What Are the Repetitive DNA Sequences Present at the Ends of Eukaryotic Chromosomes Called?

29 January 2024

Venkat S. R.

This image shows the wearing down of the chromosome ends. Repetitive DNA sequences are found throughout eukaryotic genomes. One of the most critical repetitive DNA sequences is found at the ends of eukaryotic chromosomes.

The repetitive DNA sequences found at the ends of eukaryotic chromosomes are called telomeres. Telomeres protect the chromosome from degrading or fusing with other chromosomes.

Telomeres act like end caps or protective caps on shoelaces. They protect the chromosome ends from getting frayed and prevent them from sticking to other chromosomes.

The DNA sequence in telomeres consists of a repeating TTAGGG sequence in humans, and the number of repeats can vary from a few to several thousand, depending on the organism.

This article explains the telomere structure, its functions, the role of the enzyme telomerase in regulating the ends of the chromosomes, and the importance of this entire process.

Table Of Contents

Chromosome Structure and Repetitive DNA
What Are the Repetitive DNA Sequences Present at the Ends of Eukaryotic Chromosomes Called?
The Discovery of Telomeres and Telomerase
Telomere Structure and Function
Telomerase: The Enzyme that Maintains Telomeres
The Importance of Telomeres and Repetitive DNA
Frequently Asked Questions
- Sources:

Chromosome Structure and Repetitive DNA

Eukaryotic chromosomes are made of DNA and proteins, organized into a complex structure called chromatin.

Chromatin is essential for packaging and regulating DNA, which contains the genetic information that encodes all living organisms’ traits. DNA is made up of four nucleotides — adenine (A), thymine (T), cytosine (C), and guanine (G).

The sequence of these nucleotides determines the genetic code. Repetitive DNA sequences are found throughout eukaryotic genomes, and they can occur in different forms. One of the most critical repetitive DNA sequences is found at the ends of eukaryotic chromosomes.

What Are the Repetitive DNA Sequences Present at the Ends of Eukaryotic Chromosomes Called?

This image shows the repetitive DNA sequences at the chromosome ends called telomeres. They protect chromosome ends from degradation or becoming fused with other chromosomes during cell division. — The repetitive DNA sequences at the chromosome ends are called telomeres. They protect chromosome ends from degradation or becoming fused with other chromosomes during cell division.

Telomeres are specialized structures found at the ends of eukaryotic chromosomes. They protect the chromosome ends from being degraded or fused with other chromosomes during cell division. Telomeres also play a critical role in regulating cellular aging and preventing cancer development. ^{1}

The Discovery of Telomeres and Telomerase

The discovery of telomeres dates back to the early 20th century when scientists observed that the ends of chromosomes were unusually stable compared to the rest of the chromosome.

However, it wasn’t until the 1980s that Elizabeth Blackburn and Jack Szostak discovered the repetitive DNA sequences present at telomeres in the model organism, Tetrahymena.

Later, Blackburn and Carol Greider discovered the enzyme telomerase, which maintains telomere length. This discovery earned all three of them the Nobel Prize in physiology or medicine in 2009. ^{2}

Telomere Structure and Function

Telomeres are critical components in linear chromosomes composed of repetitive DNA sequences, usually with a TTAGGG motif in vertebrates. The length of telomeres varies between species, but they typically range from a few hundred to a few thousand base pairs. ^{3}

Telomeres function as a buffer zone for chromosome ends, protecting them from being mistaken as DNA breaks and triggering a cellular response that can lead to DNA damage and mutations. Telomeres also play a role in regulating cellular aging and senescence.

As cells divide, their telomeres gradually shorten. Once telomeres reach a critical length, cells enter a state of senescence or stop dividing, leading to cellular aging.

Telomere shortening and dysfunction is associated with various age-related diseases, such as cancer, cardiovascular disease, and Alzheimer’s disease.

Mutations in genes involved in telomere maintenance, such as telomerase components or shelterin proteins, can lead to telomere dysfunction and disease.

Telomeres are not the only repetitive DNA sequences present in eukaryotic genomes. Other types of repetitive DNA include transposable elements, satellite DNA, and minisatellites. These repetitive DNA sequences play a variety of roles in genome organization, regulation, and evolution.

