What are Telomeres?

Table of Contents

• Telomeres – Definition
• Telomere Structure
• Telomere Function
• Shortening and Role in Ageing and Cancer
• Telomere replication
• Importance of Telomeres
• Frequently Asked Questions

Telomeres – Definition

What is a Telomere?

Telomeres are the repetitive nucleotide sequences that are present on the chromosomal endings. It is present in eukaryotic chromosomes. These are non-coding regions and do not code for any protein.

Muller coined the term ‘telomere’. Barbara McClintock showed that broken chromosomes have sticky ends as compared to natural chromosomal ends, which are stable and do not show the tendency to fuse. Telomeres ensure that the chromosomes do not stick together and protect from deterioration.

Telomeres contain non-coding repetitive sequences, which are rich in Guanine nucleotides. In humans, the repetitive sequence is 5’-TTAGGG-3’, which is repeated multiple times.

Telomere Structure

Telomeres are present at the chromosomes terminal in all the eukaryotic organisms. It consists of short nucleotide sequences, which are repeated multiple times. They do not code for any protein.

The sequence, which gets repeated varies in different species. The number of copies of the repeated units differs from chromosomes to chromosomes or even in the same chromosomes of different cells. In normal somatic cells of humans, around 500 to 3000 repeats are present, which shorten gradually. In some of the cells such as germ line cells, cancerous cells, telomeres do not shorten with age.

The basic pattern of the repeating unit is 5’-T_1-4A_0-1G_1-8-3’ in most of the species. Telomeres mostly end with guanine rich single strands at 3’ end. In humans, t-loops are formed at the end by 3’ single strands. The protein Shelterin protects the telomeres from degradation or modification.

The repeating sequence in humans is 5’-TTAGGG-3’. In the plant Arabidopsis thaliana, it is TTTAGGG. In most of the species, additional sequences associated with telomeres are found.

Telomeres with the tandem repeats of TTAGGG are very common in vertebrates. It is found in more than a hundred species including birds, reptiles, amphibians, fishes and mammals.

In most of the prokaryotes, DNA is circular so telomeres are not found. In some of the prokaryotes with linear DNA, telomeres are found but the structure is different from those of eukaryotic cells. They are in the form of a hairpin loop formed by a single strand or bound to proteins.

Also Read: Chromosome Structure

Shortening and Role in Ageing and Cancer

Telomeres play a vital role in the ageing of the cells. With each replication, telomeres become shorter and shorter and when they become too short, the cells stop replicating and lead to senescence and apoptosis. Therefore, it plays the role of a biological clock for cellular ageing. It also leads to oncogenic transformation of cells.

The rate of telomere shortening can be decreased by better lifestyle, diet and activities. It delays the onset of age-related diseases and also increases lifespan.

A special enzyme known as ‘Telomerase’ can increase the length of telomeres. It is present in the cells which divide unlimited times, e.g. unicellular eukaryotes, egg and sperm cells, blood cells and also cancer cells.

In cancer cells scientists have observed that telomeres are too short and when it reaches the critical point, telomerase gets reactivated thus cancer cells multiply in an uncontrolled manner. Most of the cancer cells, e.g. breast, prostate, lung, pancreas, etc. contain telomerase, which maintains the telomere length and prevents apoptosis. Anti-cancer drug is being researched, which targets telomerase.

Explore: Chromosomal Disorders in Humans

Telomere Replication

Telomeres are the terminals of the linear chromosomes. They are the repetitive sequences coding for not a specific gene. The telomeres are involved in protecting the vital genes from deletion during cell division and shortening of DNA strands at the time of replication.

Some of the telomeric sequences after every round of replication tend to get lost at the 5′ terminal of the synthesized strand on each of the daughter DNA. Since, these are noncoding sequences, its loss has no serious impact on the cell. These sequences however are not limitless. After adequate rounds of replication, the telomeric repeats get lost. There is a risk of DNA losing the coding sequences in the following rounds.

Thus, telomeres play an important role in preserving genes on DNA and cellular ageing. It protects the genome from degradation, unnecessary repair and recombination and fusion between two chromosomes.

Importance of Telomeres

• Functions as a mitotic clock of the cell as it shorten with every round of cell division
• In humans, these are distinct chromosomal end structures comprising of repeat sequences
• Essential in maintaining the chromosomal integrity. Its excess shortening is linked with instability in DNA

Conclusion

• Telomeres are distinct sequences typically present at the terminal of chromosomes
• It comprises the same short DNA sequences which are repetitive in nature
• TTAGGG is the telomere sequence in humans
• It is repeated approximately 3000 times and can extend its length up to 15,000 base pairs
• Telomere cap chromosome ends are functional in preventing the cell from identifying it as accidental DNA breaks
• They avert chromosome erosion at its terminals by involving in traditional DNA replication. It gets started with a RNA primer whose elimination cannot be restored by the DNA terminals.

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