray B., Lewis J. et al. Molecular biology of cells. M.: Mir, 1994. T. 1-3.

2. Telomere, telomerase, cancer and aging // Biochemistry. 1997. T. 62, No. 11.

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