Telomerase: The Enzyme that Maintains Telomeres

Telomerase is an enzyme that adds DNA sequences to the ends of chromosomes, maintaining telomere length. In humans, telomerase is primarily active during embryonic development and in stem cells but becomes repressed in most somatic cells.

Reactivating telomerase in adult cells has been proposed as a strategy to combat cellular aging and age-related diseases.

The Importance of Telomeres and Repetitive DNA

Telomeres and other repetitive DNA sequences are essential components of eukaryotic chromosomes. They play critical roles in maintaining genome stability, regulating gene expression, and protecting chromosome ends from degradation and fusion during cell division.

Telomeres, in particular, are vital to prevent cellular aging and cancer development. The discovery of telomeres and telomerase has revolutionized our understanding of chromosome biology and opened up new avenues for research into age-related diseases and cancer.

While much is still unknown about the role of repetitive DNA sequences in the genome, ongoing research promises to shed more light on their function and significance in biology.

They are just one example of the many repetitive DNA sequences that are found throughout eukaryotic genomes, each with their own unique functions and roles.

Frequently Asked Questions

Q) What are repetitive DNA sequences called?

A) Repetitive DNA sequences are stretches of DNA that occur multiple times in the genome. Some of the common examples of repetitive DNA sequences are called telomeres and centromeres.

Telomeres occur at the end of the chromosomes. Centromeres are typically regions on a chromosome that join sister chromatids whose primary function is for the attachment of spindle fibers in cell division.

While centromeres are typically located in the central area of a chromosome, they can also be located near the mid-region or at a number of different positions on the chromosome. ^{4}

Repetitive DNA sequences play a vital role in genome structure and function and can vary in length. They can be classified into different types based on their function, location, and organization in the genome.

Q) What are the types of repetitive DNA in eukaryotes?

A) Repetitive DNA sequences can be classified into two types — tandem repetitive sequences (known as satellite DNA) and the interspersed repeats.

Tandem repetitive sequences typically occur in three variations:

Macrosatellite sequences. Small units (without any specific number) of DNA repeated in tandem thousands of times. These are known as variable number of tandem repeats (VNTR). This large polymorphism is found mostly in centromeres and telomeres.
Minisatellite sequences. These are repeated units larger than macrosatellites, but are usually found less frequently as compared to macrosatellite sequences.
Microsatellite sequences. These involve small tandem repeats typically containing two to six base pairs. They’re also known as short tandem repeat (SSR) or simple sequence repeat (SSR).

Interspersed DNA repeats are of two types:

Long interspersed elements (LINES). LINES make up around 15% of the total human genome.
Short interspersed elements (SINES). SINES are obtained from LINES and make up roughly 10% of the human genome and are usually about 300 base pairs long. ^{5}

Q) Why are there repeats on the ends of eukaryotic chromosomes?

A) Repeats on the ends of eukaryotic chromosomes are necessary to protect the chromosome ends from being degraded or fused with other chromosomes during cell division.

The repetitive DNA sequences that make up telomeres are specifically designed to buffer the ends of chromosomes, preventing the chromosome ends from being recognized as DNA breaks and triggering a cellular response that can lead to DNA damage and mutations.

Q) Why is repetitive DNA called satellite DNA?

A) Repetitive DNA is called satellite DNA because of its characteristic appearance on DNA sequencing gels. When DNA is centrifuged on a gradient, repetitive DNA sequences will separate from non-repetitive DNA, forming distinct bands that resemble a satellite.

This is due to the fact that the repetitive sequences have a higher molecular weight and thus migrate differently through the gradient than non-repetitive DNA.

Satellite DNA can be further divided into different classes based on their length, organization, and sequence composition, and can have a range of functions in the genome, including chromosome structure, gene regulation, and genome stability.

Sources:

1 – Experimental Cell Research: “Telomeres: The beginnings and ends of eukaryotic chromosomes.”

2 – Arizona State University: “Elizabeth Blackburn, Carol Greider and Jack Szostak’s Telomere and Telomerase Experiments (1982-1989).”

3 – Your Genome: “What is a telomere?”

4 – Genome Biology: “The role of DNA sequence in centromere formation.”

5 – Molecular Medicine: “Genomics to Personalized Healthcare.”